Beefy Boxes and Bandwidth Generously Provided by pair Networks
Keep It Simple, Stupid

Comment on

( #3333=superdoc: print w/ replies, xml ) Need Help??
You should always be very suspicious if your benchmark shows results of 8823529.41 runs/second. Specially when it comes to non-trivial tasks like sprintf() - after all, than requires perl to parse a format.

Another thing that should ring loud bells is that you are doing sprintf() in void context. That's not a natural operation. Perhaps Perl optimizes that away for you - totally screwing up your benchmark. It's a simple test:

$ perl -MO=Deparse -wce 'sprintf "%x%x%x", ord ("a"), ord ("b"), o +rd ("c")' Useless use of a constant in void context at -e line 1. BEGIN { $^W = 1; } '???'; -e syntax OK $
Indeed, you just benchmarked how fast perl can do an empty loop. Not very useful. Your benchmark should include assigning the result to a variable. So, you might want to do:
#!/usr/bin/perl use strict; use warnings 'all'; use Benchmark; timethese -10 => { unpack => '$_ = unpack "H*" => "abc"', sprintf => '$_ = sprintf "%x%x%x", ord ("a"), ord ("b"), ord ( +"c")', } __END__ Benchmark: running sprintf, unpack for at least 10 CPU seconds... sprintf: 11 wallclock secs (10.25 usr + 0.00 sys = 10.25 CPU) @ 77 +5053.56/s (n=7944299) unpack: 11 wallclock secs (10.48 usr + 0.01 sys = 10.49 CPU) @ 33 +1145.09/s (n=3473712)
It looks like sprintf is still a winner. But is it? Let's check the deparser again:
$ perl -MO=Deparse -wce '$_ = sprintf "%x%x%x", ord "a", ord "b", +ord "c"' BEGIN { $^W = 1; } $_ = '616263'; -e syntax OK $
Oops. Perl is so smart, it figured out at compile time the result of the sprintf. We'd have to make the arguments of sprintf variable to make Perl actually do work at run time:
$ perl -MO=Deparse -wce '($a, $b, $c) = split // => "abc"; $_ = sprintf "%x%x%x", ord $a, ord $b, ord $c' BEGIN { $^W = 1; } ($a, $b, $c) = split(//, 'abc', 4); $_ = sprintf('%x%x%x', ord $a, ord $b, ord $c); -e syntax OK $
And only now we can run a fair benchmark:
#!/usr/bin/perl use strict; use warnings 'all'; use Benchmark; use vars qw /$a $b $c $abc/; $abc = "abc"; ($a, $b, $c) = split // => $abc; timethese -10 => { unpack => '$_ = unpack "H*" => $::abc', sprintf => '$_ = sprintf "%x%x%x", ord $::a, ord $::b, ord $:: +c', } __END__ Benchmark: running sprintf, unpack for at least 10 CPU seconds... sprintf: 11 wallclock secs (10.51 usr + 0.01 sys = 10.52 CPU) @ 20 +8379.75/s (n=2192155) unpack: 10 wallclock secs (10.10 usr + 0.00 sys = 10.10 CPU) @ 32 +3836.04/s (n=3270744)
And guess what? unpack is the winner!

The moral: no benchmark is better than a bad benchmark.


In reply to No Benchmark is better than a bad Benchmark by Abigail-II
in thread Optimizing existing Perl code (in practise) by JaWi

Use:  <p> text here (a paragraph) </p>
and:  <code> code here </code>
to format your post; it's "PerlMonks-approved HTML":

  • Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!
  • Titles consisting of a single word are discouraged, and in most cases are disallowed outright.
  • Read Where should I post X? if you're not absolutely sure you're posting in the right place.
  • Please read these before you post! —
  • Posts may use any of the Perl Monks Approved HTML tags:
    a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr
  • You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)
            For:     Use:
    & &amp;
    < &lt;
    > &gt;
    [ &#91;
    ] &#93;
  • Link using PerlMonks shortcuts! What shortcuts can I use for linking?
  • See Writeup Formatting Tips and other pages linked from there for more info.
  • Log In?

    What's my password?
    Create A New User
    and the web crawler heard nothing...

    How do I use this? | Other CB clients
    Other Users?
    Others chilling in the Monastery: (3)
    As of 2015-11-27 19:30 GMT
    Find Nodes?
      Voting Booth?

      What would be the most significant thing to happen if a rope (or wire) tied the Earth and the Moon together?

      Results (731 votes), past polls