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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
e4866e2e 8which can be used with particular reference to perl programs. While many perl
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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::DProf> 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, miniscully 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 programmers choice which to use in the
218circumstances.
219
220Check the C<Benchmark> docs for further useful techniques.
221
222=head1 PROFILING TOOLS
223
224A slightly larger piece of code will provide something on which a profiler can
225produce more extensive reporting statistics. This example uses the simplistic
226C<wordmatch> program which parses a given input file and spews out a short
227report on the contents.
228
229# wordmatch
230
231 #!/usr/bin/perl
232
233 use strict;
234 use warnings;
235
236 =head1 NAME
237
238 filewords - word analysis of input file
239
240 =head1 SYNOPSIS
241
242 filewords -f inputfilename [-d]
243
244 =head1 DESCRIPTION
245
246 This program parses the given filename, specified with C<-f>, and displays a
247 simple analysis of the words found therein. Use the C<-d> switch to enable
248 debugging messages.
249
250 =cut
251
252 use FileHandle;
253 use Getopt::Long;
254
255 my $debug = 0;
256 my $file = '';
257
258 my $result = GetOptions (
259 'debug' => \$debug,
260 'file=s' => \$file,
261 );
262 die("invalid args") unless $result;
263
264 unless ( -f $file ) {
265 die("Usage: $0 -f filename [-d]");
266 }
267 my $FH = FileHandle->new("< $file") or die("unable to open file($file): $!");
268
269 my $i_LINES = 0;
270 my $i_WORDS = 0;
271 my %count = ();
272
273 my @lines = <$FH>;
274 foreach my $line ( @lines ) {
275 $i_LINES++;
276 $line =~ s/\n//;
277 my @words = split(/ +/, $line);
278 my $i_words = scalar(@words);
279 $i_WORDS = $i_WORDS + $i_words;
280 debug("line: $i_LINES supplying $i_words words: @words");
281 my $i_word = 0;
282 foreach my $word ( @words ) {
283 $i_word++;
284 $count{$i_LINES}{spec} += matches($i_word, $word, '[^a-zA-Z0-9]');
285 $count{$i_LINES}{only} += matches($i_word, $word, '^[^a-zA-Z0-9]+$');
286 $count{$i_LINES}{cons} += matches($i_word, $word, '^[(?i:bcdfghjklmnpqrstvwxyz)]+$');
287 $count{$i_LINES}{vows} += matches($i_word, $word, '^[(?i:aeiou)]+$');
288 $count{$i_LINES}{caps} += matches($i_word, $word, '^[(A-Z)]+$');
289 }
290 }
291
292 print report( %count );
293
294 sub matches {
295 my $i_wd = shift;
296 my $word = shift;
297 my $regex = shift;
298 my $has = 0;
299
300 if ( $word =~ /($regex)/ ) {
301 $has++ if $1;
302 }
303
304 debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/");
305
306 return $has;
307 }
308
309 sub report {
310 my %report = @_;
311 my %rep;
312
313 foreach my $line ( keys %report ) {
314 foreach my $key ( keys %{ $report{$line} } ) {
315 $rep{$key} += $report{$line}{$key};
316 }
317 }
318
319 my $report = qq|
320 $0 report for $file:
321 lines in file: $i_LINES
322 words in file: $i_WORDS
323 words with special (non-word) characters: $i_spec
324 words with only special (non-word) characters: $i_only
325 words with only consonants: $i_cons
326 words with only capital letters: $i_caps
327 words with only vowels: $i_vows
328 |;
329
330 return $report;
331 }
332
333 sub debug {
334 my $message = shift;
335
336 if ( $debug ) {
337 print STDERR "DBG: $message\n";
338 }
339 }
340
341 exit 0;
342
343=head2 Devel::DProf
344
345This venerable module has been the de-facto standard for Perl code profiling
346for more than a decade, but has been replaced by a number of other modules
347which have brought us back to the 21st century. Although you're recommended to
348evaluate your tool from the several mentioned here and from the CPAN list at
349the base of this document, (and currently L<Devel::NYTProf> seems to be the
350weapon of choice - see below), we'll take a quick look at the output from
351L<Devel::DProf> first, to set a baseline for Perl profiling tools. Run the
352above program under the control of C<Devel::DProf> by using the C<-d> switch on
353the command-line.
354
355 $> perl -d:DProf wordmatch -f perl5db.pl
356
357 <...multiple lines snipped...>
358
359 wordmatch report for perl5db.pl:
360 lines in file: 9428
361 words in file: 50243
362 words with special (non-word) characters: 20480
363 words with only special (non-word) characters: 7790
364 words with only consonants: 4801
365 words with only capital letters: 1316
366 words with only vowels: 1701
367
368C<Devel::DProf> produces a special file, called F<tmon.out> by default, and
369this file is read by the C<dprofpp> program, which is already installed as part
370of the C<Devel::DProf> distribution. If you call C<dprofpp> with no options,
371it will read the F<tmon.out> file in the current directory and produce a human
372readable statistics report of the run of your program. Note that this may take
373a little time.
374
375 $> dprofpp
376
377 Total Elapsed Time = 2.951677 Seconds
378 User+System Time = 2.871677 Seconds
379 Exclusive Times
380 %Time ExclSec CumulS #Calls sec/call Csec/c Name
381 102. 2.945 3.003 251215 0.0000 0.0000 main::matches
382 2.40 0.069 0.069 260643 0.0000 0.0000 main::debug
383 1.74 0.050 0.050 1 0.0500 0.0500 main::report
384 1.04 0.030 0.049 4 0.0075 0.0123 main::BEGIN
385 0.35 0.010 0.010 3 0.0033 0.0033 Exporter::as_heavy
386 0.35 0.010 0.010 7 0.0014 0.0014 IO::File::BEGIN
387 0.00 - -0.000 1 - - Getopt::Long::FindOption
388 0.00 - -0.000 1 - - Symbol::BEGIN
389 0.00 - -0.000 1 - - Fcntl::BEGIN
390 0.00 - -0.000 1 - - Fcntl::bootstrap
391 0.00 - -0.000 1 - - warnings::BEGIN
392 0.00 - -0.000 1 - - IO::bootstrap
393 0.00 - -0.000 1 - - Getopt::Long::ConfigDefaults
394 0.00 - -0.000 1 - - Getopt::Long::Configure
395 0.00 - -0.000 1 - - Symbol::gensym
396
397C<dprofpp> will produce some quite detailed reporting on the activity of the
398C<wordmatch> program. The wallclock, user and system, times are at the top of
399the analysis, and after this are the main columns defining which define the
400report. Check the C<dprofpp> docs for details of the many options it supports.
401
402See also C<Apache::DProf> which hooks C<Devel::DProf> into C<mod_perl>.
403
404=head2 Devel::Profiler
405
406Let's take a look at the same program using a different profiler:
407C<Devel::Profiler>, a drop-in Perl-only replacement for C<Devel::DProf>. The
408usage is very slightly different in that instead of using the special C<-d:>
409flag, you pull C<Devel::Profiler> in directly as a module using C<-M>.
410
411 $> perl -MDevel::Profiler wordmatch -f perl5db.pl
412
413 <...multiple lines snipped...>
414
415 wordmatch report for perl5db.pl:
416 lines in file: 9428
417 words in file: 50243
418 words with special (non-word) characters: 20480
419 words with only special (non-word) characters: 7790
420 words with only consonants: 4801
421 words with only capital letters: 1316
422 words with only vowels: 1701
423
424
425C<Devel::Profiler> generates a tmon.out file which is compatible with the
426C<dprofpp> program, thus saving the construction of a dedicated statistics
427reader program. C<dprofpp> usage is therefore identical to the above example.
428
429 $> dprofpp
430
431 Total Elapsed Time = 20.984 Seconds
432 User+System Time = 19.981 Seconds
433 Exclusive Times
434 %Time ExclSec CumulS #Calls sec/call Csec/c Name
435 49.0 9.792 14.509 251215 0.0000 0.0001 main::matches
436 24.4 4.887 4.887 260643 0.0000 0.0000 main::debug
437 0.25 0.049 0.049 1 0.0490 0.0490 main::report
438 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::GetOptions
439 0.00 0.000 0.000 2 0.0000 0.0000 Getopt::Long::ParseOptionSpec
440 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::FindOption
441 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::new
442 0.00 0.000 0.000 1 0.0000 0.0000 IO::Handle::new
443 0.00 0.000 0.000 1 0.0000 0.0000 Symbol::gensym
444 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::open
445
446Interestingly we get slightly different results, which is mostly because the
447algorithm which generates the report is different, even though the output file
448format was allegedly identical. The elapsed, user and system times are clearly
449showing the time it took for C<Devel::Profiler> to execute it's own run, but
450the column listings feel more accurate somehow than the ones we had earlier
451from C<Devel::DProf>. The 102% figure has disappeared, for example. This is
452where we have to use the tools at our disposal, and recognise their pros and
453cons, before using them. Interestingly, the numbers of calls for each
454subroutine are identical in the two reports, it's the percentages which differ.
455As the author of C<Devel::Proviler> writes:
456
457 ...running HTML::Template's test suite under Devel::DProf shows output()
458 taking NO time but Devel::Profiler shows around 10% of the time is in output().
459 I don't know which to trust but my gut tells me something is wrong with
460 Devel::DProf. HTML::Template::output() is a big routine that's called for
461 every test. Either way, something needs fixing.
462
463YMMV.
464
465See also C<Devel::Apache::Profiler> which hooks C<Devel::Profiler> into C<mod_perl>.
466
467=head2 Devel::SmallProf
468
469The C<Devel::SmallProf> profiler examines the runtime of your Perl program and
470produces a line-by-line listing to show how many times each line was called,
471and how long each line took to execute. It is called by supplying the familiar
472C<-d> flag to Perl at runtime.
473
474 $> perl -d:SmallProf wordmatch -f perl5db.pl
475
476 <...multiple lines snipped...>
477
478 wordmatch report for perl5db.pl:
479 lines in file: 9428
480 words in file: 50243
481 words with special (non-word) characters: 20480
482 words with only special (non-word) characters: 7790
483 words with only consonants: 4801
484 words with only capital letters: 1316
485 words with only vowels: 1701
486
487C<Devel::SmallProf> writes it's output into a file called F<smallprof.out>, by
488default. The format of the file looks like this:
489
490 <num> <time> <ctime> <line>:<text>
491
492When the program has terminated, the output may be examined and sorted using
493any standard text filtering utilities. Something like the following may be
494sufficient:
495
496 $> cat smallprof.out | grep \d*: | sort -k3 | tac | head -n20
497
498 251215 1.65674 7.68000 75: if ( $word =~ /($regex)/ ) {
499 251215 0.03264 4.40000 79: debug("word: $i_wd ".($has ? 'matches' :
500 251215 0.02693 4.10000 81: return $has;
501 260643 0.02841 4.07000 128: if ( $debug ) {
502 260643 0.02601 4.04000 126: my $message = shift;
503 251215 0.02641 3.91000 73: my $has = 0;
504 251215 0.03311 3.71000 70: my $i_wd = shift;
505 251215 0.02699 3.69000 72: my $regex = shift;
506 251215 0.02766 3.68000 71: my $word = shift;
507 50243 0.59726 1.00000 59: $count{$i_LINES}{cons} =
508 50243 0.48175 0.92000 61: $count{$i_LINES}{spec} =
509 50243 0.00644 0.89000 56: my $i_cons = matches($i_word, $word,
510 50243 0.48837 0.88000 63: $count{$i_LINES}{caps} =
511 50243 0.00516 0.88000 58: my $i_caps = matches($i_word, $word, '^[(A-
512 50243 0.00631 0.81000 54: my $i_spec = matches($i_word, $word, '[^a-
513 50243 0.00496 0.80000 57: my $i_vows = matches($i_word, $word,
514 50243 0.00688 0.80000 53: $i_word++;
515 50243 0.48469 0.79000 62: $count{$i_LINES}{only} =
516 50243 0.48928 0.77000 60: $count{$i_LINES}{vows} =
517 50243 0.00683 0.75000 55: my $i_only = matches($i_word, $word, '^[^a-
518
519You can immediately see a slightly different focus to the subroutine profiling
520modules, and we start to see exactly which line of code is taking the most
521time. That regex line is looking a bit suspicious, for example. Remember that
522these tools are supposed to be used together, there is no single best way to
523profile your code, you need to use the best tools for the job.
524
525See also C<Apache::SmallProf> which hooks C<Devel::SmallProf> into C<mod_perl>.
526
527=head2 Devel::FastProf
528
529C<Devel::FastProf> is another Perl line profiler. This was written with a view
530to getting a faster line profiler, than is possible with for example
531C<Devel::SmallProf>, because it's written in C<C>. To use C<Devel::FastProf>,
532supply the C<-d> argument to Perl:
533
534 $> perl -d:FastProf wordmatch -f perl5db.pl
535
536 <...multiple lines snipped...>
537
538 wordmatch report for perl5db.pl:
539 lines in file: 9428
540 words in file: 50243
541 words with special (non-word) characters: 20480
542 words with only special (non-word) characters: 7790
543 words with only consonants: 4801
544 words with only capital letters: 1316
545 words with only vowels: 1701
546
547C<Devel::FastProf> writes statistics to the file F<fastprof.out> in the current
548directory. The output file, which can be specified, can be interpreted by using
549the C<fprofpp> command-line program.
550
551 $> fprofpp | head -n20
552
553 # fprofpp output format is:
554 # filename:line time count: source
555 wordmatch:75 3.93338 251215: if ( $word =~ /($regex)/ ) {
556 wordmatch:79 1.77774 251215: debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/");
557 wordmatch:81 1.47604 251215: return $has;
558 wordmatch:126 1.43441 260643: my $message = shift;
559 wordmatch:128 1.42156 260643: if ( $debug ) {
560 wordmatch:70 1.36824 251215: my $i_wd = shift;
561 wordmatch:71 1.36739 251215: my $word = shift;
562 wordmatch:72 1.35939 251215: my $regex = shift;
563
564Straightaway we can see that the number of times each line has been called is
565identical to the C<Devel::SmallProf> output, and the sequence is only very
566slightly different based on the ordering of the amount of time each line took
567to execute, C<if ( $debug ) { > and C<my $message = shift;>, for example. The
568differences in the actual times recorded might be in the algorithm used
569internally, or it could be due to system resource limitations or contention.
570
571See also the L<DBIx::Profiler> which will profile database queries running
572under the C<DBIx::*> namespace.
573
574=head2 Devel::NYTProf
575
576C<Devel::NYTProf> is the B<next generation> of Perl code profiler, fixing many
577shortcomings in other tools and implementing many cool features. First of all it
578can be used as either a I<line> profiler, a I<block> or a I<subroutine>
579profiler, all at once. It can also use sub-microsecond (100ns) resolution on
580systems which provide C<clock_gettime()>. It can be started and stopped even
581by the program being profiled. It's a one-line entry to profile C<mod_perl>
582applications. It's written in C<c> and is probably the fastest profiler
583available for Perl. The list of coolness just goes on. Enough of that, let's
584see how to it works - just use the familiar C<-d> switch to plug it in and run
585the code.
586
587 $> perl -d:NYTProf wordmatch -f perl5db.pl
588
589 wordmatch report for perl5db.pl:
590 lines in file: 9427
591 words in file: 50243
592 words with special (non-word) characters: 20480
593 words with only special (non-word) characters: 7790
594 words with only consonants: 4801
595 words with only capital letters: 1316
596 words with only vowels: 1701
597
598C<NYTProf> will generate a report database into the file F<nytprof.out> by
599default. Human readable reports can be generated from here by using the
600supplied C<nytprofhtml> (HTML output) and C<nytprofcsv> (CSV output) programs.
601We've used the unix sytem C<html2text> utility to convert the
602F<nytprof/index.html> file for convenience here.
603
604 $> html2text nytprof/index.html
605
606 Performance Profile Index
607 For wordmatch
608 Run on Fri Sep 26 13:46:39 2008
609 Reported on Fri Sep 26 13:47:23 2008
610
611 Top 15 Subroutines -- ordered by exclusive time
612 |Calls |P |F |Inclusive|Exclusive|Subroutine |
613 | | | |Time |Time | |
614 |251215|5 |1 |13.09263 |10.47692 |main:: |matches |
615 |260642|2 |1 |2.71199 |2.71199 |main:: |debug |
616 |1 |1 |1 |0.21404 |0.21404 |main:: |report |
617 |2 |2 |2 |0.00511 |0.00511 |XSLoader:: |load (xsub) |
618 |14 |14|7 |0.00304 |0.00298 |Exporter:: |import |
619 |3 |1 |1 |0.00265 |0.00254 |Exporter:: |as_heavy |
620 |10 |10|4 |0.00140 |0.00140 |vars:: |import |
621 |13 |13|1 |0.00129 |0.00109 |constant:: |import |
622 |1 |1 |1 |0.00360 |0.00096 |FileHandle:: |import |
623 |3 |3 |3 |0.00086 |0.00074 |warnings::register::|import |
624 |9 |3 |1 |0.00036 |0.00036 |strict:: |bits |
625 |13 |13|13|0.00032 |0.00029 |strict:: |import |
626 |2 |2 |2 |0.00020 |0.00020 |warnings:: |import |
627 |2 |1 |1 |0.00020 |0.00020 |Getopt::Long:: |ParseOptionSpec|
628 |7 |7 |6 |0.00043 |0.00020 |strict:: |unimport |
629
630 For more information see the full list of 189 subroutines.
631
632The first part of the report already shows the critical information regarding
633which subroutines are using the most time. The next gives some statistics
634about the source files profiled.
635
636 Source Code Files -- ordered by exclusive time then name
637 |Stmts |Exclusive|Avg. |Reports |Source File |
638 | |Time | | | |
639 |2699761|15.66654 |6e-06 |line . block . sub|wordmatch |
640 |35 |0.02187 |0.00062|line . block . sub|IO/Handle.pm |
641 |274 |0.01525 |0.00006|line . block . sub|Getopt/Long.pm |
642 |20 |0.00585 |0.00029|line . block . sub|Fcntl.pm |
643 |128 |0.00340 |0.00003|line . block . sub|Exporter/Heavy.pm |
644 |42 |0.00332 |0.00008|line . block . sub|IO/File.pm |
645 |261 |0.00308 |0.00001|line . block . sub|Exporter.pm |
646 |323 |0.00248 |8e-06 |line . block . sub|constant.pm |
647 |12 |0.00246 |0.00021|line . block . sub|File/Spec/Unix.pm |
648 |191 |0.00240 |0.00001|line . block . sub|vars.pm |
649 |77 |0.00201 |0.00003|line . block . sub|FileHandle.pm |
650 |12 |0.00198 |0.00016|line . block . sub|Carp.pm |
651 |14 |0.00175 |0.00013|line . block . sub|Symbol.pm |
652 |15 |0.00130 |0.00009|line . block . sub|IO.pm |
653 |22 |0.00120 |0.00005|line . block . sub|IO/Seekable.pm |
654 |198 |0.00085 |4e-06 |line . block . sub|warnings/register.pm|
655 |114 |0.00080 |7e-06 |line . block . sub|strict.pm |
656 |47 |0.00068 |0.00001|line . block . sub|warnings.pm |
657 |27 |0.00054 |0.00002|line . block . sub|overload.pm |
658 |9 |0.00047 |0.00005|line . block . sub|SelectSaver.pm |
659 |13 |0.00045 |0.00003|line . block . sub|File/Spec.pm |
660 |2701595|15.73869 | |Total |
661 |128647 |0.74946 | |Average |
662 | |0.00201 |0.00003|Median |
663 | |0.00121 |0.00003|Deviation |
664
665 Report produced by the NYTProf 2.03 Perl profiler, developed by Tim Bunce and
666 Adam Kaplan.
667
668At this point, if you're using the I<html> report, you can click through the
669various links to bore down into each subroutine and each line of code. Because
670we're using the text reporting here, and there's a whole directory full of
671reports built for each source file, we'll just display a part of the
672corresponding F<wordmatch-line.html> file, sufficient to give an idea of the
673sort of output you can expect from this cool tool.
674
675 $> html2text nytprof/wordmatch-line.html
676
677 Performance Profile -- -block view-.-line view-.-sub view-
678 For wordmatch
679 Run on Fri Sep 26 13:46:39 2008
680 Reported on Fri Sep 26 13:47:22 2008
681
682 File wordmatch
683
684 Subroutines -- ordered by exclusive time
685 |Calls |P|F|Inclusive|Exclusive|Subroutine |
686 | | | |Time |Time | |
687 |251215|5|1|13.09263 |10.47692 |main::|matches|
688 |260642|2|1|2.71199 |2.71199 |main::|debug |
689 |1 |1|1|0.21404 |0.21404 |main::|report |
690 |0 |0|0|0 |0 |main::|BEGIN |
691
692
693 |Line|Stmts.|Exclusive|Avg. |Code |
694 | | |Time | | |
695 |1 | | | |#!/usr/bin/perl |
696 |2 | | | | |
697 | | | | |use strict; |
698 |3 |3 |0.00086 |0.00029|# spent 0.00003s making 1 calls to strict:: |
699 | | | | |import |
700 | | | | |use warnings; |
701 |4 |3 |0.01563 |0.00521|# spent 0.00012s making 1 calls to warnings:: |
702 | | | | |import |
703 |5 | | | | |
704 |6 | | | |=head1 NAME |
705 |7 | | | | |
706 |8 | | | |filewords - word analysis of input file |
707 <...snip...>
708 |62 |1 |0.00445 |0.00445|print report( %count ); |
709 | | | | |# spent 0.21404s making 1 calls to main::report|
710 |63 | | | | |
711 | | | | |# spent 23.56955s (10.47692+2.61571) within |
712 | | | | |main::matches which was called 251215 times, |
713 | | | | |avg 0.00005s/call: # 50243 times |
714 | | | | |(2.12134+0.51939s) at line 57 of wordmatch, avg|
715 | | | | |0.00005s/call # 50243 times (2.17735+0.54550s) |
716 |64 | | | |at line 56 of wordmatch, avg 0.00005s/call # |
717 | | | | |50243 times (2.10992+0.51797s) at line 58 of |
718 | | | | |wordmatch, avg 0.00005s/call # 50243 times |
719 | | | | |(2.12696+0.51598s) at line 55 of wordmatch, avg|
720 | | | | |0.00005s/call # 50243 times (1.94134+0.51687s) |
721 | | | | |at line 54 of wordmatch, avg 0.00005s/call |
722 | | | | |sub matches { |
723 <...snip...>
724 |102 | | | | |
725 | | | | |# spent 2.71199s within main::debug which was |
726 | | | | |called 260642 times, avg 0.00001s/call: # |
727 | | | | |251215 times (2.61571+0s) by main::matches at |
728 |103 | | | |line 74 of wordmatch, avg 0.00001s/call # 9427 |
729 | | | | |times (0.09628+0s) at line 50 of wordmatch, avg|
730 | | | | |0.00001s/call |
731 | | | | |sub debug { |
732 |104 |260642|0.58496 |2e-06 |my $message = shift; |
733 |105 | | | | |
734 |106 |260642|1.09917 |4e-06 |if ( $debug ) { |
735 |107 | | | |print STDERR "DBG: $message\n"; |
736 |108 | | | |} |
737 |109 | | | |} |
738 |110 | | | | |
739 |111 |1 |0.01501 |0.01501|exit 0; |
740 |112 | | | | |
741
742Oodles of very useful information in there - this seems to be the way forward.
743
744See also C<Devel::NYTProf::Apache> which hooks C<Devel::NYTProf> into C<mod_perl>.
745
746=head1 SORTING
747
748Perl modules are not the only tools a performance analyst has at their
749disposal, system tools like C<time> should not be overlooked as the next
750example shows, where we take a quick look at sorting. Many books, theses and
751articles, have been written about efficient sorting algorithms, and this is not
752the place to repeat such work, there's several good sorting modules which
753deserve taking a look at too: C<Sort::Maker>, C<Sort::Key> spring to mind.
754However, it's still possible to make some observations on certain Perl specific
755interpretations on issues relating to sorting data sets and give an example or
756two with regard to how sorting large data volumes can effect performance.
757Firstly, an often overlooked point when sorting large amounts of data, one can
758attempt to reduce the data set to be dealt with and in many cases C<grep()> can
759be quite useful as a simple filter:
760
761 @data = sort grep { /$filter/ } @incoming
762
763A command such as this can vastly reduce the volume of material to actually
764sort through in the first place, and should not be too lightly disregarded
765purely on the basis of it's simplicity. The C<KISS> principle is too often
766overlooked - the next example uses the simple system C<time> utility to
767demonstrate. Let's take a look at an actual example of sorting the contents of
768a large file, an apache logfile would do. This one has over a quarter of a
769million lines, is 50M in size, and a snippet of it looks like this:
770
771# logfile
772
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)"
774 188.209-65-87.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)"
775 151.56.71.198 - - [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"
776 151.56.71.198 - - [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"
777 151.56.71.198 - - [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"
778 217.113.68.60 - - [08/Feb/2007:13:02:15 +0000] "GET / HTTP/1.1" 304 - "-" "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1)"
779 217.113.68.60 - - [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)"
780 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)"
781 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)"
782 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)"
783 195.24.196.99 - - [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"
784 195.24.196.99 - - [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"
785 195.24.196.99 - - [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"
786 crawl1.cosmixcorp.com - - [08/Feb/2007:13:27:57 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "voyager/1.0"
787 crawl1.cosmixcorp.com - - [08/Feb/2007:13:28:25 +0000] "GET /links.html HTTP/1.0" 200 3413 "-" "voyager/1.0"
788 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)"
789 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)"
790 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)"
791 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)"
792 pop.compuscan.co.za - - [08/Feb/2007:14:10:43 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net"
793 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)"
794 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)"
795 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"
796 196.201.92.41 - - [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"
797
798The specific task here is to sort the 286,525 lines of this file by Response
799Code, Query, Browser, Referring Url, and lastly Date. One solution might be to
800use the following code, which iterates over the files given on the
801command-line.
802
803# sort-apache-log
804
805 #!/usr/bin/perl -n
806
807 use strict;
808 use warnings;
809
810 my @data;
811
812 LINE:
813 while ( <> ) {
814 my $line = $_;
815 if (
816 $line =~ m/^(
817 ([\w\.\-]+) # client
818 \s*-\s*-\s*\[
819 ([^]]+) # date
820 \]\s*"\w+\s*
821 (\S+) # query
822 [^"]+"\s*
823 (\d+) # status
824 \s+\S+\s+"[^"]*"\s+"
825 ([^"]*) # browser
826 "
827 .*
828 )$/x
829 ) {
830 my @chunks = split(/ +/, $line);
831 my $ip = $1;
832 my $date = $2;
833 my $query = $3;
834 my $status = $4;
835 my $browser = $5;
836
837 push(@data, [$ip, $date, $query, $status, $browser, $line]);
838 }
839 }
840
841 my @sorted = sort {
842 $a->[3] cmp $b->[3]
843 ||
844 $a->[2] cmp $b->[2]
845 ||
846 $a->[0] cmp $b->[0]
847 ||
848 $a->[1] cmp $b->[1]
849 ||
850 $a->[4] cmp $b->[4]
851 } @data;
852
853 foreach my $data ( @sorted ) {
854 print $data->[5];
855 }
856
857 exit 0;
858
859When running this program, redirect C<STDOUT> so it is possible to check the
860output is correct from following test runs and use the system C<time> utility
861to check the overall runtime.
862
863 $> time ./sort-apache-log logfile > out-sort
864
865 real 0m17.371s
866 user 0m15.757s
867 sys 0m0.592s
868
869The program took just over 17 wallclock seconds to run. Note the different
870values C<time> outputs, it's important to always use the same one, and to not
871confuse what each one means.
872
873=over 4
874
875=item Elapsed Real Time
876
877The overall, or wallclock, time between when C<time> was called, and when it
878terminates. The elapsed time includes both user and system times, and time
879spent waiting for other users and processes on the system. Inevitably, this is
880the most approximate of the measurements given.
881
882=item User CPU Time
883
884The user time is the amount of time the entire process spent on behalf of the
885user on this system executing this program.
886
887=item System CPU Time
888
889The system time is the amount of time the kernel itself spent executing
890routines, or system calls, on behalf of this process user.
891
892=back
893
894Running this same process as a C<Schwarzian Transform> it is possible to
895eliminate the input and output arrays for storing all the data, and work on the
896input directly as it arrives too. Otherwise, the code looks fairly similar:
897
898# sort-apache-log-schwarzian
899
900 #!/usr/bin/perl -n
901
902 use strict;
903 use warnings;
904
905 print
906
907 map $_->[0] =>
908
909 sort {
910 $a->[4] cmp $b->[4]
911 ||
912 $a->[3] cmp $b->[3]
913 ||
914 $a->[1] cmp $b->[1]
915 ||
916 $a->[2] cmp $b->[2]
917 ||
918 $a->[5] cmp $b->[5]
919 }
920 map [ $_, m/^(
921 ([\w\.\-]+) # client
922 \s*-\s*-\s*\[
923 ([^]]+) # date
924 \]\s*"\w+\s*
925 (\S+) # query
926 [^"]+"\s*
927 (\d+) # status
928 \s+\S+\s+"[^"]*"\s+"
929 ([^"]*) # browser
930 "
931 .*
932 )$/xo ]
933
934 => <>;
935
936 exit 0;
937
938Run the new code against the same logfile, as above, to check the new time.
939
940 $> time ./sort-apache-log-schwarzian logfile > out-schwarz
941
942 real 0m9.664s
943 user 0m8.873s
944 sys 0m0.704s
945
946The time has been cut in half, which is a respectable speed improvement by any
947standard. Naturally, it is important to check the output is consistent with
948the first program run, this is where the unix system C<cksum> utility comes in.
949
950 $> cksum out-sort out-schwarz
951 3044173777 52029194 out-sort
952 3044173777 52029194 out-schwarz
953
954BTW. Beware too of pressure from managers who see you speed a program up by 50%
955of the runtime once, only to get a request one month later to do the same again
956(true story) - you'll just have to point out your only human, even if you are a
957Perl programmer, and you'll see what you can do...
958
959=head1 LOGGING
960
961An essential part of any good development process is appropriate error handling
962with appropriately informative messages, however there exists a school of
963thought which suggests that log files should be I<chatty>, as if the chain of
964unbroken output somehow ensures the survival of the program. If speed is in
965any way an issue, this approach is wrong.
966
967A common sight is code which looks something like this:
968
969 logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) )
970
971The problem is that this code will always be parsed and executed, even when the
972debug level set in the logging configuration file is zero. Once the debug()
973subroutine has been entered, and the internal C<$debug> variable confirmed to
974be zero, for example, the message which has been sent in will be discarded and
975the program will continue. In the example given though, the \%INC hash will
976already have been dumped, and the message string constructed, all of which work
977could be bypassed by a debug variable at the statement level, like this:
978
979 logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) if $DEBUG;
980
981This effect can be demonstrated by setting up a test script with both forms,
982including a C<debug()> subroutine to emulate typical C<logger()> functionality.
983
984# ifdebug
985
986 #!/usr/bin/perl
987
988 use strict;
989 use warnings;
990
991 use Benchmark;
992 use Data::Dumper;
993 my $DEBUG = 0;
994
995 sub debug {
996 my $msg = shift;
997
998 if ( $DEBUG ) {
999 print "DEBUG: $msg\n";
1000 }
1001 };
1002
1003 timethese(100000, {
1004 'debug' => sub {
1005 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) )
1006 },
1007 'ifdebug' => sub {
1008 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if $DEBUG
1009 },
1010 });
1011
1012Let's see what C<Benchmark> makes of this:
1013
1014 $> perl ifdebug
1015 Benchmark: timing 100000 iterations of constant, sub...
1016 ifdebug: 0 wallclock secs ( 0.01 usr + 0.00 sys = 0.01 CPU) @ 10000000.00/s (n=100000)
1017 (warning: too few iterations for a reliable count)
1018 debug: 14 wallclock secs (13.18 usr + 0.04 sys = 13.22 CPU) @ 7564.30/s (n=100000)
1019
1020In the one case the code, which does exactly the same thing as far as
1021outputting any debugging information is concerned, in other words nothing,
1022takes 14 seconds, and in the other case the code takes one hundredth of a
1023second. Looks fairly definitive. Use a C<$DEBUG> variable BEFORE you call the
1024subroutine, rather than relying on the smart functionality inside it.
1025
1026=head2 Logging if DEBUG (constant)
1027
1028It's possible to take the previous idea a little further, by using a compile
1029time C<DEBUG> constant.
1030
1031# ifdebug-constant
1032
1033 #!/usr/bin/perl
1034
1035 use strict;
1036 use warnings;
1037
1038 use Benchmark;
1039 use Data::Dumper;
1040 use constant
1041 DEBUG => 0
1042 ;
1043
1044 sub debug {
1045 if ( DEBUG ) {
1046 my $msg = shift;
1047 print "DEBUG: $msg\n";
1048 }
1049 };
1050
1051 timethese(100000, {
1052 'debug' => sub {
1053 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) )
1054 },
1055 'constant' => sub {
1056 debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if DEBUG
1057 },
1058 });
1059
1060Running this program produces the following output:
1061
1062 $> perl ifdebug-constant
1063 Benchmark: timing 100000 iterations of constant, sub...
1064 constant: 0 wallclock secs (-0.00 usr + 0.00 sys = -0.00 CPU) @ -7205759403792793600000.00/s (n=100000)
1065 (warning: too few iterations for a reliable count)
1066 sub: 14 wallclock secs (13.09 usr + 0.00 sys = 13.09 CPU) @ 7639.42/s (n=100000)
1067
1068The C<DEBUG> constant wipes the floor with even the C<$debug> variable,
1069clocking in at minus zero seconds, and generates a "warning: too few iterations
1070for a reliable count" message into the bargain. To see what is really going
1071on, and why we had too few iterations when we thought we asked for 100000, we
1072can use the very useful C<B::Deparse> to inspect the new code:
1073
1074 $> perl -MO=Deparse ifdebug-constant
1075
1076 use Benchmark;
1077 use Data::Dumper;
1078 use constant ('DEBUG', 0);
1079 sub debug {
1080 use warnings;
1081 use strict 'refs';
1082 0;
1083 }
1084 use warnings;
1085 use strict 'refs';
1086 timethese(100000, {'sub', sub {
1087 debug "A $0 logging message via process-id: $$" . Dumper(\%INC);
1088 }
1089 , 'constant', sub {
1090 0;
1091 }
1092 });
1093 ifdebug-constant syntax OK
1094
1095The output shows the constant() subroutine we're testing being replaced with
1096the value of the C<DEBUG> constant: zero. The line to be tested has been
1097completely optimized away, and you can't get much more efficient than that.
1098
1099=head1 POSTSCRIPT
1100
1101This document has provided several way to go about identifying hot-spots, and
1102checking whether any modifications have improved the runtime of the code.
1103
1104As a final thought, remember that it's not (at the time of writing) possible to
1105produce a useful program which will run in zero or negative time and this basic
1106principle can be written as: I<useful programs are slow> by their very
1107definition. It is of course possible to write a nearly instantaneous program,
1108but it's not going to do very much, here's a very efficient one:
1109
1110 $> perl -e 0
1111
1112Optimizing that any further is a job for C<p5p>.
1113
1114=head1 SEE ALSO
1115
1116Further reading can be found using the modules and links below.
1117
1118=head2 PERLDOCS
1119
1120For example: perldoc -f sort
1121
1122 L<perlfaq4>
1123 L<perlfork>
1124 L<perlfunc>
1125 L<perlretut>
1126 L<perlthrtut>
1127 L<threads>
1128
1129=head2 MAN PAGES
1130
1131 L<time>
1132
1133=head2 MODULES
1134
1135It's not possible to individually showcase all the performance related code for
1136Perl here, naturally, but here's a short list of modules from the CPAN which
1137deserve further attention.
1138
1139 L<Apache::DProf>
1140 L<Apache::SmallProf>
1141 L<Benchmark>
1142 L<DBIx::Profiler>
1143 L<Devel::AutoProfiler>
1144 L<Devel::DProf>
1145 L<Devel::DProfLB>
1146 L<Devel::FastProf>
1147 L<Devel::GraphVizProf>
1148 L<Devel::NYTProf>
1149 L<Devel::NYTProf::Apache>
1150 L<Devel::Profiler>
1151 L<Devel::Profile>
1152 L<Devel::Profit>
1153 L<Devel::SmallProf>
1154 L<Devel::WxProf>
1155 L<POE::Devel::Profiler>
1156 L<Sort::Key>
1157 L<Sort::Maker>
1158
1159=head2 URLS
1160
1161Very useful online reference material:
1162
1163 http://www.ccl4.org/~nick/P/Fast_Enough/
1164
1165 http://www-128.ibm.com/developerworks/library/l-optperl.html
1166
1167 http://perlbuzz.com/2007/11/bind-output-variables-in-dbi-for-speed-and-safety.html
1168
1169 http://en.wikipedia.org/wiki/Performance_analysis
1170
1171 http://apache.perl.org/docs/1.0/guide/performance.html
1172
1173 http://perlgolf.sourceforge.net/
1174
1175 http://www.sysarch.com/Perl/sort_paper.html
1176
1177 http://www.unix.org.ua/orelly/perl/prog/ch08_03.htm
1178
1179=head1 AUTHOR
1180
1181Richard Foley <richard.foley@rfi.net> Copyright (c) 2008
1182
1183=cut