| =head1 NAME |
| |
| perlperf - Perl Performance and Optimization Techniques |
| |
| =head1 DESCRIPTION |
| |
| This is an introduction to the use of performance and optimization techniques |
| which can be used with particular reference to perl programs. While many perl |
| developers have come from other languages, and can use their prior knowledge |
| where appropriate, there are many other people who might benefit from a few |
| perl specific pointers. If you want the condensed version, perhaps the best |
| advice comes from the renowned Japanese Samurai, Miyamoto Musashi, who said: |
| |
| "Do Not Engage in Useless Activity" |
| |
| in 1645. |
| |
| =head1 OVERVIEW |
| |
| Perhaps the most common mistake programmers make is to attempt to optimize |
| their code before a program actually does anything useful - this is a bad idea. |
| There's no point in having an extremely fast program that doesn't work. The |
| first job is to get a program to I<correctly> do something B<useful>, (not to |
| mention ensuring the test suite is fully functional), and only then to consider |
| optimizing it. Having decided to optimize existing working code, there are |
| several simple but essential steps to consider which are intrinsic to any |
| optimization process. |
| |
| =head2 ONE STEP SIDEWAYS |
| |
| Firstly, you need to establish a baseline time for the existing code, which |
| timing needs to be reliable and repeatable. You'll probably want to use the |
| C<Benchmark> or C<Devel::NYTProf> modules, or something similar, for this step, |
| or perhaps the Unix system C<time> utility, whichever is appropriate. See the |
| base of this document for a longer list of benchmarking and profiling modules, |
| and recommended further reading. |
| |
| =head2 ONE STEP FORWARD |
| |
| Next, having examined the program for I<hot spots>, (places where the code |
| seems to run slowly), change the code with the intention of making it run |
| faster. Using version control software, like C<subversion>, will ensure no |
| changes are irreversible. It's too easy to fiddle here and fiddle there - |
| don't change too much at any one time or you might not discover which piece of |
| code B<really> was the slow bit. |
| |
| =head2 ANOTHER STEP SIDEWAYS |
| |
| It's not enough to say: "that will make it run faster", you have to check it. |
| Rerun the code under control of the benchmarking or profiling modules, from the |
| first step above, and check that the new code executed the B<same task> in |
| I<less time>. Save your work and repeat... |
| |
| =head1 GENERAL GUIDELINES |
| |
| The critical thing when considering performance is to remember there is no such |
| thing as a C<Golden Bullet>, which is why there are no rules, only guidelines. |
| |
| It is clear that inline code is going to be faster than subroutine or method |
| calls, because there is less overhead, but this approach has the disadvantage |
| of being less maintainable and comes at the cost of greater memory usage - |
| there is no such thing as a free lunch. If you are searching for an element in |
| a list, it can be more efficient to store the data in a hash structure, and |
| then simply look to see whether the key is defined, rather than to loop through |
| the entire array using grep() for instance. substr() may be (a lot) faster |
| than grep() but not as flexible, so you have another trade-off to access. Your |
| code may contain a line which takes 0.01 of a second to execute which if you |
| call it 1,000 times, quite likely in a program parsing even medium sized files |
| for instance, you already have a 10 second delay, in just one single code |
| location, and if you call that line 100,000 times, your entire program will |
| slow down to an unbearable crawl. |
| |
| Using a subroutine as part of your sort is a powerful way to get exactly what |
| you want, but will usually be slower than the built-in I<alphabetic> C<cmp> and |
| I<numeric> C<E<lt>=E<gt>> sort operators. It is possible to make multiple |
| passes over your data, building indices to make the upcoming sort more |
| efficient, and to use what is known as the C<OM> (Orcish Maneuver) to cache the |
| sort keys in advance. The cache lookup, while a good idea, can itself be a |
| source of slowdown by enforcing a double pass over the data - once to setup the |
| cache, and once to sort the data. Using C<pack()> to extract the required sort |
| key into a consistent string can be an efficient way to build a single string |
| to compare, instead of using multiple sort keys, which makes it possible to use |
| the standard, written in C<c> and fast, perl C<sort()> function on the output, |
| and is the basis of the C<GRT> (Guttman Rossler Transform). Some string |
| combinations can slow the C<GRT> down, by just being too plain complex for it's |
| own good. |
| |
| For applications using database backends, the standard C<DBIx> namespace has |
| tries to help with keeping things nippy, not least because it tries to I<not> |
| query the database until the latest possible moment, but always read the docs |
| which come with your choice of libraries. Among the many issues facing |
| developers dealing with databases should remain aware of is to always use |
| C<SQL> placeholders and to consider pre-fetching data sets when this might |
| prove advantageous. Splitting up a large file by assigning multiple processes |
| to parsing a single file, using say C<POE>, C<threads> or C<fork> can also be a |
| useful way of optimizing your usage of the available C<CPU> resources, though |
| this technique is fraught with concurrency issues and demands high attention to |
| detail. |
| |
| Every case has a specific application and one or more exceptions, and there is |
| no replacement for running a few tests and finding out which method works best |
| for your particular environment, this is why writing optimal code is not an |
| exact science, and why we love using Perl so much - TMTOWTDI. |
| |
| =head1 BENCHMARKS |
| |
| Here are a few examples to demonstrate usage of Perl's benchmarking tools. |
| |
| =head2 Assigning and Dereferencing Variables. |
| |
| I'm sure most of us have seen code which looks like, (or worse than), this: |
| |
| if ( $obj->{_ref}->{_myscore} >= $obj->{_ref}->{_yourscore} ) { |
| ... |
| |
| This sort of code can be a real eyesore to read, as well as being very |
| sensitive to typos, and it's much clearer to dereference the variable |
| explicitly. We're side-stepping the issue of working with object-oriented |
| programming techniques to encapsulate variable access via methods, only |
| accessible through an object. Here we're just discussing the technical |
| implementation of choice, and whether this has an effect on performance. We |
| can see whether this dereferencing operation, has any overhead by putting |
| comparative code in a file and running a C<Benchmark> test. |
| |
| # dereference |
| |
| #!/usr/bin/perl |
| |
| use strict; |
| use warnings; |
| |
| use Benchmark; |
| |
| my $ref = { |
| 'ref' => { |
| _myscore => '100 + 1', |
| _yourscore => '102 - 1', |
| }, |
| }; |
| |
| timethese(1000000, { |
| 'direct' => sub { |
| my $x = $ref->{ref}->{_myscore} . $ref->{ref}->{_yourscore} ; |
| }, |
| 'dereference' => sub { |
| my $ref = $ref->{ref}; |
| my $myscore = $ref->{_myscore}; |
| my $yourscore = $ref->{_yourscore}; |
| my $x = $myscore . $yourscore; |
| }, |
| }); |
| |
| It's essential to run any timing measurements a sufficient number of times so |
| the numbers settle on a numerical average, otherwise each run will naturally |
| fluctuate due to variations in the environment, to reduce the effect of |
| contention for C<CPU> resources and network bandwidth for instance. Running |
| the above code for one million iterations, we can take a look at the report |
| output by the C<Benchmark> module, to see which approach is the most effective. |
| |
| $> perl dereference |
| |
| Benchmark: timing 1000000 iterations of dereference, direct... |
| dereference: 2 wallclock secs ( 1.59 usr + 0.00 sys = 1.59 CPU) @ 628930.82/s (n=1000000) |
| direct: 1 wallclock secs ( 1.20 usr + 0.00 sys = 1.20 CPU) @ 833333.33/s (n=1000000) |
| |
| The difference is clear to see and the dereferencing approach is slower. While |
| it managed to execute an average of 628,930 times a second during our test, the |
| direct approach managed to run an additional 204,403 times, unfortunately. |
| Unfortunately, because there are many examples of code written using the |
| multiple layer direct variable access, and it's usually horrible. It is, |
| however, minusculy faster. The question remains whether the minute gain is |
| actually worth the eyestrain, or the loss of maintainability. |
| |
| =head2 Search and replace or tr |
| |
| If we have a string which needs to be modified, while a regex will almost |
| always be much more flexible, C<tr>, an oft underused tool, can still be a |
| useful. One scenario might be replace all vowels with another character. The |
| regex solution might look like this: |
| |
| $str =~ s/[aeiou]/x/g |
| |
| The C<tr> alternative might look like this: |
| |
| $str =~ tr/aeiou/xxxxx/ |
| |
| We can put that into a test file which we can run to check which approach is |
| the fastest, using a global C<$STR> variable to assign to the C<my $str> |
| variable so as to avoid perl trying to optimize any of the work away by |
| noticing it's assigned only the once. |
| |
| # regex-transliterate |
| |
| #!/usr/bin/perl |
| |
| use strict; |
| use warnings; |
| |
| use Benchmark; |
| |
| my $STR = "$$-this and that"; |
| |
| timethese( 1000000, { |
| 'sr' => sub { my $str = $STR; $str =~ s/[aeiou]/x/g; return $str; }, |
| 'tr' => sub { my $str = $STR; $str =~ tr/aeiou/xxxxx/; return $str; }, |
| }); |
| |
| Running the code gives us our results: |
| |
| $> perl regex-transliterate |
| |
| Benchmark: timing 1000000 iterations of sr, tr... |
| sr: 2 wallclock secs ( 1.19 usr + 0.00 sys = 1.19 CPU) @ 840336.13/s (n=1000000) |
| tr: 0 wallclock secs ( 0.49 usr + 0.00 sys = 0.49 CPU) @ 2040816.33/s (n=1000000) |
| |
| The C<tr> version is a clear winner. One solution is flexible, the other is |
| fast - and it's appropriately the programmer's choice which to use. |
| |
| Check the C<Benchmark> docs for further useful techniques. |
| |
| =head1 PROFILING TOOLS |
| |
| A slightly larger piece of code will provide something on which a profiler can |
| produce more extensive reporting statistics. This example uses the simplistic |
| C<wordmatch> program which parses a given input file and spews out a short |
| report on the contents. |
| |
| # wordmatch |
| |
| #!/usr/bin/perl |
| |
| use strict; |
| use warnings; |
| |
| =head1 NAME |
| |
| filewords - word analysis of input file |
| |
| =head1 SYNOPSIS |
| |
| filewords -f inputfilename [-d] |
| |
| =head1 DESCRIPTION |
| |
| This program parses the given filename, specified with C<-f>, and displays a |
| simple analysis of the words found therein. Use the C<-d> switch to enable |
| debugging messages. |
| |
| =cut |
| |
| use FileHandle; |
| use Getopt::Long; |
| |
| my $debug = 0; |
| my $file = ''; |
| |
| my $result = GetOptions ( |
| 'debug' => \$debug, |
| 'file=s' => \$file, |
| ); |
| die("invalid args") unless $result; |
| |
| unless ( -f $file ) { |
| die("Usage: $0 -f filename [-d]"); |
| } |
| my $FH = FileHandle->new("< $file") or die("unable to open file($file): $!"); |
| |
| my $i_LINES = 0; |
| my $i_WORDS = 0; |
| my %count = (); |
| |
| my @lines = <$FH>; |
| foreach my $line ( @lines ) { |
| $i_LINES++; |
| $line =~ s/\n//; |
| my @words = split(/ +/, $line); |
| my $i_words = scalar(@words); |
| $i_WORDS = $i_WORDS + $i_words; |
| debug("line: $i_LINES supplying $i_words words: @words"); |
| my $i_word = 0; |
| foreach my $word ( @words ) { |
| $i_word++; |
| $count{$i_LINES}{spec} += matches($i_word, $word, '[^a-zA-Z0-9]'); |
| $count{$i_LINES}{only} += matches($i_word, $word, '^[^a-zA-Z0-9]+$'); |
| $count{$i_LINES}{cons} += matches($i_word, $word, '^[(?i:bcdfghjklmnpqrstvwxyz)]+$'); |
| $count{$i_LINES}{vows} += matches($i_word, $word, '^[(?i:aeiou)]+$'); |
| $count{$i_LINES}{caps} += matches($i_word, $word, '^[(A-Z)]+$'); |
| } |
| } |
| |
| print report( %count ); |
| |
| sub matches { |
| my $i_wd = shift; |
| my $word = shift; |
| my $regex = shift; |
| my $has = 0; |
| |
| if ( $word =~ /($regex)/ ) { |
| $has++ if $1; |
| } |
| |
| debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/"); |
| |
| return $has; |
| } |
| |
| sub report { |
| my %report = @_; |
| my %rep; |
| |
| foreach my $line ( keys %report ) { |
| foreach my $key ( keys %{ $report{$line} } ) { |
| $rep{$key} += $report{$line}{$key}; |
| } |
| } |
| |
| my $report = qq| |
| $0 report for $file: |
| lines in file: $i_LINES |
| words in file: $i_WORDS |
| words with special (non-word) characters: $i_spec |
| words with only special (non-word) characters: $i_only |
| words with only consonants: $i_cons |
| words with only capital letters: $i_caps |
| words with only vowels: $i_vows |
| |; |
| |
| return $report; |
| } |
| |
| sub debug { |
| my $message = shift; |
| |
| if ( $debug ) { |
| print STDERR "DBG: $message\n"; |
| } |
| } |
| |
| exit 0; |
| |
| =head2 Devel::DProf |
| |
| This venerable module has been the de-facto standard for Perl code profiling |
| for more than a decade, but has been replaced by a number of other modules |
| which have brought us back to the 21st century. Although you're recommended to |
| evaluate your tool from the several mentioned here and from the CPAN list at |
| the base of this document, (and currently L<Devel::NYTProf> seems to be the |
| weapon of choice - see below), we'll take a quick look at the output from |
| L<Devel::DProf> first, to set a baseline for Perl profiling tools. Run the |
| above program under the control of C<Devel::DProf> by using the C<-d> switch on |
| the command-line. |
| |
| $> perl -d:DProf wordmatch -f perl5db.pl |
| |
| <...multiple lines snipped...> |
| |
| wordmatch report for perl5db.pl: |
| lines in file: 9428 |
| words in file: 50243 |
| words with special (non-word) characters: 20480 |
| words with only special (non-word) characters: 7790 |
| words with only consonants: 4801 |
| words with only capital letters: 1316 |
| words with only vowels: 1701 |
| |
| C<Devel::DProf> produces a special file, called F<tmon.out> by default, and |
| this file is read by the C<dprofpp> program, which is already installed as part |
| of the C<Devel::DProf> distribution. If you call C<dprofpp> with no options, |
| it will read the F<tmon.out> file in the current directory and produce a human |
| readable statistics report of the run of your program. Note that this may take |
| a little time. |
| |
| $> dprofpp |
| |
| Total Elapsed Time = 2.951677 Seconds |
| User+System Time = 2.871677 Seconds |
| Exclusive Times |
| %Time ExclSec CumulS #Calls sec/call Csec/c Name |
| 102. 2.945 3.003 251215 0.0000 0.0000 main::matches |
| 2.40 0.069 0.069 260643 0.0000 0.0000 main::debug |
| 1.74 0.050 0.050 1 0.0500 0.0500 main::report |
| 1.04 0.030 0.049 4 0.0075 0.0123 main::BEGIN |
| 0.35 0.010 0.010 3 0.0033 0.0033 Exporter::as_heavy |
| 0.35 0.010 0.010 7 0.0014 0.0014 IO::File::BEGIN |
| 0.00 - -0.000 1 - - Getopt::Long::FindOption |
| 0.00 - -0.000 1 - - Symbol::BEGIN |
| 0.00 - -0.000 1 - - Fcntl::BEGIN |
| 0.00 - -0.000 1 - - Fcntl::bootstrap |
| 0.00 - -0.000 1 - - warnings::BEGIN |
| 0.00 - -0.000 1 - - IO::bootstrap |
| 0.00 - -0.000 1 - - Getopt::Long::ConfigDefaults |
| 0.00 - -0.000 1 - - Getopt::Long::Configure |
| 0.00 - -0.000 1 - - Symbol::gensym |
| |
| C<dprofpp> will produce some quite detailed reporting on the activity of the |
| C<wordmatch> program. The wallclock, user and system, times are at the top of |
| the analysis, and after this are the main columns defining which define the |
| report. Check the C<dprofpp> docs for details of the many options it supports. |
| |
| See also C<Apache::DProf> which hooks C<Devel::DProf> into C<mod_perl>. |
| |
| =head2 Devel::Profiler |
| |
| Let's take a look at the same program using a different profiler: |
| C<Devel::Profiler>, a drop-in Perl-only replacement for C<Devel::DProf>. The |
| usage is very slightly different in that instead of using the special C<-d:> |
| flag, you pull C<Devel::Profiler> in directly as a module using C<-M>. |
| |
| $> perl -MDevel::Profiler wordmatch -f perl5db.pl |
| |
| <...multiple lines snipped...> |
| |
| wordmatch report for perl5db.pl: |
| lines in file: 9428 |
| words in file: 50243 |
| words with special (non-word) characters: 20480 |
| words with only special (non-word) characters: 7790 |
| words with only consonants: 4801 |
| words with only capital letters: 1316 |
| words with only vowels: 1701 |
| |
| |
| C<Devel::Profiler> generates a tmon.out file which is compatible with the |
| C<dprofpp> program, thus saving the construction of a dedicated statistics |
| reader program. C<dprofpp> usage is therefore identical to the above example. |
| |
| $> dprofpp |
| |
| Total Elapsed Time = 20.984 Seconds |
| User+System Time = 19.981 Seconds |
| Exclusive Times |
| %Time ExclSec CumulS #Calls sec/call Csec/c Name |
| 49.0 9.792 14.509 251215 0.0000 0.0001 main::matches |
| 24.4 4.887 4.887 260643 0.0000 0.0000 main::debug |
| 0.25 0.049 0.049 1 0.0490 0.0490 main::report |
| 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::GetOptions |
| 0.00 0.000 0.000 2 0.0000 0.0000 Getopt::Long::ParseOptionSpec |
| 0.00 0.000 0.000 1 0.0000 0.0000 Getopt::Long::FindOption |
| 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::new |
| 0.00 0.000 0.000 1 0.0000 0.0000 IO::Handle::new |
| 0.00 0.000 0.000 1 0.0000 0.0000 Symbol::gensym |
| 0.00 0.000 0.000 1 0.0000 0.0000 IO::File::open |
| |
| Interestingly we get slightly different results, which is mostly because the |
| algorithm which generates the report is different, even though the output file |
| format was allegedly identical. The elapsed, user and system times are clearly |
| showing the time it took for C<Devel::Profiler> to execute its own run, but |
| the column listings feel more accurate somehow than the ones we had earlier |
| from C<Devel::DProf>. The 102% figure has disappeared, for example. This is |
| where we have to use the tools at our disposal, and recognise their pros and |
| cons, before using them. Interestingly, the numbers of calls for each |
| subroutine are identical in the two reports, it's the percentages which differ. |
| As the author of C<Devel::Proviler> writes: |
| |
| ...running HTML::Template's test suite under Devel::DProf shows output() |
| taking NO time but Devel::Profiler shows around 10% of the time is in output(). |
| I don't know which to trust but my gut tells me something is wrong with |
| Devel::DProf. HTML::Template::output() is a big routine that's called for |
| every test. Either way, something needs fixing. |
| |
| YMMV. |
| |
| See also C<Devel::Apache::Profiler> which hooks C<Devel::Profiler> into C<mod_perl>. |
| |
| =head2 Devel::SmallProf |
| |
| The C<Devel::SmallProf> profiler examines the runtime of your Perl program and |
| produces a line-by-line listing to show how many times each line was called, |
| and how long each line took to execute. It is called by supplying the familiar |
| C<-d> flag to Perl at runtime. |
| |
| $> perl -d:SmallProf wordmatch -f perl5db.pl |
| |
| <...multiple lines snipped...> |
| |
| wordmatch report for perl5db.pl: |
| lines in file: 9428 |
| words in file: 50243 |
| words with special (non-word) characters: 20480 |
| words with only special (non-word) characters: 7790 |
| words with only consonants: 4801 |
| words with only capital letters: 1316 |
| words with only vowels: 1701 |
| |
| C<Devel::SmallProf> writes it's output into a file called F<smallprof.out>, by |
| default. The format of the file looks like this: |
| |
| <num> <time> <ctime> <line>:<text> |
| |
| When the program has terminated, the output may be examined and sorted using |
| any standard text filtering utilities. Something like the following may be |
| sufficient: |
| |
| $> cat smallprof.out | grep \d*: | sort -k3 | tac | head -n20 |
| |
| 251215 1.65674 7.68000 75: if ( $word =~ /($regex)/ ) { |
| 251215 0.03264 4.40000 79: debug("word: $i_wd ".($has ? 'matches' : |
| 251215 0.02693 4.10000 81: return $has; |
| 260643 0.02841 4.07000 128: if ( $debug ) { |
| 260643 0.02601 4.04000 126: my $message = shift; |
| 251215 0.02641 3.91000 73: my $has = 0; |
| 251215 0.03311 3.71000 70: my $i_wd = shift; |
| 251215 0.02699 3.69000 72: my $regex = shift; |
| 251215 0.02766 3.68000 71: my $word = shift; |
| 50243 0.59726 1.00000 59: $count{$i_LINES}{cons} = |
| 50243 0.48175 0.92000 61: $count{$i_LINES}{spec} = |
| 50243 0.00644 0.89000 56: my $i_cons = matches($i_word, $word, |
| 50243 0.48837 0.88000 63: $count{$i_LINES}{caps} = |
| 50243 0.00516 0.88000 58: my $i_caps = matches($i_word, $word, '^[(A- |
| 50243 0.00631 0.81000 54: my $i_spec = matches($i_word, $word, '[^a- |
| 50243 0.00496 0.80000 57: my $i_vows = matches($i_word, $word, |
| 50243 0.00688 0.80000 53: $i_word++; |
| 50243 0.48469 0.79000 62: $count{$i_LINES}{only} = |
| 50243 0.48928 0.77000 60: $count{$i_LINES}{vows} = |
| 50243 0.00683 0.75000 55: my $i_only = matches($i_word, $word, '^[^a- |
| |
| You can immediately see a slightly different focus to the subroutine profiling |
| modules, and we start to see exactly which line of code is taking the most |
| time. That regex line is looking a bit suspicious, for example. Remember that |
| these tools are supposed to be used together, there is no single best way to |
| profile your code, you need to use the best tools for the job. |
| |
| See also C<Apache::SmallProf> which hooks C<Devel::SmallProf> into C<mod_perl>. |
| |
| =head2 Devel::FastProf |
| |
| C<Devel::FastProf> is another Perl line profiler. This was written with a view |
| to getting a faster line profiler, than is possible with for example |
| C<Devel::SmallProf>, because it's written in C<C>. To use C<Devel::FastProf>, |
| supply the C<-d> argument to Perl: |
| |
| $> perl -d:FastProf wordmatch -f perl5db.pl |
| |
| <...multiple lines snipped...> |
| |
| wordmatch report for perl5db.pl: |
| lines in file: 9428 |
| words in file: 50243 |
| words with special (non-word) characters: 20480 |
| words with only special (non-word) characters: 7790 |
| words with only consonants: 4801 |
| words with only capital letters: 1316 |
| words with only vowels: 1701 |
| |
| C<Devel::FastProf> writes statistics to the file F<fastprof.out> in the current |
| directory. The output file, which can be specified, can be interpreted by using |
| the C<fprofpp> command-line program. |
| |
| $> fprofpp | head -n20 |
| |
| # fprofpp output format is: |
| # filename:line time count: source |
| wordmatch:75 3.93338 251215: if ( $word =~ /($regex)/ ) { |
| wordmatch:79 1.77774 251215: debug("word: $i_wd ".($has ? 'matches' : 'does not match')." chars: /$regex/"); |
| wordmatch:81 1.47604 251215: return $has; |
| wordmatch:126 1.43441 260643: my $message = shift; |
| wordmatch:128 1.42156 260643: if ( $debug ) { |
| wordmatch:70 1.36824 251215: my $i_wd = shift; |
| wordmatch:71 1.36739 251215: my $word = shift; |
| wordmatch:72 1.35939 251215: my $regex = shift; |
| |
| Straightaway we can see that the number of times each line has been called is |
| identical to the C<Devel::SmallProf> output, and the sequence is only very |
| slightly different based on the ordering of the amount of time each line took |
| to execute, C<if ( $debug ) { > and C<my $message = shift;>, for example. The |
| differences in the actual times recorded might be in the algorithm used |
| internally, or it could be due to system resource limitations or contention. |
| |
| See also the L<DBIx::Profile> which will profile database queries running |
| under the C<DBIx::*> namespace. |
| |
| =head2 Devel::NYTProf |
| |
| C<Devel::NYTProf> is the B<next generation> of Perl code profiler, fixing many |
| shortcomings in other tools and implementing many cool features. First of all it |
| can be used as either a I<line> profiler, a I<block> or a I<subroutine> |
| profiler, all at once. It can also use sub-microsecond (100ns) resolution on |
| systems which provide C<clock_gettime()>. It can be started and stopped even |
| by the program being profiled. It's a one-line entry to profile C<mod_perl> |
| applications. It's written in C<c> and is probably the fastest profiler |
| available for Perl. The list of coolness just goes on. Enough of that, let's |
| see how to it works - just use the familiar C<-d> switch to plug it in and run |
| the code. |
| |
| $> perl -d:NYTProf wordmatch -f perl5db.pl |
| |
| wordmatch report for perl5db.pl: |
| lines in file: 9427 |
| words in file: 50243 |
| words with special (non-word) characters: 20480 |
| words with only special (non-word) characters: 7790 |
| words with only consonants: 4801 |
| words with only capital letters: 1316 |
| words with only vowels: 1701 |
| |
| C<NYTProf> will generate a report database into the file F<nytprof.out> by |
| default. Human readable reports can be generated from here by using the |
| supplied C<nytprofhtml> (HTML output) and C<nytprofcsv> (CSV output) programs. |
| We've used the Unix system C<html2text> utility to convert the |
| F<nytprof/index.html> file for convenience here. |
| |
| $> html2text nytprof/index.html |
| |
| Performance Profile Index |
| For wordmatch |
| Run on Fri Sep 26 13:46:39 2008 |
| Reported on Fri Sep 26 13:47:23 2008 |
| |
| Top 15 Subroutines -- ordered by exclusive time |
| |Calls |P |F |Inclusive|Exclusive|Subroutine | |
| | | | |Time |Time | | |
| |251215|5 |1 |13.09263 |10.47692 |main:: |matches | |
| |260642|2 |1 |2.71199 |2.71199 |main:: |debug | |
| |1 |1 |1 |0.21404 |0.21404 |main:: |report | |
| |2 |2 |2 |0.00511 |0.00511 |XSLoader:: |load (xsub) | |
| |14 |14|7 |0.00304 |0.00298 |Exporter:: |import | |
| |3 |1 |1 |0.00265 |0.00254 |Exporter:: |as_heavy | |
| |10 |10|4 |0.00140 |0.00140 |vars:: |import | |
| |13 |13|1 |0.00129 |0.00109 |constant:: |import | |
| |1 |1 |1 |0.00360 |0.00096 |FileHandle:: |import | |
| |3 |3 |3 |0.00086 |0.00074 |warnings::register::|import | |
| |9 |3 |1 |0.00036 |0.00036 |strict:: |bits | |
| |13 |13|13|0.00032 |0.00029 |strict:: |import | |
| |2 |2 |2 |0.00020 |0.00020 |warnings:: |import | |
| |2 |1 |1 |0.00020 |0.00020 |Getopt::Long:: |ParseOptionSpec| |
| |7 |7 |6 |0.00043 |0.00020 |strict:: |unimport | |
| |
| For more information see the full list of 189 subroutines. |
| |
| The first part of the report already shows the critical information regarding |
| which subroutines are using the most time. The next gives some statistics |
| about the source files profiled. |
| |
| Source Code Files -- ordered by exclusive time then name |
| |Stmts |Exclusive|Avg. |Reports |Source File | |
| | |Time | | | | |
| |2699761|15.66654 |6e-06 |line . block . sub|wordmatch | |
| |35 |0.02187 |0.00062|line . block . sub|IO/Handle.pm | |
| |274 |0.01525 |0.00006|line . block . sub|Getopt/Long.pm | |
| |20 |0.00585 |0.00029|line . block . sub|Fcntl.pm | |
| |128 |0.00340 |0.00003|line . block . sub|Exporter/Heavy.pm | |
| |42 |0.00332 |0.00008|line . block . sub|IO/File.pm | |
| |261 |0.00308 |0.00001|line . block . sub|Exporter.pm | |
| |323 |0.00248 |8e-06 |line . block . sub|constant.pm | |
| |12 |0.00246 |0.00021|line . block . sub|File/Spec/Unix.pm | |
| |191 |0.00240 |0.00001|line . block . sub|vars.pm | |
| |77 |0.00201 |0.00003|line . block . sub|FileHandle.pm | |
| |12 |0.00198 |0.00016|line . block . sub|Carp.pm | |
| |14 |0.00175 |0.00013|line . block . sub|Symbol.pm | |
| |15 |0.00130 |0.00009|line . block . sub|IO.pm | |
| |22 |0.00120 |0.00005|line . block . sub|IO/Seekable.pm | |
| |198 |0.00085 |4e-06 |line . block . sub|warnings/register.pm| |
| |114 |0.00080 |7e-06 |line . block . sub|strict.pm | |
| |47 |0.00068 |0.00001|line . block . sub|warnings.pm | |
| |27 |0.00054 |0.00002|line . block . sub|overload.pm | |
| |9 |0.00047 |0.00005|line . block . sub|SelectSaver.pm | |
| |13 |0.00045 |0.00003|line . block . sub|File/Spec.pm | |
| |2701595|15.73869 | |Total | |
| |128647 |0.74946 | |Average | |
| | |0.00201 |0.00003|Median | |
| | |0.00121 |0.00003|Deviation | |
| |
| Report produced by the NYTProf 2.03 Perl profiler, developed by Tim Bunce and |
| Adam Kaplan. |
| |
| At this point, if you're using the I<html> report, you can click through the |
| various links to bore down into each subroutine and each line of code. Because |
| we're using the text reporting here, and there's a whole directory full of |
| reports built for each source file, we'll just display a part of the |
| corresponding F<wordmatch-line.html> file, sufficient to give an idea of the |
| sort of output you can expect from this cool tool. |
| |
| $> html2text nytprof/wordmatch-line.html |
| |
| Performance Profile -- -block view-.-line view-.-sub view- |
| For wordmatch |
| Run on Fri Sep 26 13:46:39 2008 |
| Reported on Fri Sep 26 13:47:22 2008 |
| |
| File wordmatch |
| |
| Subroutines -- ordered by exclusive time |
| |Calls |P|F|Inclusive|Exclusive|Subroutine | |
| | | | |Time |Time | | |
| |251215|5|1|13.09263 |10.47692 |main::|matches| |
| |260642|2|1|2.71199 |2.71199 |main::|debug | |
| |1 |1|1|0.21404 |0.21404 |main::|report | |
| |0 |0|0|0 |0 |main::|BEGIN | |
| |
| |
| |Line|Stmts.|Exclusive|Avg. |Code | |
| | | |Time | | | |
| |1 | | | |#!/usr/bin/perl | |
| |2 | | | | | |
| | | | | |use strict; | |
| |3 |3 |0.00086 |0.00029|# spent 0.00003s making 1 calls to strict:: | |
| | | | | |import | |
| | | | | |use warnings; | |
| |4 |3 |0.01563 |0.00521|# spent 0.00012s making 1 calls to warnings:: | |
| | | | | |import | |
| |5 | | | | | |
| |6 | | | |=head1 NAME | |
| |7 | | | | | |
| |8 | | | |filewords - word analysis of input file | |
| <...snip...> |
| |62 |1 |0.00445 |0.00445|print report( %count ); | |
| | | | | |# spent 0.21404s making 1 calls to main::report| |
| |63 | | | | | |
| | | | | |# spent 23.56955s (10.47692+2.61571) within | |
| | | | | |main::matches which was called 251215 times, | |
| | | | | |avg 0.00005s/call: # 50243 times | |
| | | | | |(2.12134+0.51939s) at line 57 of wordmatch, avg| |
| | | | | |0.00005s/call # 50243 times (2.17735+0.54550s) | |
| |64 | | | |at line 56 of wordmatch, avg 0.00005s/call # | |
| | | | | |50243 times (2.10992+0.51797s) at line 58 of | |
| | | | | |wordmatch, avg 0.00005s/call # 50243 times | |
| | | | | |(2.12696+0.51598s) at line 55 of wordmatch, avg| |
| | | | | |0.00005s/call # 50243 times (1.94134+0.51687s) | |
| | | | | |at line 54 of wordmatch, avg 0.00005s/call | |
| | | | | |sub matches { | |
| <...snip...> |
| |102 | | | | | |
| | | | | |# spent 2.71199s within main::debug which was | |
| | | | | |called 260642 times, avg 0.00001s/call: # | |
| | | | | |251215 times (2.61571+0s) by main::matches at | |
| |103 | | | |line 74 of wordmatch, avg 0.00001s/call # 9427 | |
| | | | | |times (0.09628+0s) at line 50 of wordmatch, avg| |
| | | | | |0.00001s/call | |
| | | | | |sub debug { | |
| |104 |260642|0.58496 |2e-06 |my $message = shift; | |
| |105 | | | | | |
| |106 |260642|1.09917 |4e-06 |if ( $debug ) { | |
| |107 | | | |print STDERR "DBG: $message\n"; | |
| |108 | | | |} | |
| |109 | | | |} | |
| |110 | | | | | |
| |111 |1 |0.01501 |0.01501|exit 0; | |
| |112 | | | | | |
| |
| Oodles of very useful information in there - this seems to be the way forward. |
| |
| See also C<Devel::NYTProf::Apache> which hooks C<Devel::NYTProf> into C<mod_perl>. |
| |
| =head1 SORTING |
| |
| Perl modules are not the only tools a performance analyst has at their |
| disposal, system tools like C<time> should not be overlooked as the next |
| example shows, where we take a quick look at sorting. Many books, theses and |
| articles, have been written about efficient sorting algorithms, and this is not |
| the place to repeat such work, there's several good sorting modules which |
| deserve taking a look at too: C<Sort::Maker>, C<Sort::Key> spring to mind. |
| However, it's still possible to make some observations on certain Perl specific |
| interpretations on issues relating to sorting data sets and give an example or |
| two with regard to how sorting large data volumes can effect performance. |
| Firstly, an often overlooked point when sorting large amounts of data, one can |
| attempt to reduce the data set to be dealt with and in many cases C<grep()> can |
| be quite useful as a simple filter: |
| |
| @data = sort grep { /$filter/ } @incoming |
| |
| A command such as this can vastly reduce the volume of material to actually |
| sort through in the first place, and should not be too lightly disregarded |
| purely on the basis of its simplicity. The C<KISS> principle is too often |
| overlooked - the next example uses the simple system C<time> utility to |
| demonstrate. Let's take a look at an actual example of sorting the contents of |
| a large file, an apache logfile would do. This one has over a quarter of a |
| million lines, is 50M in size, and a snippet of it looks like this: |
| |
| # logfile |
| |
| 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)" |
| 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)" |
| 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" |
| 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" |
| 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" |
| 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)" |
| 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)" |
| 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)" |
| 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)" |
| 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)" |
| 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" |
| 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" |
| 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" |
| crawl1.cosmixcorp.com - - [08/Feb/2007:13:27:57 +0000] "GET /robots.txt HTTP/1.0" 200 179 "-" "voyager/1.0" |
| crawl1.cosmixcorp.com - - [08/Feb/2007:13:28:25 +0000] "GET /links.html HTTP/1.0" 200 3413 "-" "voyager/1.0" |
| 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)" |
| 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)" |
| 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)" |
| 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)" |
| pop.compuscan.co.za - - [08/Feb/2007:14:10:43 +0000] "GET / HTTP/1.1" 200 3309 "-" "www.clamav.net" |
| 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)" |
| 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)" |
| 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" |
| 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" |
| |
| The specific task here is to sort the 286,525 lines of this file by Response |
| Code, Query, Browser, Referring Url, and lastly Date. One solution might be to |
| use the following code, which iterates over the files given on the |
| command-line. |
| |
| # sort-apache-log |
| |
| #!/usr/bin/perl -n |
| |
| use strict; |
| use warnings; |
| |
| my @data; |
| |
| LINE: |
| while ( <> ) { |
| my $line = $_; |
| if ( |
| $line =~ m/^( |
| ([\w\.\-]+) # client |
| \s*-\s*-\s*\[ |
| ([^]]+) # date |
| \]\s*"\w+\s* |
| (\S+) # query |
| [^"]+"\s* |
| (\d+) # status |
| \s+\S+\s+"[^"]*"\s+" |
| ([^"]*) # browser |
| " |
| .* |
| )$/x |
| ) { |
| my @chunks = split(/ +/, $line); |
| my $ip = $1; |
| my $date = $2; |
| my $query = $3; |
| my $status = $4; |
| my $browser = $5; |
| |
| push(@data, [$ip, $date, $query, $status, $browser, $line]); |
| } |
| } |
| |
| my @sorted = sort { |
| $a->[3] cmp $b->[3] |
| || |
| $a->[2] cmp $b->[2] |
| || |
| $a->[0] cmp $b->[0] |
| || |
| $a->[1] cmp $b->[1] |
| || |
| $a->[4] cmp $b->[4] |
| } @data; |
| |
| foreach my $data ( @sorted ) { |
| print $data->[5]; |
| } |
| |
| exit 0; |
| |
| When running this program, redirect C<STDOUT> so it is possible to check the |
| output is correct from following test runs and use the system C<time> utility |
| to check the overall runtime. |
| |
| $> time ./sort-apache-log logfile > out-sort |
| |
| real 0m17.371s |
| user 0m15.757s |
| sys 0m0.592s |
| |
| The program took just over 17 wallclock seconds to run. Note the different |
| values C<time> outputs, it's important to always use the same one, and to not |
| confuse what each one means. |
| |
| =over 4 |
| |
| =item Elapsed Real Time |
| |
| The overall, or wallclock, time between when C<time> was called, and when it |
| terminates. The elapsed time includes both user and system times, and time |
| spent waiting for other users and processes on the system. Inevitably, this is |
| the most approximate of the measurements given. |
| |
| =item User CPU Time |
| |
| The user time is the amount of time the entire process spent on behalf of the |
| user on this system executing this program. |
| |
| =item System CPU Time |
| |
| The system time is the amount of time the kernel itself spent executing |
| routines, or system calls, on behalf of this process user. |
| |
| =back |
| |
| Running this same process as a C<Schwarzian Transform> it is possible to |
| eliminate the input and output arrays for storing all the data, and work on the |
| input directly as it arrives too. Otherwise, the code looks fairly similar: |
| |
| # sort-apache-log-schwarzian |
| |
| #!/usr/bin/perl -n |
| |
| use strict; |
| use warnings; |
| |
| print |
| |
| map $_->[0] => |
| |
| sort { |
| $a->[4] cmp $b->[4] |
| || |
| $a->[3] cmp $b->[3] |
| || |
| $a->[1] cmp $b->[1] |
| || |
| $a->[2] cmp $b->[2] |
| || |
| $a->[5] cmp $b->[5] |
| } |
| map [ $_, m/^( |
| ([\w\.\-]+) # client |
| \s*-\s*-\s*\[ |
| ([^]]+) # date |
| \]\s*"\w+\s* |
| (\S+) # query |
| [^"]+"\s* |
| (\d+) # status |
| \s+\S+\s+"[^"]*"\s+" |
| ([^"]*) # browser |
| " |
| .* |
| )$/xo ] |
| |
| => <>; |
| |
| exit 0; |
| |
| Run the new code against the same logfile, as above, to check the new time. |
| |
| $> time ./sort-apache-log-schwarzian logfile > out-schwarz |
| |
| real 0m9.664s |
| user 0m8.873s |
| sys 0m0.704s |
| |
| The time has been cut in half, which is a respectable speed improvement by any |
| standard. Naturally, it is important to check the output is consistent with |
| the first program run, this is where the Unix system C<cksum> utility comes in. |
| |
| $> cksum out-sort out-schwarz |
| 3044173777 52029194 out-sort |
| 3044173777 52029194 out-schwarz |
| |
| BTW. Beware too of pressure from managers who see you speed a program up by 50% |
| of the runtime once, only to get a request one month later to do the same again |
| (true story) - you'll just have to point out your only human, even if you are a |
| Perl programmer, and you'll see what you can do... |
| |
| =head1 LOGGING |
| |
| An essential part of any good development process is appropriate error handling |
| with appropriately informative messages, however there exists a school of |
| thought which suggests that log files should be I<chatty>, as if the chain of |
| unbroken output somehow ensures the survival of the program. If speed is in |
| any way an issue, this approach is wrong. |
| |
| A common sight is code which looks something like this: |
| |
| logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) |
| |
| The problem is that this code will always be parsed and executed, even when the |
| debug level set in the logging configuration file is zero. Once the debug() |
| subroutine has been entered, and the internal C<$debug> variable confirmed to |
| be zero, for example, the message which has been sent in will be discarded and |
| the program will continue. In the example given though, the \%INC hash will |
| already have been dumped, and the message string constructed, all of which work |
| could be bypassed by a debug variable at the statement level, like this: |
| |
| logger->debug( "A logging message via process-id: $$ INC: " . Dumper(\%INC) ) if $DEBUG; |
| |
| This effect can be demonstrated by setting up a test script with both forms, |
| including a C<debug()> subroutine to emulate typical C<logger()> functionality. |
| |
| # ifdebug |
| |
| #!/usr/bin/perl |
| |
| use strict; |
| use warnings; |
| |
| use Benchmark; |
| use Data::Dumper; |
| my $DEBUG = 0; |
| |
| sub debug { |
| my $msg = shift; |
| |
| if ( $DEBUG ) { |
| print "DEBUG: $msg\n"; |
| } |
| }; |
| |
| timethese(100000, { |
| 'debug' => sub { |
| debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) |
| }, |
| 'ifdebug' => sub { |
| debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if $DEBUG |
| }, |
| }); |
| |
| Let's see what C<Benchmark> makes of this: |
| |
| $> perl ifdebug |
| Benchmark: timing 100000 iterations of constant, sub... |
| ifdebug: 0 wallclock secs ( 0.01 usr + 0.00 sys = 0.01 CPU) @ 10000000.00/s (n=100000) |
| (warning: too few iterations for a reliable count) |
| debug: 14 wallclock secs (13.18 usr + 0.04 sys = 13.22 CPU) @ 7564.30/s (n=100000) |
| |
| In the one case the code, which does exactly the same thing as far as |
| outputting any debugging information is concerned, in other words nothing, |
| takes 14 seconds, and in the other case the code takes one hundredth of a |
| second. Looks fairly definitive. Use a C<$DEBUG> variable BEFORE you call the |
| subroutine, rather than relying on the smart functionality inside it. |
| |
| =head2 Logging if DEBUG (constant) |
| |
| It's possible to take the previous idea a little further, by using a compile |
| time C<DEBUG> constant. |
| |
| # ifdebug-constant |
| |
| #!/usr/bin/perl |
| |
| use strict; |
| use warnings; |
| |
| use Benchmark; |
| use Data::Dumper; |
| use constant |
| DEBUG => 0 |
| ; |
| |
| sub debug { |
| if ( DEBUG ) { |
| my $msg = shift; |
| print "DEBUG: $msg\n"; |
| } |
| }; |
| |
| timethese(100000, { |
| 'debug' => sub { |
| debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) |
| }, |
| 'constant' => sub { |
| debug( "A $0 logging message via process-id: $$" . Dumper(\%INC) ) if DEBUG |
| }, |
| }); |
| |
| Running this program produces the following output: |
| |
| $> perl ifdebug-constant |
| Benchmark: timing 100000 iterations of constant, sub... |
| constant: 0 wallclock secs (-0.00 usr + 0.00 sys = -0.00 CPU) @ -7205759403792793600000.00/s (n=100000) |
| (warning: too few iterations for a reliable count) |
| sub: 14 wallclock secs (13.09 usr + 0.00 sys = 13.09 CPU) @ 7639.42/s (n=100000) |
| |
| The C<DEBUG> constant wipes the floor with even the C<$debug> variable, |
| clocking in at minus zero seconds, and generates a "warning: too few iterations |
| for a reliable count" message into the bargain. To see what is really going |
| on, and why we had too few iterations when we thought we asked for 100000, we |
| can use the very useful C<B::Deparse> to inspect the new code: |
| |
| $> perl -MO=Deparse ifdebug-constant |
| |
| use Benchmark; |
| use Data::Dumper; |
| use constant ('DEBUG', 0); |
| sub debug { |
| use warnings; |
| use strict 'refs'; |
| 0; |
| } |
| use warnings; |
| use strict 'refs'; |
| timethese(100000, {'sub', sub { |
| debug "A $0 logging message via process-id: $$" . Dumper(\%INC); |
| } |
| , 'constant', sub { |
| 0; |
| } |
| }); |
| ifdebug-constant syntax OK |
| |
| The output shows the constant() subroutine we're testing being replaced with |
| the value of the C<DEBUG> constant: zero. The line to be tested has been |
| completely optimized away, and you can't get much more efficient than that. |
| |
| =head1 POSTSCRIPT |
| |
| This document has provided several way to go about identifying hot-spots, and |
| checking whether any modifications have improved the runtime of the code. |
| |
| As a final thought, remember that it's not (at the time of writing) possible to |
| produce a useful program which will run in zero or negative time and this basic |
| principle can be written as: I<useful programs are slow> by their very |
| definition. It is of course possible to write a nearly instantaneous program, |
| but it's not going to do very much, here's a very efficient one: |
| |
| $> perl -e 0 |
| |
| Optimizing that any further is a job for C<p5p>. |
| |
| =head1 SEE ALSO |
| |
| Further reading can be found using the modules and links below. |
| |
| =head2 PERLDOCS |
| |
| For example: C<perldoc -f sort>. |
| |
| L<perlfaq4>. |
| |
| L<perlfork>, L<perlfunc>, L<perlretut>, L<perlthrtut>. |
| |
| L<threads>. |
| |
| =head2 MAN PAGES |
| |
| C<time>. |
| |
| =head2 MODULES |
| |
| It's not possible to individually showcase all the performance related code for |
| Perl here, naturally, but here's a short list of modules from the CPAN which |
| deserve further attention. |
| |
| Apache::DProf |
| Apache::SmallProf |
| Benchmark |
| DBIx::Profile |
| Devel::AutoProfiler |
| Devel::DProf |
| Devel::DProfLB |
| Devel::FastProf |
| Devel::GraphVizProf |
| Devel::NYTProf |
| Devel::NYTProf::Apache |
| Devel::Profiler |
| Devel::Profile |
| Devel::Profit |
| Devel::SmallProf |
| Devel::WxProf |
| POE::Devel::Profiler |
| Sort::Key |
| Sort::Maker |
| |
| =head2 URLS |
| |
| Very useful online reference material: |
| |
| http://www.ccl4.org/~nick/P/Fast_Enough/ |
| |
| http://www-128.ibm.com/developerworks/library/l-optperl.html |
| |
| http://perlbuzz.com/2007/11/bind-output-variables-in-dbi-for-speed-and-safety.html |
| |
| http://en.wikipedia.org/wiki/Performance_analysis |
| |
| http://apache.perl.org/docs/1.0/guide/performance.html |
| |
| http://perlgolf.sourceforge.net/ |
| |
| http://www.sysarch.com/Perl/sort_paper.html |
| |
| =head1 AUTHOR |
| |
| Richard Foley <richard.foley@rfi.net> Copyright (c) 2008 |
| |
| =cut |