[Perl.sig] Perl-tip: Profiling Perl

[Litss Coordinator]

Sent on behalf of Jacinta Richardson.


==== Object Oriented Perl course ====

             Sydney 21st - 22nd July

     Do you write CGI code or do any sort of web development? Do you
     interface with databases? Does your code deal with user information? Do
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     When working on large projects, or writing reusable components, or
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approach
     can be invaluable.

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     features, and sub-classing and extending them represents a huge saving
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==== Profiling ====

     Profiling is used to determine which parts of your code are taking up
     the most execution time. This allows us to best decide the which
     locations to focus our optimising efforts. For example, pretend some
     code uses two subroutines A and B. Through testing we determine that
     subroutine A takes 50 time-units to execute whereas subroutine B takes
     only 2 time-units.

     The naive approach would be to assume that our optimising efforts are
     best spent on speeding up subroutine A. However, if we profile our
     subroutines when used under normal conditions, we may discover that
     subroutine A is only being called once per program invocation. On the
     other hand, subroutine B may be called hundreds of times. With such
     results, a 25% speed increase to subroutine B is likely to
significantly
     outweigh a 50% speed increase to subroutine A.

     Good profiling tools will provide information both on which subroutines
     take the longest to execute and which subroutines are called most
often.
     Should you need to optimise your code, use these tools to identify
     potential bottlenecks and focus your efforts where they can do most
     good.


==   Profiling Perl   ==

     There are a few tools to profile Perl code and which one is *best*
     depends on your current focus. A commonly used one is ``DProf'' and its
     friend ``dprofpp'' (these both come standard with Perl). To use these
     type:

             % perl -d:DProf some_program.pl
             % dprofpp

     This will give you results similar to:

             Total Elapsed Time = 1.229688 Seconds
               User+System Time = 0.559792 Seconds
             Exclusive Times
             %Time ExclSec CumulS #Calls sec/call Csec/c  Name
              85.7   0.480  0.480    100   0.0048 0.0048  main::sub_b
              14.2   0.080  0.560      1   0.0799 0.5599  main::sub_a
              0.00   0.000 -0.000      1   0.0000      -  main::BEGIN
              0.00   0.000 -0.000      1   0.0000      -  strict::import
              0.00   0.000 -0.000      1   0.0000      -  strict::bits

     The first two lines tell us how many seconds the program took to run.
     The first line says that the program finished 1.2 seconds after we
     started it and the second line tells us that had the the code been the
     only other process on the machine, it would have taken only 0.56
     seconds. For the remaining 0.64 seconds the CPU was working on other
     things.

     The first column is the percentage of time of total program invocation
     that the subroutine took. The exclusive seconds (second column) mark
the
     time that the subroutine used itself without including data from
     subroutines it called. The cumulative seconds (third column) represent
     the entire time for the subroutine including further subroutine calls.

     By looking at these columns we can determine that our ``sub_b''
     subroutine was called 100 times, took an average of 0.0048 seconds each
     time for a total of 0.48 seconds. This resulted in it taking up 85.7%
of
     the program execution time.

     On the other hand, each call to ``sub_a'' took 0.08 seconds
     (exclusively), 16 times longer than ``sub_b''. However, as ``sub_a''
was
     only called once this resulted in ``sub_a'' only taking up 14.2% of
     program execution time.

     Removing ``sub_a'' would yield a 14% improvement. Increasing it's speed
     by 50% would yield a speed up of 7%. However, improving ``sub_b'' by
     only 25% would result in a massive 29% speed up for our program.

     These results tell us that although ``sub_a'' takes up much more time,
     per call, than ``sub_b'', ``sub_b'' still makes the best first
candidate
     for optimisation.


==   Further information   ==

     For further information about profiling Perl read the perl.com article:
     http://www.perl.com/pub/a/2004/06/25/profiling.html


==== Upcoming Courses in Canberra ====

     http://perltraining.com.au/bookings/Canberra.html

             Introduction to Perl:               1st - 2nd November
             Intermediate Perl:                  3rd - 4th November


==== Upcoming Courses in Melbourne ====

     http://perltraining.com.au/bookings/Melbourne.html

             Introduction to Perl:               11th - 12th August
             Intermediate Perl:                  18th - 19th August
             Perl Best Practices:                14th - 15th November
             Understanding Regular Expressions:  22nd November


==== Upcoming Courses in Sydney ====

     http://perltraining.com.au/bookings/Sydney.html

             Object Oriented Perl:               21st - 22nd July
             Introduction to Perl:               20th - 21st September
             Intermediate Perl:                  22nd - 23rd September


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