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Re: Re: Re: Performance of Perl references

by Felonious (Chaplain)
on Jul 22, 2002 at 16:32 UTC ( #184116=note: print w/replies, xml ) Need Help??

in reply to Re: Re: Performance of Perl references
in thread Performance of Perl references

Any time you derefernce to get to a value pointed to by a reference, it's called a layer of indirection. Every layer of indirection adds some overhead, so if you are acccessing data deep inside a complex data structure, it adds up. Normally this is negligable, but if you are doing so inside loops you can gain more speed by removing as many layers of indirecion as possible.
Take the following code sample. The structure '$data' is two levels deep, and the subs 'foo' and 'bar' both loop thru and access all the data at the deepest level. 'foo' uses a naive aproach of multiple layers of indirection at the deepest level 'my $val = $data->{$key1}->{$key2}', while 'bar' reduces the indirection by assigning the inner data struct to a variable 'my $data2 = $data->{$key1}' and looping over it, accessing that data with 'my $val = $data2->{$key2}'.


Benchmark: timing 100000 iterations of multiple_indirection, reduced_indirection...
multiple_indirection: 29 wallclock secs (28.08 usr +  0.00 sys = 28.08 CPU) @ 3561.25/s (n=100000)
reduced_indirection: 25 wallclock secs (24.19 usr +  0.00 sys = 24.19 CPU) @ 4133.94/s (n=100000)
                       Rate multiple_indirection  reduced_indirection
multiple_indirection 3561/s                   --                 -14%
reduced_indirection  4134/s                  16%                   --

#!/usr/bin/perl my %innerdata; @innerdata{a..z} = a..z; my $data = { foo => { %innerdata }, bar => { %innerdata } }; sub foo { my $data = $_[0]; for $key1 (keys %$data) { for $key2 (keys %{$data->{$key1}}) { my $val = $data->{$key1}->{$key2}; # process $val; # print $val; } } } sub bar { my $data = $_[0]; for $key1 (keys %$data) { my $data2 = $data->{$key1}; for $key2 (keys %$data2) { my $val = $data2->{$key2}; # process $val; # print $val; } } } use Benchmark qw(cmpthese); cmpthese( 100000, { multiple_indirection => sub { foo($data) }, reduced_indirection => sub { bar($data) }, } )
-- O thievish Night, Why should'st thou, but for some felonious end, In thy dark lantern thus close up the stars? --Milton

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[ambrus]: (Alternately a deep source dive and then rewrite that event loop to make it better, and then as a bonus you get an idle method.)
[ambrus]: The 0.2 seconds wakeup is likely a workaround for some bug, but I can't guess what bug that is.
[ambrus]: It's been there since Prima 1.00 iirc
[Corion]: Hmmm... Weird. Maybe it needs that for doing its timers or something. Still weird.
[Corion]: But I guess I'll find out soonish, as my application seems to use Prima for more than just displaying the OpenGL output soonish ;))
[ambrus]: No, I think the timers are handled fine in the event loop, as far as I can see.
[Corion]: So far, I liked Prima quite well as it simply seems to do what I want even if it has an upside-down coordinate system. But it installs from CPAN out of the box and is cross-platform

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