1=head1 NAME 2 3perlperf - Perl Performance and Optimization Techniques 4 5=head1 DESCRIPTION 6 7This is an introduction to the use of performance and optimization techniques 8which can be used with particular reference to perl programs. While many perl 9developers have come from other languages, and can use their prior knowledge 10where appropriate, there are many other people who might benefit from a few 11perl specific pointers. If you want the condensed version, perhaps the best 12advice comes from the renowned Japanese Samurai, Miyamoto Musashi, who said: 13 14 "Do Not Engage in Useless Activity" 15 16in 1645. 17 18=head1 OVERVIEW 19 20Perhaps the most common mistake programmers make is to attempt to optimize 21their code before a program actually does anything useful - this is a bad idea. 22There's no point in having an extremely fast program that doesn't work. The 23first job is to get a program to I<correctly> do something B<useful>, (not to 24mention ensuring the test suite is fully functional), and only then to consider 25optimizing it. Having decided to optimize existing working code, there are 26several simple but essential steps to consider which are intrinsic to any 27optimization process. 28 29=head2 ONE STEP SIDEWAYS 30 31Firstly, you need to establish a baseline time for the existing code, which 32timing needs to be reliable and repeatable. You'll probably want to use the 33C<Benchmark> or C<Devel::NYTProf> modules, or something similar, for this step, 34or perhaps the Unix system C<time> utility, whichever is appropriate. See the 35base of this document for a longer list of benchmarking and profiling modules, 36and recommended further reading. 37 38=head2 ONE STEP FORWARD 39 40Next, having examined the program for I<hot spots>, (places where the code 41seems to run slowly), change the code with the intention of making it run 42faster. Using version control software, like C<subversion>, will ensure no 43changes are irreversible. It's too easy to fiddle here and fiddle there - 44don't change too much at any one time or you might not discover which piece of 45code B<really> was the slow bit. 46 47=head2 ANOTHER STEP SIDEWAYS 48 49It's not enough to say: "that will make it run faster", you have to check it. 50Rerun the code under control of the benchmarking or profiling modules, from the 51first step above, and check that the new code executed the B<same task> in 52I<less time>. Save your work and repeat... 53 54=head1 GENERAL GUIDELINES 55 56The critical thing when considering performance is to remember there is no such 57thing as a C<Golden Bullet>, which is why there are no rules, only guidelines. 58 59It is clear that inline code is going to be faster than subroutine or method 60calls, because there is less overhead, but this approach has the disadvantage 61of being less maintainable and comes at the cost of greater memory usage - 62there is no such thing as a free lunch. If you are searching for an element in 63a list, it can be more efficient to store the data in a hash structure, and 64then simply look to see whether the key is defined, rather than to loop through 65the entire array using grep() for instance. substr() may be (a lot) faster 66than grep() but not as flexible, so you have another trade-off to access. Your 67code may contain a line which takes 0.01 of a second to execute which if you 68call it 1,000 times, quite likely in a program parsing even medium sized files 69for instance, you already have a 10 second delay, in just one single code 70location, and if you call that line 100,000 times, your entire program will 71slow down to an unbearable crawl. 72 73Using a subroutine as part of your sort is a powerful way to get exactly what 74you want, but will usually be slower than the built-in I<alphabetic> C<cmp> and 75I<numeric> C<E<lt>=E<gt>> sort operators. It is possible to make multiple 76passes over your data, building indices to make the upcoming sort more 77efficient, and to use what is known as the C<OM> (Orcish Maneuver) to cache the 78sort keys in advance. The cache lookup, while a good idea, can itself be a 79source of slowdown by enforcing a double pass over the data - once to setup the 80cache, and once to sort the data. Using C<pack()> to extract the required sort 81key into a consistent string can be an efficient way to build a single string 82to compare, instead of using multiple sort keys, which makes it possible to use 83the standard, written in C<c> and fast, perl C<sort()> function on the output, 84and is the basis of the C<GRT> (Guttman Rossler Transform). Some string 85combinations can slow the C<GRT> down, by just being too plain complex for it's 86own good. 87 88For applications using database backends, the standard C<DBIx> namespace has 89tries to help with keeping things nippy, not least because it tries to I<not> 90query the database until the latest possible moment, but always read the docs 91which come with your choice of libraries. Among the many issues facing 92developers dealing with databases should remain aware of is to always use 93C<SQL> placeholders and to consider pre-fetching data sets when this might 94prove advantageous. Splitting up a large file by assigning multiple processes 95to parsing a single file, using say C<POE>, C<threads> or C<fork> can also be a 96useful way of optimizing your usage of the available C<CPU> resources, though 97this technique is fraught with concurrency issues and demands high attention to 98detail. 99 100Every case has a specific application and one or more exceptions, and there is 101no replacement for running a few tests and finding out which method works best 102for your particular environment, this is why writing optimal code is not an 103exact science, and why we love using Perl so much - TMTOWTDI. 104 105=head1 BENCHMARKS 106 107Here are a few examples to demonstrate usage of Perl's benchmarking tools. 108 109=head2 Assigning and Dereferencing Variables. 110 111I'm sure most of us have seen code which looks like, (or worse than), this: 112 113 if ( $obj->{_ref}->{_myscore} >= $obj->{_ref}->{_yourscore} ) { 114 ... 115 116This sort of code can be a real eyesore to read, as well as being very 117sensitive to typos, and it's much clearer to dereference the variable 118explicitly. We're side-stepping the issue of working with object-oriented 119programming techniques to encapsulate variable access via methods, only 120accessible through an object. Here we're just discussing the technical 121implementation of choice, and whether this has an effect on performance. We 122can see whether this dereferencing operation, has any overhead by putting 123comparative code in a file and running a C<Benchmark> test. 124 125# dereference 126 127 #!/usr/bin/perl 128 129 use strict; 130 use warnings; 131 132 use Benchmark; 133 134 my $ref = { 135 'ref' => { 136 _myscore => '100 + 1', 137 _yourscore => '102 - 1', 138 }, 139 }; 140 141 timethese(1000000, { 142 'direct' => sub { 143 my $x = $ref->{ref}->{_myscore} . $ref->{ref}->{_yourscore} ; 144 }, 145 'dereference' => sub { 146 my $ref = $ref->{ref}; 147 my $myscore = $ref->{_myscore}; 148 my $yourscore = $ref->{_yourscore}; 149 my $x = $myscore . $yourscore; 150 }, 151 }); 152 153It's essential to run any timing measurements a sufficient number of times so 154the numbers settle on a numerical average, otherwise each run will naturally 155fluctuate due to variations in the environment, to reduce the effect of 156contention for C<CPU> resources and network bandwidth for instance. Running 157the above code for one million iterations, we can take a look at the report 158output by the C<Benchmark> module, to see which approach is the most effective. 159 160 $> perl dereference 161 162 Benchmark: timing 1000000 iterations of dereference, direct... 163 dereference: 2 wallclock secs ( 1.59 usr + 0.00 sys = 1.59 CPU) @ 628930.82/s (n=1000000) 164 direct: 1 wallclock secs ( 1.20 usr + 0.00 sys = 1.20 CPU) @ 833333.33/s (n=1000000) 165 166The difference is clear to see and the dereferencing approach is slower. While 167it managed to execute an average of 628,930 times a second during our test, the 168direct approach managed to run an additional 204,403 times, unfortunately. 169Unfortunately, because there are many examples of code written using the 170multiple layer direct variable access, and it's usually horrible. It is, 171however, minusculy faster. The question remains whether the minute gain is 172actually worth the eyestrain, or the loss of maintainability. 173 174=head2 Search and replace or tr 175 176If we have a string which needs to be modified, while a regex will almost 177always be much more flexible, C<tr>, an oft underused tool, can still be a 178useful. One scenario might be replace all vowels with another character. The 179regex solution might look like this: 180 181 $str =~ s/[aeiou]/x/g 182 183The C<tr> alternative might look like this: 184 185 $str =~ tr/aeiou/xxxxx/ 186 187We can put that into a test file which we can run to check which approach is 188the fastest, using a global C<$STR> variable to assign to the C<my $str> 189variable so as to avoid perl trying to optimize any of the work away by 190noticing it's assigned only the once. 191 192# regex-transliterate 193 194 #!/usr/bin/perl 195 196 use strict; 197 use warnings; 198 199 use Benchmark; 200 201 my $STR = "$$-this and that"; 202 203 timethese( 1000000, { 204 'sr' => sub { my $str = $STR; $str =~ s/[aeiou]/x/g; return $str; }, 205 'tr' => sub { my $str = $STR; $str =~ tr/aeiou/xxxxx/; return $str; }, 206 }); 207 208Running the code gives us our results: 209 210 $> perl regex-transliterate 211 212 Benchmark: timing 1000000 iterations of sr, tr... 213 sr: 2 wallclock secs ( 1.19 usr + 0.00 sys = 1.19 CPU) @ 840336.13/s (n=1000000) 214 tr: 0 wallclock secs ( 0.49 usr + 0.00 sys = 0.49 CPU) @ 2040816.33/s (n=1000000) 215 216The C<tr> version is a clear winner. One solution is flexible, the other is 217fast - and it's appropriately the programmer's choice which to use. 218 219Check the C<Benchmark> docs for further useful techniques. 220 221=head1 PROFILING TOOLS 222 223A slightly larger piece of code will provide something on which a profiler can 224produce more extensive reporting statistics. This example uses the simplistic 225C<wordmatch> program which parses a given input file and spews out a short 226report on the contents. 227 228# wordmatch 229 230 #!/usr/bin/perl 231 232 use strict; 233 use warnings; 234 235 =head1 NAME 236 237 filewords - word analysis of input file 238 239 =head1 SYNOPSIS 240 241 filewords -f inputfilename [-d] 242 243 =head1 DESCRIPTION 244 245 This program parses the given filename, specified with C<-f>, and displays a 246 simple analysis of the words found therein. Use the C<-d> switch to enable 247 debugging messages. 248 249 =cut 250 251 use FileHandle; 252 use Getopt::Long; 253 254 my $debug = 0; 255 my $file = ''; 256 257 my $result = GetOptions ( 258 'debug' => \$debug, 259 'file=s' => \$file, 260 ); 261 die("invalid args") unless $result; 262 263 unless ( -f $file ) { 264 die("Usage: $0 -f filename [-d]"); 265 } 266 my $FH = FileHandle->new("< $file") or die("unable to open file($file): $!"); 267 268 my $i_LINES = 0; 269 my $i_WORDS = 0; 270 my %count = (); 271 272 my @lines = <$FH>; 273 foreach my $line ( @lines ) { 274 $i_LINES++; 275 $line =~ s/\n//; 276 my @words = split(/ +/, $line); 277 my $i_words = scalar(@words); 278 $i_WORDS = $i_WORDS + $i_words; 279 debug("line: $i_LINES supplying $i_words words: @words"); 280 my $i_word = 0; 281 foreach my $word ( @words ) { 282 $i_word++; 283 $count{$i_LINES}{spec} += matches($i_word, $word, '[^a-zA-Z0-9]'); 284 $count{$i_LINES}{only} += matches($i_word, $word, '^[^a-zA-Z0-9]+$'); 285 $count{$i_LINES}{cons} += matches($i_word, $word, '^[(?i:bcdfghjklmnpqrstvwxyz)]+$'); 286 $count{$i_LINES}{vows} += matches($i_word, $word, '^[(?i:aeiou)]+$'); 287 $count{$i_LINES}{caps} += matches($i_word, $word, '^[(A-Z)]+$'); 288 } 289 } 290 291 print report( %count ); 292 293 sub matches { 294 my $i_wd = shift; 295 my $word = shift; 296 my $regex = shift; 297 my $has = 0; 298 299 if ( $word =~ /($regex)/ ) { 300 $has++ if $1; 301 } 302 303 debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/"); 304 305 return $has; 306 } 307 308 sub report { 309 my %report = @_; 310 my %rep; 311 312 foreach my $line ( keys %report ) { 313 foreach my $key ( keys %{ $report{$line} } ) { 314 $rep{$key} += $report{$line}{$key}; 315 } 316 } 317 318 my $report = qq| 319 $0 report for $file: 320 lines in file: $i_LINES 321 words in file: $i_WORDS 322 words with special (non-word) characters: $i_spec 323 words with only special (non-word) characters: $i_only 324 words with only consonants: $i_cons 325 words with only capital letters: $i_caps 326 words with only vowels: $i_vows 327 |; 328 329 return $report; 330 } 331 332 sub debug { 333 my $message = shift; 334 335 if ( $debug ) { 336 print STDERR "DBG: $message\n"; 337 } 338 } 339 340 exit 0; 341 342=head2 Devel::DProf 343 344This venerable module has been the de-facto standard for Perl code profiling 345for more than a decade, but has been replaced by a number of other modules 346which have brought us back to the 21st century. Although you're recommended to 347evaluate your tool from the several mentioned here and from the CPAN list at 348the base of this document, (and currently L<Devel::NYTProf> seems to be the 349weapon of choice - see below), we'll take a quick look at the output from 350L<Devel::DProf> first, to set a baseline for Perl profiling tools. Run the 351above program under the control of C<Devel::DProf> by using the C<-d> switch on 352the command-line. 353 354 $> perl -d:DProf wordmatch -f perl5db.pl 355 356 <...multiple lines snipped...> 357 358 wordmatch report for perl5db.pl: 359 lines in file: 9428 360 words in file: 50243 361 words with special (non-word) characters: 20480 362 words with only special (non-word) characters: 7790 363 words with only consonants: 4801 364 words with only capital letters: 1316 365 words with only vowels: 1701 366 367C<Devel::DProf> produces a special file, called F<tmon.out> by default, and 368this file is read by the C<dprofpp> program, which is already installed as part 369of the C<Devel::DProf> distribution. If you call C<dprofpp> with no options, 370it will read the F<tmon.out> file in the current directory and produce a human 371readable statistics report of the run of your program. Note that this may take 372a little time. 373 374 $> dprofpp 375 376 Total Elapsed Time = 2.951677 Seconds 377 User+System Time = 2.871677 Seconds 378 Exclusive Times 379 %Time ExclSec CumulS #Calls sec/call Csec/c Name 380 102. 2.945 3.003 251215 0.0000 0.0000 main::matches 381 2.40 0.069 0.069 260643 0.0000 0.0000 main::debug 382 1.74 0.050 0.050 1 0.0500 0.0500 main::report 383 1.04 0.030 0.049 4 0.0075 0.0123 main::BEGIN 384 0.35 0.010 0.010 3 0.0033 0.0033 Exporter::as_heavy 385 0.35 0.010 0.010 7 0.0014 0.0014 IO::File::BEGIN 386 0.00 - -0.000 1 - - Getopt::Long::FindOption 387 0.00 - -0.000 1 - - Symbol::BEGIN 388 0.00 - -0.000 1 - - Fcntl::BEGIN 389 0.00 - -0.000 1 - - Fcntl::bootstrap 390 0.00 - -0.000 1 - - warnings::BEGIN 391 0.00 - -0.000 1 - - IO::bootstrap 392 0.00 - -0.000 1 - - Getopt::Long::ConfigDefaults 393 0.00 - -0.000 1 - - Getopt::Long::Configure 394 0.00 - -0.000 1 - - Symbol::gensym 395 396C<dprofpp> will produce some quite detailed reporting on the activity of the 397C<wordmatch> program. The wallclock, user and system, times are at the top of 398the analysis, and after this are the main columns defining which define the 399report. Check the C<dprofpp> docs for details of the many options it supports. 400 401See also C<Apache::DProf> which hooks C<Devel::DProf> into C<mod_perl>. 402 403=head2 Devel::Profiler 404 405Let's take a look at the same program using a different profiler: 406C<Devel::Profiler>, a drop-in Perl-only replacement for C<Devel::DProf>. The 407usage is very slightly different in that instead of using the special C<-d:> 408flag, you pull C<Devel::Profiler> in directly as a module using C<-M>. 409 410 $> perl -MDevel::Profiler wordmatch -f perl5db.pl 411 412 <...multiple lines snipped...> 413 414 wordmatch report for perl5db.pl: 415 lines in file: 9428 416 words in file: 50243 417 words with special (non-word) characters: 20480 418 words with only special (non-word) characters: 7790 419 words with only consonants: 4801 420 words with only capital letters: 1316 421 words with only vowels: 1701 422 423 424C<Devel::Profiler> generates a tmon.out file which is compatible with the 425C<dprofpp> program, thus saving the construction of a dedicated statistics 426reader program. C<dprofpp> usage is therefore identical to the above example. 427 428 $> dprofpp 429 430 Total Elapsed Time = 20.984 Seconds 431 User+System Time = 19.981 Seconds 432 Exclusive Times 433 %Time ExclSec CumulS #Calls sec/call Csec/c Name 434 49.0 9.792 14.509 251215 0.0000 0.0001 main::matches 435 24.4 4.887 4.887 260643 0.0000 0.0000 main::debug 436 0.25 0.049 0.049 1 0.0490 0.0490 main::report 437 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::GetOptions 438 0.00 0.000 0.000 2 0.0000 0.0000 Getopt::Long::ParseOptionSpec 439 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::FindOption 440 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::new 441 0.00 0.000 0.000 1 0.0000 0.0000 IO::Handle::new 442 0.00 0.000 0.000 1 0.0000 0.0000 Symbol::gensym 443 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::open 444 445Interestingly we get slightly different results, which is mostly because the 446algorithm which generates the report is different, even though the output file 447format was allegedly identical. The elapsed, user and system times are clearly 448showing the time it took for C<Devel::Profiler> to execute its own run, but 449the column listings feel more accurate somehow than the ones we had earlier 450from C<Devel::DProf>. The 102% figure has disappeared, for example. This is 451where we have to use the tools at our disposal, and recognise their pros and 452cons, before using them. Interestingly, the numbers of calls for each 453subroutine are identical in the two reports, it's the percentages which differ. 454As the author of C<Devel::Proviler> writes: 455 456 ...running HTML::Template's test suite under Devel::DProf shows output() 457 taking NO time but Devel::Profiler shows around 10% of the time is in output(). 458 I don't know which to trust but my gut tells me something is wrong with 459 Devel::DProf. HTML::Template::output() is a big routine that's called for 460 every test. Either way, something needs fixing. 461 462YMMV. 463 464See also C<Devel::Apache::Profiler> which hooks C<Devel::Profiler> into C<mod_perl>. 465 466=head2 Devel::SmallProf 467 468The C<Devel::SmallProf> profiler examines the runtime of your Perl program and 469produces a line-by-line listing to show how many times each line was called, 470and how long each line took to execute. It is called by supplying the familiar 471C<-d> flag to Perl at runtime. 472 473 $> perl -d:SmallProf wordmatch -f perl5db.pl 474 475 <...multiple lines snipped...> 476 477 wordmatch report for perl5db.pl: 478 lines in file: 9428 479 words in file: 50243 480 words with special (non-word) characters: 20480 481 words with only special (non-word) characters: 7790 482 words with only consonants: 4801 483 words with only capital letters: 1316 484 words with only vowels: 1701 485 486C<Devel::SmallProf> writes it's output into a file called F<smallprof.out>, by 487default. The format of the file looks like this: 488 489 <num> <time> <ctime> <line>:<text> 490 491When the program has terminated, the output may be examined and sorted using 492any standard text filtering utilities. Something like the following may be 493sufficient: 494 495 $> cat smallprof.out | grep \d*: | sort -k3 | tac | head -n20 496 497 251215 1.65674 7.68000 75: if ( $word =~ /($regex)/ ) { 498 251215 0.03264 4.40000 79: debug("word: $i_wd ".($has ? 'matches' : 499 251215 0.02693 4.10000 81: return $has; 500 260643 0.02841 4.07000 128: if ( $debug ) { 501 260643 0.02601 4.04000 126: my $message = shift; 502 251215 0.02641 3.91000 73: my $has = 0; 503 251215 0.03311 3.71000 70: my $i_wd = shift; 504 251215 0.02699 3.69000 72: my $regex = shift; 505 251215 0.02766 3.68000 71: my $word = shift; 506 50243 0.59726 1.00000 59: $count{$i_LINES}{cons} = 507 50243 0.48175 0.92000 61: $count{$i_LINES}{spec} = 508 50243 0.00644 0.89000 56: my $i_cons = matches($i_word, $word, 509 50243 0.48837 0.88000 63: $count{$i_LINES}{caps} = 510 50243 0.00516 0.88000 58: my $i_caps = matches($i_word, $word, '^[(A- 511 50243 0.00631 0.81000 54: my $i_spec = matches($i_word, $word, '[^a- 512 50243 0.00496 0.80000 57: my $i_vows = matches($i_word, $word, 513 50243 0.00688 0.80000 53: $i_word++; 514 50243 0.48469 0.79000 62: $count{$i_LINES}{only} = 515 50243 0.48928 0.77000 60: $count{$i_LINES}{vows} = 516 50243 0.00683 0.75000 55: my $i_only = matches($i_word, $word, '^[^a- 517 518You can immediately see a slightly different focus to the subroutine profiling 519modules, and we start to see exactly which line of code is taking the most 520time. That regex line is looking a bit suspicious, for example. Remember that 521these tools are supposed to be used together, there is no single best way to 522profile your code, you need to use the best tools for the job. 523 524See also C<Apache::SmallProf> which hooks C<Devel::SmallProf> into C<mod_perl>. 525 526=head2 Devel::FastProf 527 528C<Devel::FastProf> is another Perl line profiler. This was written with a view 529to getting a faster line profiler, than is possible with for example 530C<Devel::SmallProf>, because it's written in C<C>. To use C<Devel::FastProf>, 531supply the C<-d> argument to Perl: 532 533 $> perl -d:FastProf wordmatch -f perl5db.pl 534 535 <...multiple lines snipped...> 536 537 wordmatch report for perl5db.pl: 538 lines in file: 9428 539 words in file: 50243 540 words with special (non-word) characters: 20480 541 words with only special (non-word) characters: 7790 542 words with only consonants: 4801 543 words with only capital letters: 1316 544 words with only vowels: 1701 545 546C<Devel::FastProf> writes statistics to the file F<fastprof.out> in the current 547directory. The output file, which can be specified, can be interpreted by using 548the C<fprofpp> command-line program. 549 550 $> fprofpp | head -n20 551 552 # fprofpp output format is: 553 # filename:line time count: source 554 wordmatch:75 3.93338 251215: if ( $word =~ /($regex)/ ) { 555 wordmatch:79 1.77774 251215: debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/"); 556 wordmatch:81 1.47604 251215: return $has; 557 wordmatch:126 1.43441 260643: my $message = shift; 558 wordmatch:128 1.42156 260643: if ( $debug ) { 559 wordmatch:70 1.36824 251215: my $i_wd = shift; 560 wordmatch:71 1.36739 251215: my $word = shift; 561 wordmatch:72 1.35939 251215: my $regex = shift; 562 563Straightaway we can see that the number of times each line has been called is 564identical to the C<Devel::SmallProf> output, and the sequence is only very 565slightly different based on the ordering of the amount of time each line took 566to execute, C<if ( $debug ) { > and C<my $message = shift;>, for example. The 567differences in the actual times recorded might be in the algorithm used 568internally, or it could be due to system resource limitations or contention. 569 570See also the L<DBIx::Profile> which will profile database queries running 571under the C<DBIx::*> namespace. 572 573=head2 Devel::NYTProf 574 575C<Devel::NYTProf> is the B<next generation> of Perl code profiler, fixing many 576shortcomings in other tools and implementing many cool features. First of all it 577can be used as either a I<line> profiler, a I<block> or a I<subroutine> 578profiler, all at once. It can also use sub-microsecond (100ns) resolution on 579systems which provide C<clock_gettime()>. It can be started and stopped even 580by the program being profiled. It's a one-line entry to profile C<mod_perl> 581applications. It's written in C<c> and is probably the fastest profiler 582available for Perl. The list of coolness just goes on. Enough of that, let's 583see how to it works - just use the familiar C<-d> switch to plug it in and run 584the code. 585 586 $> perl -d:NYTProf wordmatch -f perl5db.pl 587 588 wordmatch report for perl5db.pl: 589 lines in file: 9427 590 words in file: 50243 591 words with special (non-word) characters: 20480 592 words with only special (non-word) characters: 7790 593 words with only consonants: 4801 594 words with only capital letters: 1316 595 words with only vowels: 1701 596 597C<NYTProf> will generate a report database into the file F<nytprof.out> by 598default. Human readable reports can be generated from here by using the 599supplied C<nytprofhtml> (HTML output) and C<nytprofcsv> (CSV output) programs. 600We've used the Unix system C<html2text> utility to convert the 601F<nytprof/index.html> file for convenience here. 602 603 $> html2text nytprof/index.html 604 605 Performance Profile Index 606 For wordmatch 607 Run on Fri Sep 26 13:46:39 2008 608 Reported on Fri Sep 26 13:47:23 2008 609 610 Top 15 Subroutines -- ordered by exclusive time 611 |Calls |P |F |Inclusive|Exclusive|Subroutine | 612 | | | |Time |Time | | 613 |251215|5 |1 |13.09263 |10.47692 |main:: |matches | 614 |260642|2 |1 |2.71199 |2.71199 |main:: |debug | 615 |1 |1 |1 |0.21404 |0.21404 |main:: |report | 616 |2 |2 |2 |0.00511 |0.00511 |XSLoader:: |load (xsub) | 617 |14 |14|7 |0.00304 |0.00298 |Exporter:: |import | 618 |3 |1 |1 |0.00265 |0.00254 |Exporter:: |as_heavy | 619 |10 |10|4 |0.00140 |0.00140 |vars:: |import | 620 |13 |13|1 |0.00129 |0.00109 |constant:: |import | 621 |1 |1 |1 |0.00360 |0.00096 |FileHandle:: |import | 622 |3 |3 |3 |0.00086 |0.00074 |warnings::register::|import | 623 |9 |3 |1 |0.00036 |0.00036 |strict:: |bits | 624 |13 |13|13|0.00032 |0.00029 |strict:: |import | 625 |2 |2 |2 |0.00020 |0.00020 |warnings:: |import | 626 |2 |1 |1 |0.00020 |0.00020 |Getopt::Long:: |ParseOptionSpec| 627 |7 |7 |6 |0.00043 |0.00020 |strict:: |unimport | 628 629 For more information see the full list of 189 subroutines. 630 631The first part of the report already shows the critical information regarding 632which subroutines are using the most time. The next gives some statistics 633about the source files profiled. 634 635 Source Code Files -- ordered by exclusive time then name 636 |Stmts |Exclusive|Avg. |Reports |Source File | 637 | |Time | | | | 638 |2699761|15.66654 |6e-06 |line . block . sub|wordmatch | 639 |35 |0.02187 |0.00062|line . block . sub|IO/Handle.pm | 640 |274 |0.01525 |0.00006|line . block . sub|Getopt/Long.pm | 641 |20 |0.00585 |0.00029|line . block . sub|Fcntl.pm | 642 |128 |0.00340 |0.00003|line . block . sub|Exporter/Heavy.pm | 643 |42 |0.00332 |0.00008|line . block . sub|IO/File.pm | 644 |261 |0.00308 |0.00001|line . block . sub|Exporter.pm | 645 |323 |0.00248 |8e-06 |line . block . sub|constant.pm | 646 |12 |0.00246 |0.00021|line . block . sub|File/Spec/Unix.pm | 647 |191 |0.00240 |0.00001|line . block . sub|vars.pm | 648 |77 |0.00201 |0.00003|line . block . sub|FileHandle.pm | 649 |12 |0.00198 |0.00016|line . block . sub|Carp.pm | 650 |14 |0.00175 |0.00013|line . block . sub|Symbol.pm | 651 |15 |0.00130 |0.00009|line . block . sub|IO.pm | 652 |22 |0.00120 |0.00005|line . block . sub|IO/Seekable.pm | 653 |198 |0.00085 |4e-06 |line . block . sub|warnings/register.pm| 654 |114 |0.00080 |7e-06 |line . block . sub|strict.pm | 655 |47 |0.00068 |0.00001|line . block . sub|warnings.pm | 656 |27 |0.00054 |0.00002|line . block . sub|overload.pm | 657 |9 |0.00047 |0.00005|line . block . sub|SelectSaver.pm | 658 |13 |0.00045 |0.00003|line . block . sub|File/Spec.pm | 659 |2701595|15.73869 | |Total | 660 |128647 |0.74946 | |Average | 661 | |0.00201 |0.00003|Median | 662 | |0.00121 |0.00003|Deviation | 663 664 Report produced by the NYTProf 2.03 Perl profiler, developed by Tim Bunce and 665 Adam Kaplan. 666 667At this point, if you're using the I<html> report, you can click through the 668various links to bore down into each subroutine and each line of code. Because 669we're using the text reporting here, and there's a whole directory full of 670reports built for each source file, we'll just display a part of the 671corresponding F<wordmatch-line.html> file, sufficient to give an idea of the 672sort of output you can expect from this cool tool. 673 674 $> html2text nytprof/wordmatch-line.html 675 676 Performance Profile -- -block view-.-line view-.-sub view- 677 For wordmatch 678 Run on Fri Sep 26 13:46:39 2008 679 Reported on Fri Sep 26 13:47:22 2008 680 681 File wordmatch 682 683 Subroutines -- ordered by exclusive time 684 |Calls |P|F|Inclusive|Exclusive|Subroutine | 685 | | | |Time |Time | | 686 |251215|5|1|13.09263 |10.47692 |main::|matches| 687 |260642|2|1|2.71199 |2.71199 |main::|debug | 688 |1 |1|1|0.21404 |0.21404 |main::|report | 689 |0 |0|0|0 |0 |main::|BEGIN | 690 691 692 |Line|Stmts.|Exclusive|Avg. |Code | 693 | | |Time | | | 694 |1 | | | |#!/usr/bin/perl | 695 |2 | | | | | 696 | | | | |use strict; | 697 |3 |3 |0.00086 |0.00029|# spent 0.00003s making 1 calls to strict:: | 698 | | | | |import | 699 | | | | |use warnings; | 700 |4 |3 |0.01563 |0.00521|# spent 0.00012s making 1 calls to warnings:: | 701 | | | | |import | 702 |5 | | | | | 703 |6 | | | |=head1 NAME | 704 |7 | | | | | 705 |8 | | | |filewords - word analysis of input file | 706 <...snip...> 707 |62 |1 |0.00445 |0.00445|print report( %count ); | 708 | | | | |# spent 0.21404s making 1 calls to main::report| 709 |63 | | | | | 710 | | | | |# spent 23.56955s (10.47692+2.61571) within | 711 | | | | |main::matches which was called 251215 times, | 712 | | | | |avg 0.00005s/call: # 50243 times | 713 | | | | |(2.12134+0.51939s) at line 57 of wordmatch, avg| 714 | | | | |0.00005s/call # 50243 times (2.17735+0.54550s) | 715 |64 | | | |at line 56 of wordmatch, avg 0.00005s/call # | 716 | | | | |50243 times (2.10992+0.51797s) at line 58 of | 717 | | | | |wordmatch, avg 0.00005s/call # 50243 times | 718 | | | | |(2.12696+0.51598s) at line 55 of wordmatch, avg| 719 | | | | |0.00005s/call # 50243 times (1.94134+0.51687s) | 720 | | | | |at line 54 of wordmatch, avg 0.00005s/call | 721 | | | | |sub matches { | 722 <...snip...> 723 |102 | | | | | 724 | | | | |# spent 2.71199s within main::debug which was | 725 | | | | |called 260642 times, avg 0.00001s/call: # | 726 | | | | |251215 times (2.61571+0s) by main::matches at | 727 |103 | | | |line 74 of wordmatch, avg 0.00001s/call # 9427 | 728 | | | | |times (0.09628+0s) at line 50 of wordmatch, avg| 729 | | | | |0.00001s/call | 730 | | | | |sub debug { | 731 |104 |260642|0.58496 |2e-06 |my $message = shift; | 732 |105 | | | | | 733 |106 |260642|1.09917 |4e-06 |if ( $debug ) { | 734 |107 | | | |print STDERR "DBG: $message\n"; | 735 |108 | | | |} | 736 |109 | | | |} | 737 |110 | | | | | 738 |111 |1 |0.01501 |0.01501|exit 0; | 739 |112 | | | | | 740 741Oodles of very useful information in there - this seems to be the way forward. 742 743See also C<Devel::NYTProf::Apache> which hooks C<Devel::NYTProf> into C<mod_perl>. 744 745=head1 SORTING 746 747Perl modules are not the only tools a performance analyst has at their 748disposal, system tools like C<time> should not be overlooked as the next 749example shows, where we take a quick look at sorting. Many books, theses and 750articles, have been written about efficient sorting algorithms, and this is not 751the place to repeat such work, there's several good sorting modules which 752deserve taking a look at too: C<Sort::Maker>, C<Sort::Key> spring to mind. 753However, it's still possible to make some observations on certain Perl specific 754interpretations on issues relating to sorting data sets and give an example or 755two with regard to how sorting large data volumes can effect performance. 756Firstly, an often overlooked point when sorting large amounts of data, one can 757attempt to reduce the data set to be dealt with and in many cases C<grep()> can 758be quite useful as a simple filter: 759 760 @data = sort grep { /$filter/ } @incoming 761 762A command such as this can vastly reduce the volume of material to actually 763sort through in the first place, and should not be too lightly disregarded 764purely on the basis of its simplicity. The C<KISS> principle is too often 765overlooked - the next example uses the simple system C<time> utility to 766demonstrate. Let's take a look at an actual example of sorting the contents of 767a large file, an apache logfile would do. This one has over a quarter of a 768million lines, is 50M in size, and a snippet of it looks like this: 769 770# logfile 771 772 188.209-65-87.adsl-dyn.isp.belgacom.be - - [08/Feb/2007:12:57:16 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 773 188.209-65-87.adsl-dyn.isp.belgacom.be - - [08/Feb/2007:12:57:16 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 774 151.56.71.198 - - [08/Feb/2007:12:57:41 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1" 775 151.56.71.198 - - [08/Feb/2007:12:57:42 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1" 776 151.56.71.198 - - [08/Feb/2007:12:57:43 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.2; en-US; rv:1.8.1.1) Gecko/20061204 Firefox/2.0.0.1" 777 217.113.68.60 - - [08/Feb/2007:13:02:15 +0000] "GET / HTTP/1.1" 304 - "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 778 217.113.68.60 - - [08/Feb/2007:13:02:16 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 779 debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)" 780 debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)" 781 debora.to.isac.cnr.it - - [08/Feb/2007:13:03:58 +0000] "GET /favicon.ico HTTP/1.1" 404 209 "-" "Mozilla/5.0 (compatible; Konqueror/3.4; Linux) KHTML/3.4.0 (like Gecko)" 782 195.24.196.99 - - [08/Feb/2007:13:26:48 +0000] "GET / HTTP/1.0" 200 3309 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9" 783 195.24.196.99 - - [08/Feb/2007:13:26:58 +0000] "GET /data/css HTTP/1.0" 404 206 "http://www.rfi.net/" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9" 784 195.24.196.99 - - [08/Feb/2007:13:26:59 +0000] "GET /favicon.ico HTTP/1.0" 404 209 "-" "Mozilla/5.0 (Windows; U; Windows NT 5.1; fr; rv:1.8.0.9) Gecko/20061206 Firefox/1.5.0.9" 785 crawl1.cosmixcorp.com - - [08/Feb/2007:13:27:57 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "voyager/1.0" 786 crawl1.cosmixcorp.com - - [08/Feb/2007:13:28:25 +0000] "GET /links.html HTTP/1.0" 200 3413 "-" "voyager/1.0" 787 fhm226.internetdsl.tpnet.pl - - [08/Feb/2007:13:37:32 +0000] "GET /suse-on-vaio.html HTTP/1.1" 200 2858 "http://www.linux-on-laptops.com/sony.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 788 fhm226.internetdsl.tpnet.pl - - [08/Feb/2007:13:37:34 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net/suse-on-vaio.html" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)" 789 80.247.140.134 - - [08/Feb/2007:13:57:35 +0000] "GET / HTTP/1.1" 200 3309 "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)" 790 80.247.140.134 - - [08/Feb/2007:13:57:37 +0000] "GET /data/css HTTP/1.1" 404 206 "http://www.rfi.net" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; .NET CLR 1.1.4322)" 791 pop.compuscan.co.za - - [08/Feb/2007:14:10:43 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net" 792 livebot-207-46-98-57.search.live.com - - [08/Feb/2007:14:12:04 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "msnbot/1.0 (+http://search.msn.com/msnbot.htm)" 793 livebot-207-46-98-57.search.live.com - - [08/Feb/2007:14:12:04 +0000] "GET /html/oracle.html HTTP/1.0" 404 214 "-" "msnbot/1.0 (+http://search.msn.com/msnbot.htm)" 794 dslb-088-064-005-154.pools.arcor-ip.net - - [08/Feb/2007:14:12:15 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net" 795 196.201.92.41 - - [08/Feb/2007:14:15:01 +0000] "GET / HTTP/1.1" 200 3309 "-" "MOT-L7/08.B7.DCR MIB/2.2.1 Profile/MIDP-2.0 Configuration/CLDC-1.1" 796 797The specific task here is to sort the 286,525 lines of this file by Response 798Code, Query, Browser, Referring Url, and lastly Date. One solution might be to 799use the following code, which iterates over the files given on the 800command-line. 801 802# sort-apache-log 803 804 #!/usr/bin/perl -n 805 806 use strict; 807 use warnings; 808 809 my @data; 810 811 LINE: 812 while ( <> ) { 813 my $line = $_; 814 if ( 815 $line =~ m/^( 816 ([\w\.\-]+) # client 817 \s*-\s*-\s*\[ 818 ([^]]+) # date 819 \]\s*"\w+\s* 820 (\S+) # query 821 [^"]+"\s* 822 (\d+) # status 823 \s+\S+\s+"[^"]*"\s+" 824 ([^"]*) # browser 825 " 826 .* 827 )$/x 828 ) { 829 my @chunks = split(/ +/, $line); 830 my $ip = $1; 831 my $date = $2; 832 my $query = $3; 833 my $status = $4; 834 my $browser = $5; 835 836 push(@data, [$ip, $date, $query, $status, $browser, $line]); 837 } 838 } 839 840 my @sorted = sort { 841 $a->[3] cmp $b->[3] 842 || 843 $a->[2] cmp $b->[2] 844 || 845 $a->[0] cmp $b->[0] 846 || 847 $a->[1] cmp $b->[1] 848 || 849 $a->[4] cmp $b->[4] 850 } @data; 851 852 foreach my $data ( @sorted ) { 853 print $data->[5]; 854 } 855 856 exit 0; 857 858When running this program, redirect C<STDOUT> so it is possible to check the 859output is correct from following test runs and use the system C<time> utility 860to check the overall runtime. 861 862 $> time ./sort-apache-log logfile > out-sort 863 864 real 0m17.371s 865 user 0m15.757s 866 sys 0m0.592s 867 868The program took just over 17 wallclock seconds to run. Note the different 869values C<time> outputs, it's important to always use the same one, and to not 870confuse what each one means. 871 872=over 4 873 874=item Elapsed Real Time 875 876The overall, or wallclock, time between when C<time> was called, and when it 877terminates. The elapsed time includes both user and system times, and time 878spent waiting for other users and processes on the system. Inevitably, this is 879the most approximate of the measurements given. 880 881=item User CPU Time 882 883The user time is the amount of time the entire process spent on behalf of the 884user on this system executing this program. 885 886=item System CPU Time 887 888The system time is the amount of time the kernel itself spent executing 889routines, or system calls, on behalf of this process user. 890 891=back 892 893Running this same process as a C<Schwarzian Transform> it is possible to 894eliminate the input and output arrays for storing all the data, and work on the 895input directly as it arrives too. Otherwise, the code looks fairly similar: 896 897# sort-apache-log-schwarzian 898 899 #!/usr/bin/perl -n 900 901 use strict; 902 use warnings; 903 904 print 905 906 map $_->[0] => 907 908 sort { 909 $a->[4] cmp $b->[4] 910 || 911 $a->[3] cmp $b->[3] 912 || 913 $a->[1] cmp $b->[1] 914 || 915 $a->[2] cmp $b->[2] 916 || 917 $a->[5] cmp $b->[5] 918 } 919 map [ $_, m/^( 920 ([\w\.\-]+) # client 921 \s*-\s*-\s*\[ 922 ([^]]+) # date 923 \]\s*"\w+\s* 924 (\S+) # query 925 [^"]+"\s* 926 (\d+) # status 927 \s+\S+\s+"[^"]*"\s+" 928 ([^"]*) # browser 929 " 930 .* 931 )$/xo ] 932 933 => <>; 934 935 exit 0; 936 937Run the new code against the same logfile, as above, to check the new time. 938 939 $> time ./sort-apache-log-schwarzian logfile > out-schwarz 940 941 real 0m9.664s 942 user 0m8.873s 943 sys 0m0.704s 944 945The time has been cut in half, which is a respectable speed improvement by any 946standard. Naturally, it is important to check the output is consistent with 947the first program run, this is where the Unix system C<cksum> utility comes in. 948 949 $> cksum out-sort out-schwarz 950 3044173777 52029194 out-sort 951 3044173777 52029194 out-schwarz 952 953BTW. Beware too of pressure from managers who see you speed a program up by 50% 954of the runtime once, only to get a request one month later to do the same again 955(true story) - you'll just have to point out your only human, even if you are a 956Perl programmer, and you'll see what you can do... 957 958=head1 LOGGING 959 960An essential part of any good development process is appropriate error handling 961with appropriately informative messages, however there exists a school of 962thought which suggests that log files should be I<chatty>, as if the chain of 963unbroken output somehow ensures the survival of the program. If speed is in 964any way an issue, this approach is wrong. 965 966A common sight is code which looks something like this: 967 968 logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) 969 970The problem is that this code will always be parsed and executed, even when the 971debug level set in the logging configuration file is zero. Once the debug() 972subroutine has been entered, and the internal C<$debug> variable confirmed to 973be zero, for example, the message which has been sent in will be discarded and 974the program will continue. In the example given though, the \%INC hash will 975already have been dumped, and the message string constructed, all of which work 976could be bypassed by a debug variable at the statement level, like this: 977 978 logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) if $DEBUG; 979 980This effect can be demonstrated by setting up a test script with both forms, 981including a C<debug()> subroutine to emulate typical C<logger()> functionality. 982 983# ifdebug 984 985 #!/usr/bin/perl 986 987 use strict; 988 use warnings; 989 990 use Benchmark; 991 use Data::Dumper; 992 my $DEBUG = 0; 993 994 sub debug { 995 my $msg = shift; 996 997 if ( $DEBUG ) { 998 print "DEBUG: $msg\n"; 999 } 1000 }; 1001 1002 timethese(100000, { 1003 'debug' => sub { 1004 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) 1005 }, 1006 'ifdebug' => sub { 1007 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if $DEBUG 1008 }, 1009 }); 1010 1011Let's see what C<Benchmark> makes of this: 1012 1013 $> perl ifdebug 1014 Benchmark: timing 100000 iterations of constant, sub... 1015 ifdebug: 0 wallclock secs ( 0.01 usr + 0.00 sys = 0.01 CPU) @ 10000000.00/s (n=100000) 1016 (warning: too few iterations for a reliable count) 1017 debug: 14 wallclock secs (13.18 usr + 0.04 sys = 13.22 CPU) @ 7564.30/s (n=100000) 1018 1019In the one case the code, which does exactly the same thing as far as 1020outputting any debugging information is concerned, in other words nothing, 1021takes 14 seconds, and in the other case the code takes one hundredth of a 1022second. Looks fairly definitive. Use a C<$DEBUG> variable BEFORE you call the 1023subroutine, rather than relying on the smart functionality inside it. 1024 1025=head2 Logging if DEBUG (constant) 1026 1027It's possible to take the previous idea a little further, by using a compile 1028time C<DEBUG> constant. 1029 1030# ifdebug-constant 1031 1032 #!/usr/bin/perl 1033 1034 use strict; 1035 use warnings; 1036 1037 use Benchmark; 1038 use Data::Dumper; 1039 use constant 1040 DEBUG => 0 1041 ; 1042 1043 sub debug { 1044 if ( DEBUG ) { 1045 my $msg = shift; 1046 print "DEBUG: $msg\n"; 1047 } 1048 }; 1049 1050 timethese(100000, { 1051 'debug' => sub { 1052 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) 1053 }, 1054 'constant' => sub { 1055 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if DEBUG 1056 }, 1057 }); 1058 1059Running this program produces the following output: 1060 1061 $> perl ifdebug-constant 1062 Benchmark: timing 100000 iterations of constant, sub... 1063 constant: 0 wallclock secs (-0.00 usr + 0.00 sys = -0.00 CPU) @ -7205759403792793600000.00/s (n=100000) 1064 (warning: too few iterations for a reliable count) 1065 sub: 14 wallclock secs (13.09 usr + 0.00 sys = 13.09 CPU) @ 7639.42/s (n=100000) 1066 1067The C<DEBUG> constant wipes the floor with even the C<$debug> variable, 1068clocking in at minus zero seconds, and generates a "warning: too few iterations 1069for a reliable count" message into the bargain. To see what is really going 1070on, and why we had too few iterations when we thought we asked for 100000, we 1071can use the very useful C<B::Deparse> to inspect the new code: 1072 1073 $> perl -MO=Deparse ifdebug-constant 1074 1075 use Benchmark; 1076 use Data::Dumper; 1077 use constant ('DEBUG', 0); 1078 sub debug { 1079 use warnings; 1080 use strict 'refs'; 1081 0; 1082 } 1083 use warnings; 1084 use strict 'refs'; 1085 timethese(100000, {'sub', sub { 1086 debug "A $0 logging message via process-id: $$" . Dumper(\%INC); 1087 } 1088 , 'constant', sub { 1089 0; 1090 } 1091 }); 1092 ifdebug-constant syntax OK 1093 1094The output shows the constant() subroutine we're testing being replaced with 1095the value of the C<DEBUG> constant: zero. The line to be tested has been 1096completely optimized away, and you can't get much more efficient than that. 1097 1098=head1 POSTSCRIPT 1099 1100This document has provided several way to go about identifying hot-spots, and 1101checking whether any modifications have improved the runtime of the code. 1102 1103As a final thought, remember that it's not (at the time of writing) possible to 1104produce a useful program which will run in zero or negative time and this basic 1105principle can be written as: I<useful programs are slow> by their very 1106definition. It is of course possible to write a nearly instantaneous program, 1107but it's not going to do very much, here's a very efficient one: 1108 1109 $> perl -e 0 1110 1111Optimizing that any further is a job for C<p5p>. 1112 1113=head1 SEE ALSO 1114 1115Further reading can be found using the modules and links below. 1116 1117=head2 PERLDOCS 1118 1119For example: C<perldoc -f sort>. 1120 1121L<perlfaq4>. 1122 1123L<perlfork>, L<perlfunc>, L<perlretut>, L<perlthrtut>. 1124 1125L<threads>. 1126 1127=head2 MAN PAGES 1128 1129C<time>. 1130 1131=head2 MODULES 1132 1133It's not possible to individually showcase all the performance related code for 1134Perl here, naturally, but here's a short list of modules from the CPAN which 1135deserve further attention. 1136 1137 Apache::DProf 1138 Apache::SmallProf 1139 Benchmark 1140 DBIx::Profile 1141 Devel::AutoProfiler 1142 Devel::DProf 1143 Devel::DProfLB 1144 Devel::FastProf 1145 Devel::GraphVizProf 1146 Devel::NYTProf 1147 Devel::NYTProf::Apache 1148 Devel::Profiler 1149 Devel::Profile 1150 Devel::Profit 1151 Devel::SmallProf 1152 Devel::WxProf 1153 POE::Devel::Profiler 1154 Sort::Key 1155 Sort::Maker 1156 1157=head2 URLS 1158 1159Very useful online reference material: 1160 1161 http://www.ccl4.org/~nick/P/Fast_Enough/ 1162 1163 http://www-128.ibm.com/developerworks/library/l-optperl.html 1164 1165 http://perlbuzz.com/2007/11/bind-output-variables-in-dbi-for-speed-and-safety.html 1166 1167 http://en.wikipedia.org/wiki/Performance_analysis 1168 1169 http://apache.perl.org/docs/1.0/guide/performance.html 1170 1171 http://perlgolf.sourceforge.net/ 1172 1173 http://www.sysarch.com/Perl/sort_paper.html 1174 1175=head1 AUTHOR 1176 1177Richard Foley <richard.foley@rfi.net> Copyright (c) 2008 1178 1179=cut 1180