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Greatest-common-divisor dependency of juggling sequence rotation efficient performance

Greatest-common-divisor dependency of juggling sequence rotation efficient performance 1IntroductionRearrangement of objects to achieve some goal is common in many aspects of computing, including databases, image processing, cryptography, priority queuing of tasks for scheduling in operating and real-time systems, to mention but a few. When memory is a scarce and critical resource in the systems, such as embedded/real-time systems, the operation would be restricted to minimum or constant memory usage known as “in-place” execution [1,2]. Typical in-place objects rearrangement algorithms such as those for in-place sorting include insertion sort, quick sort, and some implementation of merging in merge sort. In-place re-arrangement operations are commonly accomplished using sequence (or array) rotation. Sequence rotation or circular shifting is also increasingly being used in internal buffer management in text editors [3,4,5], co-processor design [6], image encryption [7], permutation in Data Encryption Standard and Advanced Encryption Standard [8], and task scheduling in real-time systems [9]. Hence, sequence rotation algorithms have become so ubiquitous that they form part of programming languages standard libraries such as Java and C++ “Standard Template Libraries” (STL) [10,11]. In short, any in-place operation to rearrangement of sequence items would likely employ sequence rotation. Popular among these rotation algorithms are three-way-reversal and juggling rotation.Juggling sequence rotation is a circle shifting of sequence (or array) elements to exchange part of the sequence with another. Each element move (or assignment) is accomplished in a circular fashion at specified length or number of skipped positions on the sequence, known as cycle gap/cycle length (or offset). A move is “either assigning a value into an array[/sequence] element or copying an array[/sequence] element to elsewhere” [10]. The next element to move requires jumping/skipping one or more specified preceding/succeeding elements (depending on left/right rotation, respectively [12]) like a juggler exercise, hence the term ‘Juggling Rotation.’ It was first tagged “Dolphin algorithm” as the idea was conceived to be “like Dolphins leaping out of water and disappear again” [5] (Figure 1a). The skipped elements respective positions, which constitute the cycle gap, include the destination location but exclude the source position of the next element to be moved. The number of circles required to accomplish complete sequence block exchange with another is calculated using greatest common divisor (GCD) of the sequence size and the cycle gap, hence the alternative term ‘greatest common divisor (or simply GCD) based sequence rotation’ [13]. Thus, the juggling length (or offset) determines the GCD-computed number of circles to complete the rotation.Figure 1Illustrative diagrams of juggling and three-way-reversal sequence rotation. (a) A sequence size 7 with cycle gap 3 rotation. Number of circles = gcd(7, 3) = 1. Elements move in the direction of the arcs (like dolphins), each skipping cycle gap item positions to its destination. (b) A sequence size of 10 with cycle gap 5 rotation using three-way-reversal. The numbering at the bottom of the sequence is a mirror of the original sequence. The two sets of arcs at the top do independent reversals on their respective portions of the sequence and the set of arcs at the bottom does the final reversal after the first two have completed.Three-way-reversal, on the other hand, takes a reversal approach in rotating a sequence. A sequence reversal maps entire sequence into pairs of corresponding elements from both ends of the sequence and swap the paired elements. The pairing divides the sequence, at the middle, into two equal discrete halves. Mirrored items from opposite halves are swapped. Let the sequence S be sorted such that S={(e)i∈N∣ei⩽ei+1}S=\left\{{\left(e)}_{i\in {\mathbb{N}}}| {e}_{i}\leqslant {e}_{i+1}\right\}. We define the reversal or inversion of SSas S−1={(e)j∈N∣ej⩾ej+1}{S}^{-1}=\left\{{\left(e)}_{j\in {\mathbb{N}}}| {e}_{j}\geqslant {e}_{j+1}\right\}. For example, if S={1,2,3,4}S=\left\{1,2,3,4\right\}, then S−1={4,3,2,1}{S}^{-1}=\left\{4,3,2,1\right\}. Three-way-reversal divides the sequence into two halves at the middle and applies the reversal function on both halves separately and independently. If the sequence size is odd, the middle item is untouched. Finally, the entire resulting sequence is again reversed making the third reversal, hence the “three-way-reversal” (Figure 1b). Note that the reversal function is not stable, but three-way-reversal is, with regard to individual elements in their respective separate halves and not for identical elements across the halves. We are interested in the performance of juggling rotation in the light of its rival competitor – the three-way-reversal.The motivation for this interest is that we want to see algorithm that runs with few number of elements assignments and requires the smallest extra memory usage, making it suitable for embedded/real-time systems. In particular, hard (or even soft) real-time systems, whose timing requirements must not be compromised, require such algorithm that executes uniformly with few extra memory need. Although, other domains may not bother about duration of the algorithm execution, the real-time system domain cannot afford to gamble with timing and accuracy of algorithm. Hence, in studying the juggling algorithm, our focus is on embedded/real-time system domain that would require fewest element assignments running proportionately (and accurately) with algorithm timing in a uniform fashion. As an optimized algorithm, we expect the juggling algorithm to run fastest and uniformly – given that it assigns fewest number of elements, as against three-way-reversal algorithm. Such algorithm will be well suited for the embedded/real-time systems, whose timing constraints must not be undermined.A good performing algorithm is judged by its efficiency using computational complexity theory [14,15]. The complexity is the execution cost measured in terms of storage, time, and/or “whatever units are relevant” [16]. Sometimes, the performance is measured in terms of optimality, focusing on “best configuration” (such as number of element assignments and comparisons [10]) to achieve some goals [17,18]. On this, juggling rotation algorithm is optimal [5,19]. But this may not translate to time or space complexity efficiency, suitable for time critical systems. The following notations are used in this article: gcd-R – GCD-based rotation; twr – three-way-reversal rotation.In this article, Section 1 presents the introduction. Section 2 contains the related works that briefly review the literature and clearly states the problem. Section 3 highlights the tools and methods used in the experimentation. Section 4 presents and analyzes the results. In discussing the results, Section 5 explains the implication of the outcome, identifying the positivity and negativity of it. Finally, Section 6 concludes the article with emphasis on the positivity and negativity of the research and also gives some direction for future scope.2Related worksIn the light of memory/time criticality of systems, most embedded systems demand that any applied algorithm consumes minimum memory [20,21] to satisfy the “three common principles” [22] of embedded systems: performance, low energy consumption, and low price due to limited hardware [23]; and if the embedded systems are real-time, the execution must also be time bound [24]. While unexpected delay in execution may be tolerated in some applications such as ordinary in-place sorting, this is forbidden in some embedded/real-time system operations [25] such as ordering sequence of tasks for priority scheduling. Yet, most of the studies on real-time system scheduling have been on solving the complex, “NP-hard” [9], problem of allocating tasks to uni(multi)processor(s) for execution to meet deadline. Little attention has so far been given to efficient (re-)ordering of tasks themselves before safely allocating to processors. Interestingly, all these studies often defined task models that comprised sequence of jobs [9,26] as ordered set or vector of tuple of parameters such as arrival time, size of task, and deadline.While studying real-time “scheduling algorithms for divisible loads,” Lin et al. [27] made three important decisions, including “scheduling policy to determine the order of execution for tasks,” “number of processing nodes to allocate to each task,” and “strategy to partition the task among the allocated nodes.” Our interest here focuses on the scheduling policy. The study investigated policies for ordering of tasks to be executed – the popular FIFO (First In First Out in which tasks were scheduled in “their order of arrival”), EDF (Earliest Deadline First in which tasks were ordered for real-time scheduling by “their absolute deadlines”), and MWF (“Maximum Workload derivative First” which was “a real-time scheduling algorithm for divisible tasks”). Whichever policy or combination of policies adopted, one concept was common – ‘ordering of sequence of tasks,’ even if the policies included “processor preemption” [9]. Also, Dinh [28] carried out extensive and excellent study of scheduling parallel tasks for multiprocessor. Whether it be “Federated scheduling” (based on “heavy tasks” and “light tasks”) or “global fixed-priority (G-FP)” or “global scheduling and partitioned scheduling” for multiprocessor scheduling, there was usually associating queue (or sequence) containing “the tasks sorted” [28] in a particular order. But there was little study on the safety and efficiency of the algorithms used for sorting the tasks queue for the scheduling.Probably, these studies on scheduling already settled for existing (re-)ordering algorithms. But it is a common knowledge that the complexity of computer architecture keeps on increasing, in their power and number of processors, from “clusters” well into “cloud” of multiprocessing systems that comprises “hundreds and thousands computers” in their respective “utility computing” clusters [29]. This has correspondingly opened the flood gate of tremendously increasing size of data of different forms admissible for processing. Here comes two ever emerging concept: increasing power and number of processing units versus increasing data size. In the context of real-time systems, this would mean more than scheduling tasks for “clusters” [27] of multiprocessors and to include efficient (re-)ordering of the tasks (possibly in huge number) for safety scheduling. Now, as the sequence size of the jobs increases, we are faced with the challenge of identifying the sorting algorithms that would rearrange the tasks efficiently without compromising the deadline targetsMittermair and Puschner [30] sought to address the efficient re-ordering question by asking “Which sorting algorithms[…]” were suitable for hard real-time systems. In addressing it, they showed that of the eight sorting algorithms studied – “bubble sort, insertion sort, selection sort, merge sort, quick sort, heap sort, radix sort, and distribution counting sort” – merge sort was the most “stable” and had “the best worst-case performance” as the size of data increases. The study opined that because of the time criticality of hard real-time systems, any consideration of algorithm suitability should be based on worst-case execution. In this case, merge sort won.For merge sort, the most time intensive component of the algorithm is the merge function. Suitable implementation of merge sort for embedded/real-time systems would be the in-place merging strategy, because of the system minimum memory requirements. And, in-place merging is commonly accomplished with sequence rotation. This is where our research is anchored on the context of real-time system task scheduling.Now, given the theoretical optimality of juggling array rotation, developers who use the algorithm may choose any cycle gap to implement rearrangement of tasks requirements, including those for some hard real-time applications. This can be disastrous for time critical systems if execution is delayed, failing to satisfy one or more of the requirements stipulated in system formal specification [31].The current study examines delays in execution for juggling rotation algorithm. We use Java as simulation programming language for the rotation to simulate ordering of tasks for safety and efficiency in the domain of real-time scheduling, since Java is popular for implementing real-time systems – coined as “real-time Java.” The study is limited to GCD values that are equal to cycle gaps, since all other cycle gaps decompose to GCDs that equal cycle gap as adequately studied by ref. [10].In the study by ref. [10], two in-place rotation algorithms, implemented in the STL distribution, were analyzed. The first, STL1{{\rm{STL}}}_{1}, used swap function while the other, STL2{{\rm{STL}}}_{2}, used GCD function. The study proved theoretically that STL1{{\rm{STL}}}_{1}used n−gcd(n,Δ)n-{\rm{\gcd }}\left(n,\Delta )swaps to rotate array of size nnand cycle length Δ\Delta , noting that each swap “macro” required three assignments or movements of elements. The STL2{{\rm{STL}}}_{2}version used explicit GCD and proved, using graph theory, that total number of elements moves required were n+gcd(n,Δ)n+{\rm{\gcd }}\left(n,\Delta ). It further showed that any array of size nncan be decomposed into “disjoint classes of cycle lengths.” The study concluded that STL2{{\rm{STL}}}_{2}rotation algorithm is optimal, since it used only n+gcd(n,Δ)n+{\rm{\gcd }}\left(n,\Delta )item moves (compare ref. [19]). To further cement the finding, empirical results were obtained both for STL1{{\rm{STL}}}_{1}and STL2{{\rm{STL}}}_{2}and for additional STL3{{\rm{STL}}}_{3}and Space algorithms. STL1{{\rm{STL}}}_{1}was the version that used implicit GCD (i.e., the GCD was incorporated into the inner loop of the rotation) while Space used auxiliary memory. The outcome of the experiment is as follows: Space had the fastest running time followed by STL2{{\rm{STL}}}_{2}, while STL1{{\rm{STL}}}_{1}and STL3{{\rm{STL}}}_{3}exhibited almost same running times. The study suspected that incorporation of “variable Bound” into STL3{{\rm{STL}}}_{3}could be the reason for its low performance, since the Bound was maintained in nntimes. Even though there are other implementation styles [32,33], including non-GCD explicit version STL3{{\rm{STL}}}_{3}, the current study is motivated to use the GCD explicit version (STL2{{\rm{STL}}}_{2}) for the in-place rotation, since Space used auxiliary memory and the fact that “the extra cost of implicitly computing the GCD was significantly higher than computing it explicitly” [10] – not suitable for hard real-time systems.In using GCD rotation algorithms to exchange (or rotate, circle shift) block or section of sequence or array with another in an in-place sorting, studies have shown that its use is optimal as stated earlier [17,19] and the experimental running time result from the algorithm is believed to exhibit high performance strength [18]. This is good fit; since every computing system is desired to execute at best performance, placing more demands for selection among candidates, especially for embedded/real-time systems. As mentioned earlier, the GCD-based rotation or “vector exchange” [19] is believed to be more efficient than some other rotation algorithms. On this, Erho et al. [13] and Symvonis [34] found that “the problem solution can be reduced” to m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)moves [10] as against its three-way-reversal counterpart that used as much as 3(⌊(m+n)∕2⌋+⌊m∕2⌋+⌊n∕2⌋)3\left(\lfloor \left(m+n)/2\rfloor +\lfloor m/2\rfloor +\lfloor n/2\rfloor )moves [19], where m+nm+nwas the sequence size and nnthe cycle gap or offset [10]. However, Erho and Consul [35] had further reduced the excessive mathematical expression of three-way-reversal element moves to 3(m+n−ev)3\left(m+n-ev)and index comparison to m+n+3−evm+n+3-ev, where ev=0ev=0or 1 as determined by ref. [35] using a modular function ev=mmod2,ifmmod2=nmod21,otherwise.ev=\left\{\begin{array}{l}m\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2,\hspace{1em}{\rm{if}}\hspace{0.33em}m\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2=n\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2\\ 1,\hspace{1em}{\rm{otherwise}}.\end{array}\right.Additionally, in the study by ref. [35], a claim was made that the rotation algorithm performance might not be affected by the size of elements versus indexes variability. The study cited a student record database table that had score field, the main field of interest, ranging from −1-1to 100% but index field ranging into millions. In such case, indexes were “predominantly heavier than elements” [13]. This motivated the study in ref. [13] to choose sequence size of 20,000,000 to enable the processor spend enough time doing the rotation and for the researchers to observe the behavior of the algorithm over large input of indexes against different sizes of elements as enshrined in four primitive data types.While using the least common multiple and GCD [6,36], Erho et al. [13] not only confirmed that the GCD-based rotation actually required m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)element moves, they additionally derived that it required 2m+3n−gcd(m+n,n)2m+3n-{\rm{\gcd }}\left(m+n,n)index moves for some implementation style of explicit GCD calculated rotation – the STL2{{\rm{STL}}}_{2}[10]. Yet, the study by Erho et al. [13] showed that despite sizable number, 3(m+n−ev)3\left(m+n-ev), of element moves and smaller number, m+n+3−evm+n+3-ev, of index moves for three-way-reversal, there was significantly huge discrepancy between the average execution time of the GCD rotation and the three-way-reversal approach across all four data types (LONG, INT, SHORT, and CHAR) in Java. This conclusion was reached using “two-way ANOVA test” on the empirical data with R statistical package. By testing hypothesis on the interaction of the two factors: algorithms versus data types and their levels – (gcd-R versus twr) and (LONG, INT, SHORT, versus CHAR) – respectively, the study showed that index computing had no significant effect on the running times of both algorithms across the four data types, but element moves did. This element assignment observation had already been made by Bentley [4], pointing out that this phenomenon was due to the algorithm “poor caching behavior.” However, this general observation was not in the light of different data types and lacked sufficient details.Taking one of the data types, type LONG for instance, Erho et al. [13] showed that, on the average, execution time for three-way-reversal was only 49.66761 ms, whereas that of GCD-based rotation was as huge as 246.4394ms – approximately 496.18% difference. This study is curious about what contributes to this apparent big gap, outside the observation by ref. [4], given the theoretical derivation that GCD-based rotation used only m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)element moves against three-way-reversal rotation which used as much as 3(m+n−ev)3\left(m+n-ev)element moves. The interest of this current study is therefore to investigate the source of the huge up surge in the average execution time of the GCD-based rotation. The aim is to properly guide prospective implementation of the juggling rotation (particularly in real-time system task scheduling) should the algorithm be a choice. The objective is to establish better understanding of the behavior (in terms of time complexity in a real-time system context) of the GCD-based rotation at various ranges of cycle gaps, through experimental data analyzed both graphically and tabularly. This can only be achieved with good tools and appropriate methods.3Materials and methodsThe data presented in this study were extractions from the ones studied in ref. [13], but distinctively in different form. Also, a preliminary version (mainly the data and results) of this current form of the article has already been presented at a conference [37]. So the tools and methodology used were exactly the ones described in the previous studies. Briefly on the experiment, as reflected in refs [13,37], two algorithms (juggling and three-way-reversal) were used for each of the four data types (Figures 2 and 3). The function codes for both algorithms were copied directly from the simulation implementation in ref. [13] and briefly discussed also in ref. [37]. Execution timings of all four data types were computed in the same manner with the two algorithms encapsulated within their respective data type blocks of running loops, each type in seperate program file. The block execution iterating for 100 times was nested in an outer loop iterating for 5 times and the execution times for each appended to a text file. The 5-times-limit looping was again nested in another outer loop running for 71 times. On the whole, we had 100×5×71=35,500100\times 5\times 71=\hspace{0.1em}\text{35,500}\hspace{0.1em}records generated. However, this current study did not give attention to indexes assignments execution timing, since the concern here was on influence of cycle gaps/GCD on execution timing. Also, the sampled data used here were selection of only one record per cycle gap instead of five (each from every next 100 records) in the previous study.Figure 2GCD-based rotation function as implemented in Java program.Figure 3Three-way-reversal-based rotation function in Java program.In this study, extract of 71 records representing the different cycle gaps from the large pool of raw data, generated in ref. [13], is presented. The data are plotted in graphs using Microsoft Excel and inspected pictorially. The patterns exhibited in the graphs necessitated reinspection of the actual data from the various data types. But it turned out that inspection of one type sufficed. Thus, type LONG data were presented also in tabular form for correlation with its corresponding graph. Table 1 is extracted from the LONG data type record sample of that pool of data.Table 1Sampled data for GCD rotation and three-way-reversal rotation, one record per cycle gap of 71 (see refs [13, 37])Table page 1Table page 2Table page 3Table page 4gcd-RtwrCycle gapgcd-RtwrCycle gapgcd-RtwrCycle gapgcd-RtwrCycle gap454910,000,000711334980,00053464503,20035270481281743485,000,000701445378,12552437513,12534281481251643484,000,00069784962,50051448592,50033284491001544512,500,00068994950,00050405502,0003230348801450482,000,000671055240,00049427501,6003127048641348511,250,000662124932,00048440481,2803025748501261521,000,000651234931,25047397491,2502930549401113154800,000641025025,00046376481,000283154932107449625,000631525020,000453904880027321512596959500,000624074816,0004439850640262924820810852400,000613794915,625433825262525272491678448312,500604064912,500424114850024207501066748250,000594904810,00041376504002317349859947200,00058433478,000403814932022106485419549160,00057479506,40039303472562189494311357156,25056469496,2503834749250205449226651125,00055466485,00037309492001943481111651100,00054462494,000363104916018The table is arranged in a way that several cycle gap records can be viewed side by side. It is arranged into four columns labeled table page 1 to table page 4. Each table page holds records of cycle gaps with their corresponding algorithms running times. The table-page columns should be seen as sequential. For example, table page 2 column follows table page 1 column, in that order, if vertical arrangement of table-pages is assumed. The gray-colored-column serial numbering of the rows is the 71 cycle gaps represented in the horizontal entries of the graphs. On the table, serial numbers 1 to 5, for example, represent cycle gaps 1, 2, 4, 5, 8, and serial numbers 69, 70, 71 represent 4,000,000 and 5,000,000 and 10,000,000, respectively. Sequel to this, the horizontal entries of a corresponding graph for serial number 1, 3, and 5 represent the cycle gaps 1, 4, and 8, respectively.The three-way-reversal rotation data were also extracted and placed alongside the juggling rotation for easy comparison of deviations from uniformly efficient performance. Now, we want to figure out the outcome of these experimental data.4ResultsWe want to disclose here that the results of this work have already been published in a Conference Proceedings [37]. However, there are great variations between the conference paper and what we present here. In fact, the title of the conference paper had a limited view on the study, which also affected the keyword choices. Again, the article was barely five pages with bogus images having dotted lines. The table, too, was slightly different in shape and coloration. Little (or non) was said about the application domains and significance of the study could not be clearly ascertained in that conference article. In addition, far reaching conclusions of the research were altogether either missing or scanty at best. Now, let us analyze these results to see in depth of what they mean.In a quick look through the first few and the last few cycle gaps in the table of rotation algorithms data, the execution timing shows that GCD-based sequence exchange actually performs better than three-way-reversal rotation. Yet, the study [13] had successfully shown that, on the average, execution time for three-way-reversal was only 49.66761ms, whereas that of GCD-based rotation was as big as 246.4394ms. We are now more inquisitive than ever to know the reason for this apparent big gap.4.1Analysis of the graphsAlthough Table 1 may not really be necessary since it represents Figure 4, we present it here for the purpose of easy comparison with the figure. Conversely, Figure 4 graph would not be required for the same above reason [38]. The three-way-reversal rotation graph was plotted alongside the juggling rotation for a clear comparison and to give a feel of the deviations from efficient performance. Plotting the extracted data in Excel, the look of all four graphs tends to be like bell shapes. This meant that GCD rotation efficient performance was lowest around a consecutive range of cycle gaps – quite some points away from both ends and concentrating around the middle of the cycle gaps listing. For data type LONG, the range is between point 9 and 44 standing for cycle gaps 25–16,000. But note the high performance indication from point 45–65, for cycle gaps 20,000–1,000,000 of juggling rotation and even outperformed three-way-reversal rotation at point 66–71. This trend appeared to be similar to those of the integer data type (Figures 4 and 5).Figure 4Gcd-R for LONG data type.Figure 5Gcd-R for INT data type.The case was a little different for SHORT and CHAR data types, probably due to the circumstance of the materials and methods used. With these types the bell shapes are still visible but with some sort of down skewness close to the peak of the bells, tending to split the single bell into two, respectively. This occurs around point 18 to 31 for the cycle gaps 160–1,600, in that order. Note that even though Table 1 is meant for type LONG, the serial numbering (the gray-colored) and the cycle gap columns are the same across all four types. In similarity with types LONG and INT, the SHORT and CHAR types also had high performance indications from point 1 to 5, for the cycle gaps 1–8 and from points 50 to 71, for the cycle gaps 50,000 to 10,000,000. Type CHAR even had higher performance indication from points 1 to 3, for the cycle gaps 1, 2, and 4 (Figures 6 and 7). This is important in view of the domain under consideration. Let us have a glimpse of the implications of these results.Figure 6Gcd-R for SHORT data type.Figure 7Gcd-R for CHAR data type.5DiscussionThe previous study by Erho et al. [13] had already shown that GCD-based rotation demonstrated, on the average, a huge up surge of 246.4394ms in execution time than three-way-reversal running time of just 49.66761ms. This was approximately 496.18% difference. What the current research has shown is that the GCD rotation performance for some of the cycle gaps actually competes favorably with three-way-reversal rotation, which means a good and balanced efficiency in performance along the other competitor is maintained with these cycle gaps. This can easily be seen from the two extreme points on the xx-axis of all four graphs.Surprisingly, though, the plots begin to exhibit sharp rise after few cycle gaps and then steep fall before few remaining cycle gaps, considerably. The pattern produced appears to be bell shaped. This could not be as a result of mere outliers, otherwise it would not maintain somewhat uniform pattern across all four graphs. The conclusion reachable here is that ‘there could be a phenomenon playing out’: for some cycle gaps (around the region closest to 1 and/or near, but not higher than, half of the sequence size) the performance is quite efficient, but very poor otherwise. This could be deceptive to system developers given the known optimality of juggling rotation algorithm.A careful comparison of Figure 4 with Table 1 shows that the worst of this poor performance coincides with cycle gaps within the neighborhood of the square root of the sequence size; and all four graphs conform to this fact. For instance, the sequence size used for this experiment was 20,000,000 and the square root was approximately 4472.14. The table has that about seven (7) cycle gaps to both left and right of the square root value (4472.14) fall within the worst performing values for the GCD-based sequence rotation. These cycle gaps correspond to ranges 30–43 horizontal entries for the graphs. This range pattern appears to cut across all four data types as can be seen from the graphs. Impact of this discovery is better imagined in the development of real-time systems. A real-time system application developer, working on problem such as task scheduling, who is familiar with the theoretical optimality of juggling rotation and has also tested with the algorithm using small-size versus half-of-sequence-size ranges of cycle gaps would not imagine that some cycle gap values in between (around the square root of sequence size) will fail the system.Some form of skewness in graph is also observed with regard to some of the data types. It appears that as the data type decreased from long (LONG) to character (CHAR), the skewness became more and more pronounced. This pattern tends to split the single bell apparently into two in the case of SHORT and CHAR types. These points of skewness seem to improve the GCD rotation performance a little. This phenomenon may be language, operating system, and/or machine dependent. But our interest is in the behavior of the algorithm in Java, since Java (Real-Time Java) is popular for real-time systems. In any case, this phenomenon sound like a little good news for system developers/users if their data are of smaller sizes or around half of sequence size. Next, we want to know what to make of all these.6Conclusion and future workThis article has analyzed the relationship between sequence rotation cycle gaps, which are used to determine their GCDs, and the performance of GCD-based sequence rotation. Using randomly sampled data, extracted from the experiment data of the study in ref. [13] but presented in different format, the study found that efficient performances of the algorithm depended on the cycle gaps, which determined GCD. For cycle gaps that were close to 1 and/or close or up to half of the sequence size, the performance was quite efficiently competitive.Perhaps, this might explain why some previous studies quickly concluded that GCD rotation performed better than three-way-reversal rotation [18]. Mostly in an in-place sorting, the block to rotate with another would likely be about half of the sequence under consideration. In such a case, the implementation simply fall in the range of cycle gaps that we are designating in this study as “GCD rotation friendly” – compare Figure 7 and the data type used in ref. [10]. This can be deceptive to unsuspecting developer and dangerous for time critical systems.The study found that the situation was quite different when the cycle gaps were not in the ranges of GCD rotation friendly. In fact, the poor performance was worst when the algorithm used cycle gaps that were within about seven (7) cycle gaps to both left and right of the square root of the given sequence size. This understanding is quite significant because unsuspecting user/developer who may have used juggling rotation for in-place sorting which commonly involves swapping one half of a sequence with the other, may not imagine that cycle gaps (each much smaller than half of the sequence size) can be problematic in execution; given that algorithm complexity is measured in terms of input size. For instance, a real-time task scheduling application developer, knowing that it takes 50ms to rotate offsets of 4 or less, and similar for half of 20,000,000 sequence sizes, may implement requirement offset of just 4,500 items and places some 100ms time bound for safety. But this cycle gap runs for some 460ms (Table 1) – dangerous for time critical systems. Again, in Section 2, we stated that “[…]unexpected delay in execution may be tolerated in some applications such as ordinary in-place sorting[…]”. But can this 460ms execution time for the chosen cycle gap of 4,500 really be tolerated in sorting, where a much bigger cycle gap of 10,000,000 executes for just 50ms? In addition to this understanding, it seems not clear (or even unknown) in the literature that these peak sets of offsets are within the neighborhood of the square root of the sequence size.However, some form of skewness was also noticed that depended on the data types. SHORT and CHAR types in particular exhibited this, tending to split the single bell-shaped pattern of GCD rotation into two, respectively. These points, representing cycle gaps, within the splitting region showed slight improvement from the worst poor performance of the algorithm.Thus, the GCD-based sequence rotation efficient performance heavily depends on the greatest-common-divisor values, which are direct reflection of the cycle gaps. This research has given us a better understanding of the GCD-based sequence rotation by throwing light on the effect of GCD on performance of the algorithm. It therefore strongly recommends that if application involves sequence rotation (such as in-place sorting) with cycle gaps that are within the neighborhood to both left and right of the square root of the given sequence size, DO NOT use GCD-based vector rotation, especially as the size of input increases. For applications that are time critical, such as hard real-time systems, DO NOT use juggling rotation algorithm if the cycle gaps are not within GCD rotation friendly range.It is clear from this study that cycle gaps around the square root of sequence size are heavily slow in juggling rotation. The question is why is fewest number of element assignments not proportionate with execution time of juggling rotation? There is need for further work on this. Also, this study was limited to cycle lengths that were equal to GCD values. For instance, cycle gaps 3, 6, 7, 9, etc. that were not equal to their respective GCD were not covered in this study. Will the performance of juggling rotation on cycle gap 9, for example, be higher/lower than that of 8 and 10, and in what pattern? This is a question for investigation. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Open Computer Science de Gruyter

Greatest-common-divisor dependency of juggling sequence rotation efficient performance

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de Gruyter
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© 2022 Joseph A. Erho et al., published by De Gruyter
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2299-1093
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10.1515/comp-2022-0234
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Abstract

1IntroductionRearrangement of objects to achieve some goal is common in many aspects of computing, including databases, image processing, cryptography, priority queuing of tasks for scheduling in operating and real-time systems, to mention but a few. When memory is a scarce and critical resource in the systems, such as embedded/real-time systems, the operation would be restricted to minimum or constant memory usage known as “in-place” execution [1,2]. Typical in-place objects rearrangement algorithms such as those for in-place sorting include insertion sort, quick sort, and some implementation of merging in merge sort. In-place re-arrangement operations are commonly accomplished using sequence (or array) rotation. Sequence rotation or circular shifting is also increasingly being used in internal buffer management in text editors [3,4,5], co-processor design [6], image encryption [7], permutation in Data Encryption Standard and Advanced Encryption Standard [8], and task scheduling in real-time systems [9]. Hence, sequence rotation algorithms have become so ubiquitous that they form part of programming languages standard libraries such as Java and C++ “Standard Template Libraries” (STL) [10,11]. In short, any in-place operation to rearrangement of sequence items would likely employ sequence rotation. Popular among these rotation algorithms are three-way-reversal and juggling rotation.Juggling sequence rotation is a circle shifting of sequence (or array) elements to exchange part of the sequence with another. Each element move (or assignment) is accomplished in a circular fashion at specified length or number of skipped positions on the sequence, known as cycle gap/cycle length (or offset). A move is “either assigning a value into an array[/sequence] element or copying an array[/sequence] element to elsewhere” [10]. The next element to move requires jumping/skipping one or more specified preceding/succeeding elements (depending on left/right rotation, respectively [12]) like a juggler exercise, hence the term ‘Juggling Rotation.’ It was first tagged “Dolphin algorithm” as the idea was conceived to be “like Dolphins leaping out of water and disappear again” [5] (Figure 1a). The skipped elements respective positions, which constitute the cycle gap, include the destination location but exclude the source position of the next element to be moved. The number of circles required to accomplish complete sequence block exchange with another is calculated using greatest common divisor (GCD) of the sequence size and the cycle gap, hence the alternative term ‘greatest common divisor (or simply GCD) based sequence rotation’ [13]. Thus, the juggling length (or offset) determines the GCD-computed number of circles to complete the rotation.Figure 1Illustrative diagrams of juggling and three-way-reversal sequence rotation. (a) A sequence size 7 with cycle gap 3 rotation. Number of circles = gcd(7, 3) = 1. Elements move in the direction of the arcs (like dolphins), each skipping cycle gap item positions to its destination. (b) A sequence size of 10 with cycle gap 5 rotation using three-way-reversal. The numbering at the bottom of the sequence is a mirror of the original sequence. The two sets of arcs at the top do independent reversals on their respective portions of the sequence and the set of arcs at the bottom does the final reversal after the first two have completed.Three-way-reversal, on the other hand, takes a reversal approach in rotating a sequence. A sequence reversal maps entire sequence into pairs of corresponding elements from both ends of the sequence and swap the paired elements. The pairing divides the sequence, at the middle, into two equal discrete halves. Mirrored items from opposite halves are swapped. Let the sequence S be sorted such that S={(e)i∈N∣ei⩽ei+1}S=\left\{{\left(e)}_{i\in {\mathbb{N}}}| {e}_{i}\leqslant {e}_{i+1}\right\}. We define the reversal or inversion of SSas S−1={(e)j∈N∣ej⩾ej+1}{S}^{-1}=\left\{{\left(e)}_{j\in {\mathbb{N}}}| {e}_{j}\geqslant {e}_{j+1}\right\}. For example, if S={1,2,3,4}S=\left\{1,2,3,4\right\}, then S−1={4,3,2,1}{S}^{-1}=\left\{4,3,2,1\right\}. Three-way-reversal divides the sequence into two halves at the middle and applies the reversal function on both halves separately and independently. If the sequence size is odd, the middle item is untouched. Finally, the entire resulting sequence is again reversed making the third reversal, hence the “three-way-reversal” (Figure 1b). Note that the reversal function is not stable, but three-way-reversal is, with regard to individual elements in their respective separate halves and not for identical elements across the halves. We are interested in the performance of juggling rotation in the light of its rival competitor – the three-way-reversal.The motivation for this interest is that we want to see algorithm that runs with few number of elements assignments and requires the smallest extra memory usage, making it suitable for embedded/real-time systems. In particular, hard (or even soft) real-time systems, whose timing requirements must not be compromised, require such algorithm that executes uniformly with few extra memory need. Although, other domains may not bother about duration of the algorithm execution, the real-time system domain cannot afford to gamble with timing and accuracy of algorithm. Hence, in studying the juggling algorithm, our focus is on embedded/real-time system domain that would require fewest element assignments running proportionately (and accurately) with algorithm timing in a uniform fashion. As an optimized algorithm, we expect the juggling algorithm to run fastest and uniformly – given that it assigns fewest number of elements, as against three-way-reversal algorithm. Such algorithm will be well suited for the embedded/real-time systems, whose timing constraints must not be undermined.A good performing algorithm is judged by its efficiency using computational complexity theory [14,15]. The complexity is the execution cost measured in terms of storage, time, and/or “whatever units are relevant” [16]. Sometimes, the performance is measured in terms of optimality, focusing on “best configuration” (such as number of element assignments and comparisons [10]) to achieve some goals [17,18]. On this, juggling rotation algorithm is optimal [5,19]. But this may not translate to time or space complexity efficiency, suitable for time critical systems. The following notations are used in this article: gcd-R – GCD-based rotation; twr – three-way-reversal rotation.In this article, Section 1 presents the introduction. Section 2 contains the related works that briefly review the literature and clearly states the problem. Section 3 highlights the tools and methods used in the experimentation. Section 4 presents and analyzes the results. In discussing the results, Section 5 explains the implication of the outcome, identifying the positivity and negativity of it. Finally, Section 6 concludes the article with emphasis on the positivity and negativity of the research and also gives some direction for future scope.2Related worksIn the light of memory/time criticality of systems, most embedded systems demand that any applied algorithm consumes minimum memory [20,21] to satisfy the “three common principles” [22] of embedded systems: performance, low energy consumption, and low price due to limited hardware [23]; and if the embedded systems are real-time, the execution must also be time bound [24]. While unexpected delay in execution may be tolerated in some applications such as ordinary in-place sorting, this is forbidden in some embedded/real-time system operations [25] such as ordering sequence of tasks for priority scheduling. Yet, most of the studies on real-time system scheduling have been on solving the complex, “NP-hard” [9], problem of allocating tasks to uni(multi)processor(s) for execution to meet deadline. Little attention has so far been given to efficient (re-)ordering of tasks themselves before safely allocating to processors. Interestingly, all these studies often defined task models that comprised sequence of jobs [9,26] as ordered set or vector of tuple of parameters such as arrival time, size of task, and deadline.While studying real-time “scheduling algorithms for divisible loads,” Lin et al. [27] made three important decisions, including “scheduling policy to determine the order of execution for tasks,” “number of processing nodes to allocate to each task,” and “strategy to partition the task among the allocated nodes.” Our interest here focuses on the scheduling policy. The study investigated policies for ordering of tasks to be executed – the popular FIFO (First In First Out in which tasks were scheduled in “their order of arrival”), EDF (Earliest Deadline First in which tasks were ordered for real-time scheduling by “their absolute deadlines”), and MWF (“Maximum Workload derivative First” which was “a real-time scheduling algorithm for divisible tasks”). Whichever policy or combination of policies adopted, one concept was common – ‘ordering of sequence of tasks,’ even if the policies included “processor preemption” [9]. Also, Dinh [28] carried out extensive and excellent study of scheduling parallel tasks for multiprocessor. Whether it be “Federated scheduling” (based on “heavy tasks” and “light tasks”) or “global fixed-priority (G-FP)” or “global scheduling and partitioned scheduling” for multiprocessor scheduling, there was usually associating queue (or sequence) containing “the tasks sorted” [28] in a particular order. But there was little study on the safety and efficiency of the algorithms used for sorting the tasks queue for the scheduling.Probably, these studies on scheduling already settled for existing (re-)ordering algorithms. But it is a common knowledge that the complexity of computer architecture keeps on increasing, in their power and number of processors, from “clusters” well into “cloud” of multiprocessing systems that comprises “hundreds and thousands computers” in their respective “utility computing” clusters [29]. This has correspondingly opened the flood gate of tremendously increasing size of data of different forms admissible for processing. Here comes two ever emerging concept: increasing power and number of processing units versus increasing data size. In the context of real-time systems, this would mean more than scheduling tasks for “clusters” [27] of multiprocessors and to include efficient (re-)ordering of the tasks (possibly in huge number) for safety scheduling. Now, as the sequence size of the jobs increases, we are faced with the challenge of identifying the sorting algorithms that would rearrange the tasks efficiently without compromising the deadline targetsMittermair and Puschner [30] sought to address the efficient re-ordering question by asking “Which sorting algorithms[…]” were suitable for hard real-time systems. In addressing it, they showed that of the eight sorting algorithms studied – “bubble sort, insertion sort, selection sort, merge sort, quick sort, heap sort, radix sort, and distribution counting sort” – merge sort was the most “stable” and had “the best worst-case performance” as the size of data increases. The study opined that because of the time criticality of hard real-time systems, any consideration of algorithm suitability should be based on worst-case execution. In this case, merge sort won.For merge sort, the most time intensive component of the algorithm is the merge function. Suitable implementation of merge sort for embedded/real-time systems would be the in-place merging strategy, because of the system minimum memory requirements. And, in-place merging is commonly accomplished with sequence rotation. This is where our research is anchored on the context of real-time system task scheduling.Now, given the theoretical optimality of juggling array rotation, developers who use the algorithm may choose any cycle gap to implement rearrangement of tasks requirements, including those for some hard real-time applications. This can be disastrous for time critical systems if execution is delayed, failing to satisfy one or more of the requirements stipulated in system formal specification [31].The current study examines delays in execution for juggling rotation algorithm. We use Java as simulation programming language for the rotation to simulate ordering of tasks for safety and efficiency in the domain of real-time scheduling, since Java is popular for implementing real-time systems – coined as “real-time Java.” The study is limited to GCD values that are equal to cycle gaps, since all other cycle gaps decompose to GCDs that equal cycle gap as adequately studied by ref. [10].In the study by ref. [10], two in-place rotation algorithms, implemented in the STL distribution, were analyzed. The first, STL1{{\rm{STL}}}_{1}, used swap function while the other, STL2{{\rm{STL}}}_{2}, used GCD function. The study proved theoretically that STL1{{\rm{STL}}}_{1}used n−gcd(n,Δ)n-{\rm{\gcd }}\left(n,\Delta )swaps to rotate array of size nnand cycle length Δ\Delta , noting that each swap “macro” required three assignments or movements of elements. The STL2{{\rm{STL}}}_{2}version used explicit GCD and proved, using graph theory, that total number of elements moves required were n+gcd(n,Δ)n+{\rm{\gcd }}\left(n,\Delta ). It further showed that any array of size nncan be decomposed into “disjoint classes of cycle lengths.” The study concluded that STL2{{\rm{STL}}}_{2}rotation algorithm is optimal, since it used only n+gcd(n,Δ)n+{\rm{\gcd }}\left(n,\Delta )item moves (compare ref. [19]). To further cement the finding, empirical results were obtained both for STL1{{\rm{STL}}}_{1}and STL2{{\rm{STL}}}_{2}and for additional STL3{{\rm{STL}}}_{3}and Space algorithms. STL1{{\rm{STL}}}_{1}was the version that used implicit GCD (i.e., the GCD was incorporated into the inner loop of the rotation) while Space used auxiliary memory. The outcome of the experiment is as follows: Space had the fastest running time followed by STL2{{\rm{STL}}}_{2}, while STL1{{\rm{STL}}}_{1}and STL3{{\rm{STL}}}_{3}exhibited almost same running times. The study suspected that incorporation of “variable Bound” into STL3{{\rm{STL}}}_{3}could be the reason for its low performance, since the Bound was maintained in nntimes. Even though there are other implementation styles [32,33], including non-GCD explicit version STL3{{\rm{STL}}}_{3}, the current study is motivated to use the GCD explicit version (STL2{{\rm{STL}}}_{2}) for the in-place rotation, since Space used auxiliary memory and the fact that “the extra cost of implicitly computing the GCD was significantly higher than computing it explicitly” [10] – not suitable for hard real-time systems.In using GCD rotation algorithms to exchange (or rotate, circle shift) block or section of sequence or array with another in an in-place sorting, studies have shown that its use is optimal as stated earlier [17,19] and the experimental running time result from the algorithm is believed to exhibit high performance strength [18]. This is good fit; since every computing system is desired to execute at best performance, placing more demands for selection among candidates, especially for embedded/real-time systems. As mentioned earlier, the GCD-based rotation or “vector exchange” [19] is believed to be more efficient than some other rotation algorithms. On this, Erho et al. [13] and Symvonis [34] found that “the problem solution can be reduced” to m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)moves [10] as against its three-way-reversal counterpart that used as much as 3(⌊(m+n)∕2⌋+⌊m∕2⌋+⌊n∕2⌋)3\left(\lfloor \left(m+n)/2\rfloor +\lfloor m/2\rfloor +\lfloor n/2\rfloor )moves [19], where m+nm+nwas the sequence size and nnthe cycle gap or offset [10]. However, Erho and Consul [35] had further reduced the excessive mathematical expression of three-way-reversal element moves to 3(m+n−ev)3\left(m+n-ev)and index comparison to m+n+3−evm+n+3-ev, where ev=0ev=0or 1 as determined by ref. [35] using a modular function ev=mmod2,ifmmod2=nmod21,otherwise.ev=\left\{\begin{array}{l}m\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2,\hspace{1em}{\rm{if}}\hspace{0.33em}m\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2=n\hspace{0.33em}{\rm{mod}}\hspace{0.33em}2\\ 1,\hspace{1em}{\rm{otherwise}}.\end{array}\right.Additionally, in the study by ref. [35], a claim was made that the rotation algorithm performance might not be affected by the size of elements versus indexes variability. The study cited a student record database table that had score field, the main field of interest, ranging from −1-1to 100% but index field ranging into millions. In such case, indexes were “predominantly heavier than elements” [13]. This motivated the study in ref. [13] to choose sequence size of 20,000,000 to enable the processor spend enough time doing the rotation and for the researchers to observe the behavior of the algorithm over large input of indexes against different sizes of elements as enshrined in four primitive data types.While using the least common multiple and GCD [6,36], Erho et al. [13] not only confirmed that the GCD-based rotation actually required m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)element moves, they additionally derived that it required 2m+3n−gcd(m+n,n)2m+3n-{\rm{\gcd }}\left(m+n,n)index moves for some implementation style of explicit GCD calculated rotation – the STL2{{\rm{STL}}}_{2}[10]. Yet, the study by Erho et al. [13] showed that despite sizable number, 3(m+n−ev)3\left(m+n-ev), of element moves and smaller number, m+n+3−evm+n+3-ev, of index moves for three-way-reversal, there was significantly huge discrepancy between the average execution time of the GCD rotation and the three-way-reversal approach across all four data types (LONG, INT, SHORT, and CHAR) in Java. This conclusion was reached using “two-way ANOVA test” on the empirical data with R statistical package. By testing hypothesis on the interaction of the two factors: algorithms versus data types and their levels – (gcd-R versus twr) and (LONG, INT, SHORT, versus CHAR) – respectively, the study showed that index computing had no significant effect on the running times of both algorithms across the four data types, but element moves did. This element assignment observation had already been made by Bentley [4], pointing out that this phenomenon was due to the algorithm “poor caching behavior.” However, this general observation was not in the light of different data types and lacked sufficient details.Taking one of the data types, type LONG for instance, Erho et al. [13] showed that, on the average, execution time for three-way-reversal was only 49.66761 ms, whereas that of GCD-based rotation was as huge as 246.4394ms – approximately 496.18% difference. This study is curious about what contributes to this apparent big gap, outside the observation by ref. [4], given the theoretical derivation that GCD-based rotation used only m+n+gcd(m+n,n)m+n+{\rm{\gcd }}\left(m+n,n)element moves against three-way-reversal rotation which used as much as 3(m+n−ev)3\left(m+n-ev)element moves. The interest of this current study is therefore to investigate the source of the huge up surge in the average execution time of the GCD-based rotation. The aim is to properly guide prospective implementation of the juggling rotation (particularly in real-time system task scheduling) should the algorithm be a choice. The objective is to establish better understanding of the behavior (in terms of time complexity in a real-time system context) of the GCD-based rotation at various ranges of cycle gaps, through experimental data analyzed both graphically and tabularly. This can only be achieved with good tools and appropriate methods.3Materials and methodsThe data presented in this study were extractions from the ones studied in ref. [13], but distinctively in different form. Also, a preliminary version (mainly the data and results) of this current form of the article has already been presented at a conference [37]. So the tools and methodology used were exactly the ones described in the previous studies. Briefly on the experiment, as reflected in refs [13,37], two algorithms (juggling and three-way-reversal) were used for each of the four data types (Figures 2 and 3). The function codes for both algorithms were copied directly from the simulation implementation in ref. [13] and briefly discussed also in ref. [37]. Execution timings of all four data types were computed in the same manner with the two algorithms encapsulated within their respective data type blocks of running loops, each type in seperate program file. The block execution iterating for 100 times was nested in an outer loop iterating for 5 times and the execution times for each appended to a text file. The 5-times-limit looping was again nested in another outer loop running for 71 times. On the whole, we had 100×5×71=35,500100\times 5\times 71=\hspace{0.1em}\text{35,500}\hspace{0.1em}records generated. However, this current study did not give attention to indexes assignments execution timing, since the concern here was on influence of cycle gaps/GCD on execution timing. Also, the sampled data used here were selection of only one record per cycle gap instead of five (each from every next 100 records) in the previous study.Figure 2GCD-based rotation function as implemented in Java program.Figure 3Three-way-reversal-based rotation function in Java program.In this study, extract of 71 records representing the different cycle gaps from the large pool of raw data, generated in ref. [13], is presented. The data are plotted in graphs using Microsoft Excel and inspected pictorially. The patterns exhibited in the graphs necessitated reinspection of the actual data from the various data types. But it turned out that inspection of one type sufficed. Thus, type LONG data were presented also in tabular form for correlation with its corresponding graph. Table 1 is extracted from the LONG data type record sample of that pool of data.Table 1Sampled data for GCD rotation and three-way-reversal rotation, one record per cycle gap of 71 (see refs [13, 37])Table page 1Table page 2Table page 3Table page 4gcd-RtwrCycle gapgcd-RtwrCycle gapgcd-RtwrCycle gapgcd-RtwrCycle gap454910,000,000711334980,00053464503,20035270481281743485,000,000701445378,12552437513,12534281481251643484,000,00069784962,50051448592,50033284491001544512,500,00068994950,00050405502,0003230348801450482,000,000671055240,00049427501,6003127048641348511,250,000662124932,00048440481,2803025748501261521,000,000651234931,25047397491,2502930549401113154800,000641025025,00046376481,000283154932107449625,000631525020,000453904880027321512596959500,000624074816,0004439850640262924820810852400,000613794915,625433825262525272491678448312,500604064912,500424114850024207501066748250,000594904810,00041376504002317349859947200,00058433478,000403814932022106485419549160,00057479506,40039303472562189494311357156,25056469496,2503834749250205449226651125,00055466485,00037309492001943481111651100,00054462494,000363104916018The table is arranged in a way that several cycle gap records can be viewed side by side. It is arranged into four columns labeled table page 1 to table page 4. Each table page holds records of cycle gaps with their corresponding algorithms running times. The table-page columns should be seen as sequential. For example, table page 2 column follows table page 1 column, in that order, if vertical arrangement of table-pages is assumed. The gray-colored-column serial numbering of the rows is the 71 cycle gaps represented in the horizontal entries of the graphs. On the table, serial numbers 1 to 5, for example, represent cycle gaps 1, 2, 4, 5, 8, and serial numbers 69, 70, 71 represent 4,000,000 and 5,000,000 and 10,000,000, respectively. Sequel to this, the horizontal entries of a corresponding graph for serial number 1, 3, and 5 represent the cycle gaps 1, 4, and 8, respectively.The three-way-reversal rotation data were also extracted and placed alongside the juggling rotation for easy comparison of deviations from uniformly efficient performance. Now, we want to figure out the outcome of these experimental data.4ResultsWe want to disclose here that the results of this work have already been published in a Conference Proceedings [37]. However, there are great variations between the conference paper and what we present here. In fact, the title of the conference paper had a limited view on the study, which also affected the keyword choices. Again, the article was barely five pages with bogus images having dotted lines. The table, too, was slightly different in shape and coloration. Little (or non) was said about the application domains and significance of the study could not be clearly ascertained in that conference article. In addition, far reaching conclusions of the research were altogether either missing or scanty at best. Now, let us analyze these results to see in depth of what they mean.In a quick look through the first few and the last few cycle gaps in the table of rotation algorithms data, the execution timing shows that GCD-based sequence exchange actually performs better than three-way-reversal rotation. Yet, the study [13] had successfully shown that, on the average, execution time for three-way-reversal was only 49.66761ms, whereas that of GCD-based rotation was as big as 246.4394ms. We are now more inquisitive than ever to know the reason for this apparent big gap.4.1Analysis of the graphsAlthough Table 1 may not really be necessary since it represents Figure 4, we present it here for the purpose of easy comparison with the figure. Conversely, Figure 4 graph would not be required for the same above reason [38]. The three-way-reversal rotation graph was plotted alongside the juggling rotation for a clear comparison and to give a feel of the deviations from efficient performance. Plotting the extracted data in Excel, the look of all four graphs tends to be like bell shapes. This meant that GCD rotation efficient performance was lowest around a consecutive range of cycle gaps – quite some points away from both ends and concentrating around the middle of the cycle gaps listing. For data type LONG, the range is between point 9 and 44 standing for cycle gaps 25–16,000. But note the high performance indication from point 45–65, for cycle gaps 20,000–1,000,000 of juggling rotation and even outperformed three-way-reversal rotation at point 66–71. This trend appeared to be similar to those of the integer data type (Figures 4 and 5).Figure 4Gcd-R for LONG data type.Figure 5Gcd-R for INT data type.The case was a little different for SHORT and CHAR data types, probably due to the circumstance of the materials and methods used. With these types the bell shapes are still visible but with some sort of down skewness close to the peak of the bells, tending to split the single bell into two, respectively. This occurs around point 18 to 31 for the cycle gaps 160–1,600, in that order. Note that even though Table 1 is meant for type LONG, the serial numbering (the gray-colored) and the cycle gap columns are the same across all four types. In similarity with types LONG and INT, the SHORT and CHAR types also had high performance indications from point 1 to 5, for the cycle gaps 1–8 and from points 50 to 71, for the cycle gaps 50,000 to 10,000,000. Type CHAR even had higher performance indication from points 1 to 3, for the cycle gaps 1, 2, and 4 (Figures 6 and 7). This is important in view of the domain under consideration. Let us have a glimpse of the implications of these results.Figure 6Gcd-R for SHORT data type.Figure 7Gcd-R for CHAR data type.5DiscussionThe previous study by Erho et al. [13] had already shown that GCD-based rotation demonstrated, on the average, a huge up surge of 246.4394ms in execution time than three-way-reversal running time of just 49.66761ms. This was approximately 496.18% difference. What the current research has shown is that the GCD rotation performance for some of the cycle gaps actually competes favorably with three-way-reversal rotation, which means a good and balanced efficiency in performance along the other competitor is maintained with these cycle gaps. This can easily be seen from the two extreme points on the xx-axis of all four graphs.Surprisingly, though, the plots begin to exhibit sharp rise after few cycle gaps and then steep fall before few remaining cycle gaps, considerably. The pattern produced appears to be bell shaped. This could not be as a result of mere outliers, otherwise it would not maintain somewhat uniform pattern across all four graphs. The conclusion reachable here is that ‘there could be a phenomenon playing out’: for some cycle gaps (around the region closest to 1 and/or near, but not higher than, half of the sequence size) the performance is quite efficient, but very poor otherwise. This could be deceptive to system developers given the known optimality of juggling rotation algorithm.A careful comparison of Figure 4 with Table 1 shows that the worst of this poor performance coincides with cycle gaps within the neighborhood of the square root of the sequence size; and all four graphs conform to this fact. For instance, the sequence size used for this experiment was 20,000,000 and the square root was approximately 4472.14. The table has that about seven (7) cycle gaps to both left and right of the square root value (4472.14) fall within the worst performing values for the GCD-based sequence rotation. These cycle gaps correspond to ranges 30–43 horizontal entries for the graphs. This range pattern appears to cut across all four data types as can be seen from the graphs. Impact of this discovery is better imagined in the development of real-time systems. A real-time system application developer, working on problem such as task scheduling, who is familiar with the theoretical optimality of juggling rotation and has also tested with the algorithm using small-size versus half-of-sequence-size ranges of cycle gaps would not imagine that some cycle gap values in between (around the square root of sequence size) will fail the system.Some form of skewness in graph is also observed with regard to some of the data types. It appears that as the data type decreased from long (LONG) to character (CHAR), the skewness became more and more pronounced. This pattern tends to split the single bell apparently into two in the case of SHORT and CHAR types. These points of skewness seem to improve the GCD rotation performance a little. This phenomenon may be language, operating system, and/or machine dependent. But our interest is in the behavior of the algorithm in Java, since Java (Real-Time Java) is popular for real-time systems. In any case, this phenomenon sound like a little good news for system developers/users if their data are of smaller sizes or around half of sequence size. Next, we want to know what to make of all these.6Conclusion and future workThis article has analyzed the relationship between sequence rotation cycle gaps, which are used to determine their GCDs, and the performance of GCD-based sequence rotation. Using randomly sampled data, extracted from the experiment data of the study in ref. [13] but presented in different format, the study found that efficient performances of the algorithm depended on the cycle gaps, which determined GCD. For cycle gaps that were close to 1 and/or close or up to half of the sequence size, the performance was quite efficiently competitive.Perhaps, this might explain why some previous studies quickly concluded that GCD rotation performed better than three-way-reversal rotation [18]. Mostly in an in-place sorting, the block to rotate with another would likely be about half of the sequence under consideration. In such a case, the implementation simply fall in the range of cycle gaps that we are designating in this study as “GCD rotation friendly” – compare Figure 7 and the data type used in ref. [10]. This can be deceptive to unsuspecting developer and dangerous for time critical systems.The study found that the situation was quite different when the cycle gaps were not in the ranges of GCD rotation friendly. In fact, the poor performance was worst when the algorithm used cycle gaps that were within about seven (7) cycle gaps to both left and right of the square root of the given sequence size. This understanding is quite significant because unsuspecting user/developer who may have used juggling rotation for in-place sorting which commonly involves swapping one half of a sequence with the other, may not imagine that cycle gaps (each much smaller than half of the sequence size) can be problematic in execution; given that algorithm complexity is measured in terms of input size. For instance, a real-time task scheduling application developer, knowing that it takes 50ms to rotate offsets of 4 or less, and similar for half of 20,000,000 sequence sizes, may implement requirement offset of just 4,500 items and places some 100ms time bound for safety. But this cycle gap runs for some 460ms (Table 1) – dangerous for time critical systems. Again, in Section 2, we stated that “[…]unexpected delay in execution may be tolerated in some applications such as ordinary in-place sorting[…]”. But can this 460ms execution time for the chosen cycle gap of 4,500 really be tolerated in sorting, where a much bigger cycle gap of 10,000,000 executes for just 50ms? In addition to this understanding, it seems not clear (or even unknown) in the literature that these peak sets of offsets are within the neighborhood of the square root of the sequence size.However, some form of skewness was also noticed that depended on the data types. SHORT and CHAR types in particular exhibited this, tending to split the single bell-shaped pattern of GCD rotation into two, respectively. These points, representing cycle gaps, within the splitting region showed slight improvement from the worst poor performance of the algorithm.Thus, the GCD-based sequence rotation efficient performance heavily depends on the greatest-common-divisor values, which are direct reflection of the cycle gaps. This research has given us a better understanding of the GCD-based sequence rotation by throwing light on the effect of GCD on performance of the algorithm. It therefore strongly recommends that if application involves sequence rotation (such as in-place sorting) with cycle gaps that are within the neighborhood to both left and right of the square root of the given sequence size, DO NOT use GCD-based vector rotation, especially as the size of input increases. For applications that are time critical, such as hard real-time systems, DO NOT use juggling rotation algorithm if the cycle gaps are not within GCD rotation friendly range.It is clear from this study that cycle gaps around the square root of sequence size are heavily slow in juggling rotation. The question is why is fewest number of element assignments not proportionate with execution time of juggling rotation? There is need for further work on this. Also, this study was limited to cycle lengths that were equal to GCD values. For instance, cycle gaps 3, 6, 7, 9, etc. that were not equal to their respective GCD were not covered in this study. Will the performance of juggling rotation on cycle gap 9, for example, be higher/lower than that of 8 and 10, and in what pattern? This is a question for investigation.

Journal

Open Computer Science – de Gruyter

Published: Jan 1, 2022

Keywords: array rotation; real-time tasks priority sorting; juggling rotation algorithm; array circular shifting; juggling rotation performance

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