Analysis of Parallelization Effects on Textual Data Compression

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1 Analysis of Parallelization Effects on Textual Data GORAN MARTINOVIC, CASLAV LIVADA, DRAGO ZAGAR Faculty of Electrical Engineering Josip Juraj Strossmayer University of Osijek Kneza Trpimira 2b, Osijek CROATIA Abstract: - This article deals with the problem of textual data compression speed up by dividing a bigger file into smaller ones which are simultaneously compressed. This procedure is called parallel computing and this experiment uses a processor with four cores. Parallel processing is applied on five different algorithms, two of these are entropy coders and others are dictionary coders. Algorithms are mutually compared by speed and performance depending on the number of cores used. Key-Words: - data compression, dictionary coder, entropy coder, multiple cores, parallel computing. 1 Introduction According to [1], compression is used to reduce needed physical space for storing data using certain methods of data writing. The main unit for data storage is a file and depending on the data type within the file, some repetition is noticed. The data is stored once and after that only the position of repeated data is stored. In this way, it is possible to decrease needed space for data storage, which depends on data type and structure. There are two kinds of compression: lossless and lossy compression (compression without and with certain data loss). Lossless compression is carried out by data compression algorithms which ensure that reconstructed data from compressed files corresponds to starting data. Lossless compression is used when it is needed to save the original data and typical examples by which lossless compression must be used are executable files, source codes, textual data, etc. [2], [4]. The majority of programs performing lossless compression practice two steps on the data that will be compressed and these steps are: Input data statistical model generation Input data can be transformed into sequences of bits by using the statistical model so that less frequently occurring data give longer output sequences [5]. By using lossy compression, reconstructed data differs from input data, but these differences are so small and reconstructed data is still usable. According to [1] and [2], lossy compression methods are used for multimedia compression (audio, video and pictures). Two main lossy compression schemes are: Lossy transformation codecs (coder-decoder) Picture or audio samples are taken, divided into smaller parts, quantized and compressed using entropy coding [1]. Predictive lossy codecs preceding and/or following decoded data is used to assume a current picture or audio sample. The difference between the assumed and the real sample with additional data is quantized and coded. 2 Analyzed Methods Since we deal with textual data, only the lossless compression methods will be shown and analyzed. These methods are: Huffman coding, Arithmetic coding, LZ78 and LZW [3]. 2.1 Huffman Coding As in [2], Huffman coding codes symbols with variable length code words, depending on their probability of incidence. Huffman coding is based on two simple facts: 1. In optimal code, symbols with greater probabilities of incidence have smaller code words than symbols with lesser probabilities of incidence. 2. In optimal code, two symbols with smallest probabilities have code words of same length. Huffman coding algorithm is shown in Fig. 1, similarly to [2]: 1. Sort symbols by falling probabilities. 2. Link two symbols with least probabilities into one new symbol, on branching assign 0 to one branch and 1 to the second branch. 3. Sort the obtained symbols by probabilities. 4. Repeat until there is only one symbol. 5. Going back through tree read codes. Fig. 1 - Huffman coding algorithm ISSN: ISBN:

2 Algorithm data structure is binary tree. Decoding algorithm uses the same procedure as the coder for building a tree and before sending data, coder must first send the probabilities of symbol incidence. 2.2 Arithmetic Coding The algorithm takes queues of symbols as input data (message) and transforms them into a floating point number, depending on a known statistical model. The arithmetic coding is shown in Fig. 2, as in [2]: 1. Divide interval [0, 1] into M intervals that correspond to symbols, symbol length is proportional to symbols probability. 2. Select the interval of the first symbol in queue. 3. Subdivide the current symbol interval into new M subintervals, proportional to their probabilities. 4. From these subintervals, select one that matches the next symbol in queue. 5. Repeat steps 3 and 4 until all symbols are coded. 6. Output the interval value in binary form. Fig. 2 - Arithmetic coding algorithm 2.3 LZ78 Algorithm LZ78 uses a code word dictionary and tries to replace a stream of symbols with the reference to the stream dictionary location. The main assumption is that the reference to the stream in the dictionary is shorter than the stream itself. The stream is coded using an arranged pair length-distance which can be described as follows: every length symbol equals the symbol that is exactly distance symbols after uncompressed stream. The structure in which this data is held is called a sliding window; hence, LZ78 is sometimes called sliding window compression. Coder needs this data to discover if there is any correspondence and decoder needs this data for reference interpretation [6] and [7]. The coding algorithm is shown in Fig. 3, according to [1]: 1. Initialize the dictionary to a known value. 2. Read an uncoded string that is the length of the maximum allowable match. 3. Search for the longest matching string in the dictionary. 4. If a match is found to be greater than or equal to the minimum allowable match length: a. Write the encoded flag, then the offset and length to the encoded output. b. Otherwise, write the uncoded flag and the first uncoded symbol to the encoded output. 5. Shift a copy of the symbols written to the encoded output from the unencoded string to the dictionary. 6. Read a number of symbols from the uncoded input equal to the number of symbols written in Step Repeat from Step 3, until all the entire input has been encoded. Fig. 3 - LZ78 coding algorithm LZ78 reads the input stream in advance and compares it with the dictionary which is constantly updated. The input stream will be read until the corresponding data in the dictionary is found and in that moment, the position and length of data are written into dictionary. 2.4 LZW Coding LZW is a refined version of the LZ78 algorithm; hence it is faster but usually not optimal because of the constrained data analysis. The LZW coding algorithm is shown in Fig. 4, as in [8]: 1. Set w to zero. 2. For every c (character from an uncompressed file) a. If (w + c) exists in the dictionary, then w = w + c. b. Else add a dictionary code for w to output, add (w + c) in the dictionary and set w = c. 3. Add the dictionary code for w to output. Fig. 4 - LZW coding algorithm 3 Parallel Processing Parallel processing implies simultaneous execution of multiple instructions. It is based on the principle that bigger tasks can always be divided into smaller ones which are simultaneously executed. Communication and synchronization between various subtasks are the biggest restrictions for achieving good parallel processing performance. Parallel processing program execution speed-up is described by Amdahl s law [9]. Theoretically, by doubling processing elements, execution time should halve, and with the second doubling, execution time should halve again. However, a few parallel algorithms achieve this optimal speed-up. The majority of parallel algorithms have this optimal (linear) speed-up for a small number of processing ISSN: ISBN:

3 elements but with a further increase of processing elements execution time becomes constant [10]. Not every parallelization will result in execution time speed-up. As the tasks are divided in more and more threads, these threads need more time for mutual communication. It is possible that this additional communication dominates the time needed to solve problems; therefore further parallelization increases the program execution time, instead of decreasing it. This phenomenon is called parallel slowdown [11]. In this research we used a multi-core processor which consists of several processing units (cores) that can execute more instructions per cycle from different instruction streams. 4 Parallel Processing Data Data compression program will be executed on a multicore processor computer, i.e. on a computer with four cores. Computer configuration is as follows: CPU: Intel Core i GHz, RAM: 4 GB, Windows 7 OS. The coding algorithm on a multi-core system can be divided into several steps [10]: read input data stream separate input data stream in N equal parts where the number of processor cores, compress disjointed data using one of mentioned compression methods, join compressed parts into one file. program was written in C++ for command prompt. Executable file needs additional parameters, like compression type, input file and output file. was repeated ten times, so that we could obtain the best results because various processes were running in the background of the computer. After that, statistical data about compression was written in a file, from where we did our analysis. Fig. 5 shows data compression using four cores. 5 Results and Analysis Data compression algorithm analysis will be tested on a textual file showed in Table 1. The number of compression iterations is ten. Results are analyzed and minimum, maximum and mean values are shown in tables. Table 1. Source file Document name Size [in bytes] Description text.txt 5,910,599 Text 5.1 and Average of File Using Arithmetic Coding Table 2 shows mean and minimum values of the time needed for compression, compressed file size and the achieved compression. The values were acquired by analyzing results shown in Fig. 6. Table 2. times by using arithmetic coding min avg Compressed file size [byte] 3,530, ratio for the implemented compression method was calculated by using expression (1). Compressed file size ratio[ %] = *100% (1) Original file size From Table 2, but also from Fig. 6 we can conclude that compression time decreases with an increase in a utilized core number. By doubling the number of cores, compression time halves. Compressed data is stored in the file bible_arit.txt. Fig. 5 - Multi-core data compression algorithm Fig. 6 - time comparison depending on the number of cores used in arithmetic coding ISSN: ISBN:

4 5.2 File s Using Huffman Coding Table 3 shows compression times and Fig. 7 enables a comparison of compression times when using Huffman coding. Table 3. times by using Huffman coding Compressed file size [byte] min ,671, avg As well as in arithmetic coding, in Huffman coding compression time decreases proportionally with an increase in the number of cores. The compressed file size created by Huffman coding is larger than the compressed file size generated by arithmetic coding. time is longer in Huffman coding than in arithmetic coding. Fig. 7 - time comparison depending on the number of cores used in Huffman coding Fig. 8 - time comparison depending on the number of cores used in LZ78 coding 5.4 Results of LZW Coding Depending on the umber of Cores Used Table 5 shows average and minimum values of file compression times, compressed file size and a compression ratio. The values were acquired by analyzing results shown in Fig. 9. Table 5. times by using Huffman coding min avg Compressed file size [byte] Compressio n 2,434, LZW coding achieves the best compression ratio but compression time only increases with first core doubling and with further core number increase compression time doesn`t change much. 5.3 Results of LZ78 Coding Depending on the umber of Cores Used Table 4 shows compression time when using LZ78 coding and Fig. 8 shows the comparison of compression times depending on the number of cores. Table 4. s by Using LZ78 Coding min avg Compressed file size [byte] 3,140, Data compression time reduces when the number of cores is increased. time difference is very small for cases when three and four cores are used. Fig. 9 - time comparison depending on the number of cores used in LZW coding 5.5 Comparative analysis of minimal data compression times Table 6 shows compression time (minimum values in ms) and the compression ratio of the file text.txt for all coding methods. ISSN: ISBN:

5 Table 6. times for various coding methods depending on the number of cores used method Arithmetic Huffman LZ LZW Fig. 11 shows file sizes (in bytes) after compressing with a certain coding method. which was demonstrated in this paper. From the results of this paper, it can be concluded that compression time halved with doubling the number of cores. We used two types of coders: entropy coders and dictionary coders. time decreased by increasing the number of cores in all of the methods used. Entropy coders can compress a textual file faster, but they cannot compress as well as dictionary coders do. According to what we have seen so far, we can safely conclude that the parallelization procedure has the best effect on entropy coders, especially on arithmetic coding. Acknowledgment This work was supported by research project grant No from the Ministry of Science, Education and Sports of the Republic of Croatia. Fig ratio of all used coding methods Fig Compressed file size comparison depending on the coding method used 6 Conclusion The goal of this paper was to see the effects of parallelization on textual file compression. The parallelization procedure consists of splitting up the task in several smaller subtasks and their simultaneous execution for shortening the procedure execution time, i.e. shortening textual file compression time using multiple cores. It was logical to assume that compression time will decrease by increasing the number of cores, References: [1] M. Pu, Fundamental Data, Butterworth-Heinemann, [2] K.P. Subbalakshmi, Lossless Image, Lossless Handbook, Academic Press, Chapter 9, 2003, pp [3] T. Moffat, C. Bell, I. H. Witten, Lossless for Text and Images, Int. J. of High Speed Electronics and Systems, Vol. 8, No. 1, 1997, pp [4] J. Platoš, V. Snášel, of small text files, Advanced Engineering Informatics, Vol. 22, Issue 3, 2008, pp [5] J. Ziv, A. Lempel, A Universal Algorithm for Sequential Data, IEEE Trans. Information Th.., Vol. 23, No. 3, 1977, pp [6] S. C. Sahinalp, N. M. Rajpoot, Dictionary-Based Data : An Algorithmic Perspective, Lossless Handbook, Academic Press, Ch. 6, 2003, pp [7] F. Rizzo, J. A. Storer, B. Carpentieri, LZ-based image compression, Information Sciences, Vol. 135, Issues 1-2, 2001, pp [8] M. R. Nelson, LZW Data, Dr. Dobb`s Journal, Vol. 14, Issue 10, 1989, pp [9] A. Grama, G. Karypis, V. Kumar, A. Gupta, Introduction to Parallel Computing, Addison Wesley, 2 nd Ed., [10] J. A. Storer, J. H. Reif, A parallel architecture for high speed data compression, J. Parallel & Distributed Computing, Vol. 13. Issue 2, 1991, pp [11] H. P. Flatt, K. Kennedy, Performance of parallel processors, Parallel Computing, Vol. 12, Issue 1, 1989, pp ISSN: ISBN: