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diff --git a/Master/tlpkg/tlperl/lib/pods/perlperf.pod b/Master/tlpkg/tlperl/lib/pods/perlperf.pod new file mode 100644 index 00000000000..a934271088b --- /dev/null +++ b/Master/tlpkg/tlperl/lib/pods/perlperf.pod @@ -0,0 +1,1179 @@ +=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::DProf> 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::Profiler> 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 sytem 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::Profiler + 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 |