Mercurial > octave-nkf
annotate doc/interpreter/vectorize.txi @ 16772:7eff3032d144
Fix seealso HTML links when using Texinfo 4.X (bug #38862).
Use the prefix 'docX' instead of 'doc-' for Texinfo references.
* doc/interpreter/arith.txi, doc/interpreter/basics.txi,
doc/interpreter/container.txi, doc/interpreter/func.txi,
doc/interpreter/java.txi, doc/interpreter/matrix.txi,
doc/interpreter/munge-texi.pl, doc/interpreter/package.txi,
doc/interpreter/plot.txi, doc/interpreter/stats.txi,
doc/interpreter/strings.txi, doc/interpreter/system.txi,
doc/interpreter/var.txi, doc/interpreter/vectorize.txi,
libinterp/corefcn/qz.cc, libinterp/corefcn/regexp.cc,
libinterp/interpfcn/error.cc, scripts/general/accumarray.m,
scripts/general/structfun.m, scripts/miscellaneous/setfield.m,
scripts/optimization/fminbnd.m, scripts/optimization/fzero.m,
scripts/optimization/lsqnonneg.m:
Use the prefix 'docX' instead of 'doc-' for Texinfo references.
author | Rik <rik@octave.org> |
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date | Tue, 18 Jun 2013 20:35:12 -0700 |
parents | 18f8f2987205 |
children | 011119bf4868 |
rev | line source |
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1 @c Copyright (C) 2012 Jordi GutiƩrrez Hermoso |
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2 @c |
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3 @c This file is part of Octave. |
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4 @c |
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5 @c Octave is free software; you can redistribute it and/or modify it |
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6 @c under the terms of the GNU General Public License as published by the |
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7 @c Free Software Foundation; either version 3 of the License, or (at |
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8 @c your option) any later version. |
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9 @c |
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10 @c Octave is distributed in the hope that it will be useful, but WITHOUT |
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11 @c ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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12 @c FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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13 @c for more details. |
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14 @c |
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15 @c You should have received a copy of the GNU General Public License |
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16 @c along with Octave; see the file COPYING. If not, see |
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17 @c <http://www.gnu.org/licenses/>. |
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18 |
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19 @node Vectorization and Faster Code Execution |
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20 @chapter Vectorization and Faster Code Execution |
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21 @cindex vectorization |
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22 @cindex vectorize |
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23 |
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24 Vectorization is a programming technique that uses vector operations |
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25 instead of element-by-element loop-based operations. Besides frequently |
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26 producing more succinct Octave code, vectorization also allows for better |
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27 optimization in the subsequent implementation. The optimizations may occur |
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28 either in Octave's own Fortran, C, or C++ internal implementation, or even at a |
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29 lower level depending on the compiler and external numerical libraries used to |
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30 build Octave. The ultimate goal is to make use of your hardware's vector |
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31 instructions if possible or to perform other optimizations in software. |
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32 |
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33 Vectorization is not a concept unique to Octave, but it is particularly |
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34 important because Octave is a matrix-oriented language. Vectorized |
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35 Octave code will see a dramatic speed up (10X--100X) in most cases. |
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36 |
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37 This chapter discusses vectorization and other techniques for writing faster |
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38 code. |
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39 |
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40 @menu |
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41 * Basic Vectorization:: Basic techniques for code optimization |
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42 * Broadcasting:: Broadcasting operations |
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43 * Function Application:: Applying functions to arrays, cells, and structs |
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44 * Accumulation:: Accumulation functions |
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45 * Miscellaneous Techniques:: Other techniques for speeding up code |
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46 * Examples:: |
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47 @end menu |
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48 |
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49 @node Basic Vectorization |
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50 @section Basic Vectorization |
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51 |
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52 To a very good first approximation, the goal in vectorization is to |
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53 write code that avoids loops and uses whole-array operations. As a |
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54 trivial example, consider |
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55 |
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56 @example |
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57 @group |
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58 for i = 1:n |
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59 for j = 1:m |
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60 c(i,j) = a(i,j) + b(i,j); |
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61 endfor |
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62 endfor |
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63 @end group |
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64 @end example |
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65 |
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66 @noindent |
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67 compared to the much simpler |
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68 |
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69 @example |
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70 c = a + b; |
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71 @end example |
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72 |
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73 @noindent |
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74 This isn't merely easier to write; it is also internally much easier to |
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75 optimize. Octave delegates this operation to an underlying |
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76 implementation which, among other optimizations, may use special vector |
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77 hardware instructions or could conceivably even perform the additions in |
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78 parallel. In general, if the code is vectorized, the underlying |
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79 implementation has more freedom about the assumptions it can make |
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80 in order to achieve faster execution. |
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81 |
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82 This is especially important for loops with "cheap" bodies. Often it |
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83 suffices to vectorize just the innermost loop to get acceptable |
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84 performance. A general rule of thumb is that the "order" of the |
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85 vectorized body should be greater or equal to the "order" of the |
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86 enclosing loop. |
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87 |
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88 As a less trivial example, instead of |
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89 |
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90 @example |
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91 @group |
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92 for i = 1:n-1 |
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93 a(i) = b(i+1) - b(i); |
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94 endfor |
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95 @end group |
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96 @end example |
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97 |
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98 @noindent |
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99 write |
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100 |
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101 @example |
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102 a = b(2:n) - b(1:n-1); |
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103 @end example |
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104 |
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105 This shows an important general concept about using arrays for indexing |
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106 instead of looping over an index variable. @xref{Index Expressions}. |
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107 Also use boolean indexing generously. If a condition needs to be tested, |
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108 this condition can also be written as a boolean index. For instance, |
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109 instead of |
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110 |
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111 @example |
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112 @group |
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113 for i = 1:n |
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114 if a(i) > 5 |
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115 a(i) -= 20 |
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116 endif |
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117 endfor |
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118 @end group |
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119 @end example |
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120 |
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121 @noindent |
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122 write |
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123 |
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124 @example |
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125 a(a>5) -= 20; |
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126 @end example |
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127 |
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128 @noindent |
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129 which exploits the fact that @code{a > 5} produces a boolean index. |
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130 |
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131 Use elementwise vector operators whenever possible to avoid looping |
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132 (operators like @code{.*} and @code{.^}). @xref{Arithmetic Ops}. For |
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133 simple inline functions, the @code{vectorize} function can do this |
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134 automatically. |
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135 |
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136 @DOCSTRING(vectorize) |
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137 |
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138 Also exploit broadcasting in these elementwise operators both to avoid |
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139 looping and unnecessary intermediate memory allocations. |
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140 @xref{Broadcasting}. |
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141 |
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142 Use built-in and library functions if possible. Built-in and compiled |
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143 functions are very fast. Even with an m-file library function, chances |
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144 are good that it is already optimized, or will be optimized more in a |
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145 future release. |
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146 |
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147 For instance, even better than |
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148 |
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149 @example |
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150 a = b(2:n) - b(1:n-1); |
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151 @end example |
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152 |
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153 @noindent |
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154 is |
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155 |
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156 @example |
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157 a = diff (b); |
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158 @end example |
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159 |
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160 Most Octave functions are written with vector and array arguments in |
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161 mind. If you find yourself writing a loop with a very simple operation, |
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162 chances are that such a function already exists. The following functions |
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163 occur frequently in vectorized code: |
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164 |
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165 @itemize @bullet |
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166 @item |
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167 Index manipulation |
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168 |
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169 @itemize |
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170 @item |
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171 find |
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172 |
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173 @item |
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174 sub2ind |
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175 |
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176 @item |
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177 ind2sub |
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178 |
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179 @item |
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180 sort |
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181 |
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182 @item |
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183 unique |
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184 |
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185 @item |
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186 lookup |
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187 |
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188 @item |
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189 ifelse / merge |
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190 @end itemize |
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191 |
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192 @item |
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193 Repetition |
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194 |
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195 @itemize |
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196 @item |
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197 repmat |
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198 |
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199 @item |
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200 repelems |
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201 @end itemize |
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202 |
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203 @item |
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204 Vectorized arithmetic |
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205 |
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206 @itemize |
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207 @item |
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208 sum |
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209 |
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210 @item |
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211 prod |
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212 |
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213 @item |
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214 cumsum |
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215 |
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216 @item |
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217 cumprod |
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218 |
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219 @item |
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220 sumsq |
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221 |
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222 @item |
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223 diff |
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224 |
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225 @item |
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226 dot |
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227 |
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228 @item |
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229 cummax |
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230 |
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231 @item |
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232 cummin |
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233 @end itemize |
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234 |
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235 @item |
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236 Shape of higher dimensional arrays |
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237 |
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238 @itemize |
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239 @item |
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240 reshape |
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241 |
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242 @item |
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243 resize |
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244 |
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245 @item |
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246 permute |
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247 |
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248 @item |
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249 squeeze |
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250 |
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251 @item |
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252 deal |
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253 @end itemize |
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254 |
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255 @end itemize |
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256 |
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257 @node Broadcasting |
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258 @section Broadcasting |
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259 @cindex broadcast |
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260 @cindex broadcasting |
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261 @cindex BSX |
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262 @cindex recycling |
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263 @cindex SIMD |
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264 |
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265 Broadcasting refers to how Octave binary operators and functions behave |
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266 when their matrix or array operands or arguments differ in size. Since |
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267 version 3.6.0, Octave now automatically broadcasts vectors, matrices, |
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268 and arrays when using elementwise binary operators and functions. |
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269 Broadly speaking, smaller arrays are ``broadcast'' across the larger |
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270 one, until they have a compatible shape. The rule is that corresponding |
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271 array dimensions must either |
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272 |
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273 @enumerate |
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274 @item |
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275 be equal, or |
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276 |
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277 @item |
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278 one of them must be 1. |
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279 @end enumerate |
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280 |
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281 @noindent |
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282 In case all dimensions are equal, no broadcasting occurs and ordinary |
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283 element-by-element arithmetic takes place. For arrays of higher |
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284 dimensions, if the number of dimensions isn't the same, then missing |
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285 trailing dimensions are treated as 1. When one of the dimensions is 1, |
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286 the array with that singleton dimension gets copied along that dimension |
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287 until it matches the dimension of the other array. For example, consider |
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288 |
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289 @example |
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290 @group |
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291 x = [1 2 3; |
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292 4 5 6; |
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293 7 8 9]; |
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294 |
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295 y = [10 20 30]; |
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296 |
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297 x + y |
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298 @end group |
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299 @end example |
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300 |
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301 @noindent |
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302 Without broadcasting, @code{x + y} would be an error because the dimensions |
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303 do not agree. However, with broadcasting it is as if the following |
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304 operation were performed: |
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305 |
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306 @example |
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307 @group |
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308 x = [1 2 3 |
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309 4 5 6 |
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310 7 8 9]; |
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311 |
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312 y = [10 20 30 |
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313 10 20 30 |
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314 10 20 30]; |
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315 |
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316 x + y |
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317 @result{} 11 22 33 |
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318 14 25 36 |
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319 17 28 39 |
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320 @end group |
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321 @end example |
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322 |
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323 @noindent |
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324 That is, the smaller array of size @code{[1 3]} gets copied along the |
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325 singleton dimension (the number of rows) until it is @code{[3 3]}. No |
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326 actual copying takes place, however. The internal implementation reuses |
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327 elements along the necessary dimension in order to achieve the desired |
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328 effect without copying in memory. |
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329 |
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330 Both arrays can be broadcast across each other, for example, all |
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331 pairwise differences of the elements of a vector with itself: |
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332 |
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333 @example |
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334 @group |
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335 y - y' |
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336 @result{} 0 10 20 |
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337 -10 0 10 |
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338 -20 -10 0 |
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339 @end group |
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340 @end example |
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341 |
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342 @noindent |
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343 Here the vectors of size @code{[1 3]} and @code{[3 1]} both get |
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344 broadcast into matrices of size @code{[3 3]} before ordinary matrix |
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345 subtraction takes place. |
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346 |
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347 A special case of broadcasting that may be familiar is when all |
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348 dimensions of the array being broadcast are 1, i.e., the array is a |
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349 scalar. Thus for example, operations like @code{x - 42} and @code{max |
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350 (x, 2)} are basic examples of broadcasting. |
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351 |
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352 For a higher-dimensional example, suppose @code{img} is an RGB image of |
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353 size @code{[m n 3]} and we wish to multiply each color by a different |
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354 scalar. The following code accomplishes this with broadcasting, |
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355 |
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356 @example |
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357 img .*= permute ([0.8, 0.9, 1.2], [1, 3, 2]); |
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358 @end example |
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359 |
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360 @noindent |
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361 Note the usage of permute to match the dimensions of the |
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362 @code{[0.8, 0.9, 1.2]} vector with @code{img}. |
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363 |
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364 For functions that are not written with broadcasting semantics, |
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365 @code{bsxfun} can be useful for coercing them to broadcast. |
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366 |
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367 @DOCSTRING(bsxfun) |
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368 |
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369 Broadcasting is only applied if either of the two broadcasting |
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370 conditions hold. As usual, however, broadcasting does not apply when two |
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371 dimensions differ and neither is 1: |
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372 |
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373 @example |
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374 @group |
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375 x = [1 2 3 |
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376 4 5 6]; |
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377 y = [10 20 |
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378 30 40]; |
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379 x + y |
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380 @end group |
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381 @end example |
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382 |
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383 @noindent |
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384 This will produce an error about nonconformant arguments. |
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385 |
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386 Besides common arithmetic operations, several functions of two arguments |
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387 also broadcast. The full list of functions and operators that broadcast |
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388 is |
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389 |
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390 @example |
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391 plus + .+ |
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392 minus - .- |
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393 times .* |
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394 rdivide ./ |
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395 ldivide .\ |
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396 power .^ .** |
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397 lt < |
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398 le <= |
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399 eq == |
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400 gt > |
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401 ge >= |
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402 ne != ~= |
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403 and & |
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404 or | |
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405 atan2 |
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406 hypot |
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407 max |
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408 min |
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409 mod |
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410 rem |
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411 xor |
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412 |
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413 += -= .+= .-= .*= ./= .\= .^= .**= &= |= |
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414 @end example |
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415 |
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416 Beware of resorting to broadcasting if a simpler operation will suffice. |
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417 For matrices @var{a} and @var{b}, consider the following: |
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418 |
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419 @example |
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420 @var{c} = sum (permute (@var{a}, [1, 3, 2]) .* permute (@var{b}, [3, 2, 1]), 3); |
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421 @end example |
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422 |
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423 @noindent |
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424 This operation broadcasts the two matrices with permuted dimensions |
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425 across each other during elementwise multiplication in order to obtain a |
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426 larger 3-D array, and this array is then summed along the third dimension. |
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427 A moment of thought will prove that this operation is simply the much |
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428 faster ordinary matrix multiplication, @code{@var{c} = @var{a}*@var{b};}. |
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429 |
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430 A note on terminology: ``broadcasting'' is the term popularized by the |
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431 Numpy numerical environment in the Python programming language. In other |
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432 programming languages and environments, broadcasting may also be known |
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433 as @emph{binary singleton expansion} (BSX, in @sc{matlab}, and the |
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434 origin of the name of the @code{bsxfun} function), @emph{recycling} (R |
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435 programming language), @emph{single-instruction multiple data} (SIMD), |
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436 or @emph{replication}. |
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437 |
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438 @subsection Broadcasting and Legacy Code |
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439 |
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440 The new broadcasting semantics almost never affect code that worked |
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441 in previous versions of Octave. Consequently, all code inherited from |
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442 @sc{matlab} that worked in previous versions of Octave should still work |
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443 without change in Octave. The only exception is code such as |
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444 |
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445 @example |
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446 @group |
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447 try |
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448 c = a.*b; |
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449 catch |
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450 c = a.*a; |
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451 end_try_catch |
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452 @end group |
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453 @end example |
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454 |
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455 @noindent |
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456 that may have relied on matrices of different size producing an error. |
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457 Due to how broadcasting changes semantics with older versions of Octave, |
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458 by default Octave warns if a broadcasting operation is performed. To |
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459 disable this warning, refer to its ID (@pxref{docXwarning_ids}): |
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460 |
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461 @example |
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462 warning ("off", "Octave:broadcast"); |
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463 @end example |
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464 |
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465 @noindent |
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466 If you want to recover the old behavior and produce an error, turn this |
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467 warning into an error: |
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468 |
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469 @example |
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470 warning ("error", "Octave:broadcast"); |
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471 @end example |
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472 |
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473 @noindent |
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474 For broadcasting on scalars that worked in previous versions of Octave, |
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475 this warning will not be emitted. |
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476 |
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477 @node Function Application |
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478 @section Function Application |
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479 @cindex map |
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480 @cindex apply |
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481 @cindex function application |
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482 |
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483 As a general rule, functions should already be written with matrix |
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484 arguments in mind and should consider whole matrix operations in a |
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485 vectorized manner. Sometimes, writing functions in this way appears |
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486 difficult or impossible for various reasons. For those situations, |
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487 Octave provides facilities for applying a function to each element of an |
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488 array, cell, or struct. |
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489 |
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490 @DOCSTRING(arrayfun) |
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491 |
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492 @DOCSTRING(spfun) |
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493 |
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494 @DOCSTRING(cellfun) |
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495 |
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496 @DOCSTRING(structfun) |
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497 |
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498 @node Accumulation |
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499 @section Accumulation |
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500 |
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501 Whenever it's possible to categorize according to indices the elements |
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502 of an array when performing a computation, accumulation functions can be |
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503 useful. |
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504 |
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505 @DOCSTRING(accumarray) |
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506 |
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507 @DOCSTRING(accumdim) |
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508 |
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509 @node Miscellaneous Techniques |
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510 @section Miscellaneous Techniques |
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511 @cindex execution speed |
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512 @cindex speedups |
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513 @cindex optimization |
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514 |
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515 Here are some other ways of improving the execution speed of Octave |
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516 programs. |
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517 |
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518 @itemize @bullet |
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519 |
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520 @item Avoid computing costly intermediate results multiple times. |
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521 Octave currently does not eliminate common subexpressions. Also, certain |
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522 internal computation results are cached for variables. For instance, if |
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523 a matrix variable is used multiple times as an index, checking the |
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524 indices (and internal conversion to integers) is only done once. |
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525 |
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526 @item Be aware of lazy copies (copy-on-write). |
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527 @cindex copy-on-write |
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528 @cindex COW |
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529 @cindex memory management |
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530 When a copy of an object is created, the data is not immediately copied, but |
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531 rather shared. The actual copying is postponed until the copied data needs to |
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532 be modified. For example: |
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533 |
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534 @example |
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535 @group |
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536 a = zeros (1000); # create a 1000x1000 matrix |
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537 b = a; # no copying done here |
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538 b(1) = 1; # copying done here |
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539 @end group |
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540 @end example |
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541 |
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542 Lazy copying applies to whole Octave objects such as matrices, cells, |
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543 struct, and also individual cell or struct elements (not array |
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544 elements). |
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545 |
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546 Additionally, index expressions also use lazy copying when Octave can |
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547 determine that the indexed portion is contiguous in memory. For example: |
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548 |
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549 @example |
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550 @group |
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551 a = zeros (1000); # create a 1000x1000 matrix |
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552 b = a(:,10:100); # no copying done here |
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553 b = a(10:100,:); # copying done here |
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554 @end group |
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555 @end example |
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556 |
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557 This applies to arrays (matrices), cell arrays, and structs indexed |
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558 using @samp{()}. Index expressions generating comma-separated lists can also |
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559 benefit from shallow copying in some cases. In particular, when @var{a} is a |
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560 struct array, expressions like @code{@{a.x@}, @{a(:,2).x@}} will use lazy |
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561 copying, so that data can be shared between a struct array and a cell array. |
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562 |
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563 Most indexing expressions do not live longer than their parent |
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564 objects. In rare cases, however, a lazily copied slice outlasts its |
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565 parent, in which case it becomes orphaned, still occupying unnecessarily |
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566 more memory than needed. To provide a remedy working in most real cases, |
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567 Octave checks for orphaned lazy slices at certain situations, when a |
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568 value is stored into a "permanent" location, such as a named variable or |
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569 cell or struct element, and possibly economizes them. For example: |
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570 |
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571 @example |
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572 @group |
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573 a = zeros (1000); # create a 1000x1000 matrix |
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574 b = a(:,10:100); # lazy slice |
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575 a = []; # the original a array is still allocated |
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576 c@{1@} = b; # b is reallocated at this point |
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577 @end group |
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578 @end example |
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579 |
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580 @item Avoid deep recursion. |
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581 Function calls to m-file functions carry a relatively significant overhead, so |
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582 rewriting a recursion as a loop often helps. Also, note that the maximum level |
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583 of recursion is limited. |
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584 |
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585 @item Avoid resizing matrices unnecessarily. |
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586 When building a single result matrix from a series of calculations, set the |
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587 size of the result matrix first, then insert values into it. Write |
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588 |
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589 @example |
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590 @group |
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591 result = zeros (big_n, big_m) |
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592 for i = over:and_over |
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593 ridx = @dots{} |
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594 cidx = @dots{} |
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595 result(ridx, cidx) = new_value (); |
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596 endfor |
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597 @end group |
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598 @end example |
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599 |
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600 @noindent |
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601 instead of |
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602 |
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603 @example |
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604 @group |
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605 result = []; |
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606 for i = ever:and_ever |
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607 result = [ result, new_value() ]; |
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608 endfor |
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609 @end group |
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610 @end example |
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611 |
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612 Sometimes the number of items can not be computed in advance, and |
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613 stack-like operations are needed. When elements are being repeatedly |
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614 inserted or removed from the end of an array, Octave detects it as stack |
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615 usage and attempts to use a smarter memory management strategy by |
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616 pre-allocating the array in bigger chunks. This strategy is also applied |
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617 to cell and struct arrays. |
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618 |
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619 @example |
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620 @group |
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621 a = []; |
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622 while (condition) |
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623 @dots{} |
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624 a(end+1) = value; # "push" operation |
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625 @dots{} |
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626 a(end) = []; # "pop" operation |
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627 @dots{} |
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628 endwhile |
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629 @end group |
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630 @end example |
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631 |
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632 @item Avoid calling @code{eval} or @code{feval} excessively. |
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633 Parsing input or looking up the name of a function in the symbol table are |
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634 relatively expensive operations. |
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635 |
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636 If you are using @code{eval} merely as an exception handling mechanism, and not |
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637 because you need to execute some arbitrary text, use the @code{try} |
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638 statement instead. @xref{The try Statement}. |
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639 |
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640 @item Use @code{ignore_function_time_stamp} when appropriate. |
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641 If you are calling lots of functions, and none of them will need to change |
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642 during your run, set the variable @code{ignore_function_time_stamp} to |
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643 @code{"all"}. This will stop Octave from checking the time stamp of a function |
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644 file to see if it has been updated while the program is being run. |
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645 @end itemize |
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646 |
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647 @node Examples |
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648 @section Examples |
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649 |
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650 The following are examples of vectorization questions asked by actual |
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651 users of Octave and their solutions. |
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652 |
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653 @c FIXME: We need a lot more examples here. |
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654 |
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655 @itemize @bullet |
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656 @item |
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657 For a vector @code{A}, the following loop |
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658 |
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659 @example |
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660 @group |
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661 n = length (A); |
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662 B = zeros (n, 2); |
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663 for i = 1:length (A) |
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664 ## this will be two columns, the first is the difference and |
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665 ## the second the mean of the two elements used for the diff. |
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666 B(i,:) = [A(i+1)-A(i), (A(i+1) + A(i))/2)]; |
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667 endfor |
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668 @end group |
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669 @end example |
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670 |
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671 @noindent |
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672 can be turned into the following one-liner: |
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673 |
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674 @example |
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675 B = [diff(A)(:), 0.5*(A(1:end-1)+A(2:end))(:)] |
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676 @end example |
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677 |
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678 Note the usage of colon indexing to flatten an intermediate result into |
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679 a column vector. This is a common vectorization trick. |
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680 |
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681 @end itemize |