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RE (tilly) 3: sort performance

by tilly (Archbishop)
on Sep 12, 2000 at 05:59 UTC ( #32048=note: print w/replies, xml ) Need Help??


in reply to RE: Re (tilly) 1: sort performance
in thread sort performance

You are right. I just benchmarked it. The overhead of the maps far outweighs the cost of the sort. In fact it is so slow I will have to ask p5p about it. Here is my test:
use strict; use Benchmark; use vars qw(%hash %test); for (1..1000) { $hash{"key$_"}{this} = "num$_"; } %test = ( schwartz => sub { my @sorted = map $_->[0], sort {$a->[1] cmp $b->[1]} map [$_, $hash{$_}{this}], keys %hash; }, straight => sub { my @sorted = sort { $hash{$a}{this} <=> $hash{$b}{this} } keys %hash; }, stupid => sub { my @sorted = sort { my $foo = [$a, $hash{$a}{this}]; $hash{$a}{this} <=> $hash{$b}{this} } keys %hash; }, ); timethese (-1, \%test);
Believe it or not, stupid is several times faster for me than schwartz. IMO there is simply no possible good reason for that!

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RE: RE (tilly) 3: sort performance
by tye (Sage) on Sep 12, 2000 at 07:00 UTC

    I suspect that the optimizer is just removing the whole $foo bit from "stupid" for you.

    I was wondering if $a->[1] would be much faster than $hash{$a}{this}. I didn't think it would be by a big enough factor.

    If you want raw speed here, create a side array for comparison and sort a list of indices:

    my @list= keys %hash; my @sort= map { $hash{$_}{this} } @list; my @sorted= @list[ sort { $sort[$a] cmp $sort[$b] } 0..$#sort ];
            - tye (but my friends call me "Tye")
      I suspected that, but it cannot be because the straight method without that line is much faster, and as you change the $foo line you see the speed of the stupid sort change.

      I suspect a more fundamental issue with how map and grep are implemented, and it should IMO be fixable.

RE (tilly) 4: sort performance
by tilly (Archbishop) on Sep 12, 2000 at 07:42 UTC
    When faced with the ridiculous, look for the obvious. If you change all three to use the same kind of sort, Schwartz runs faster than the other two.

    *sigh*

    (Stupid user tricks.)

      I get "schartz" 5%-30% faster than "straight" with "tye" (which you didn't time) being 50%-100% faster still (all using "cmp"). (:

              - tye (but my friends call me "Tye")

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