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Re: Performance optimization (allocation inside a for loop)

From: Rob Mahurin
Subject: Re: Performance optimization (allocation inside a for loop)
Date: Wed, 1 Apr 2009 20:49:52 -0400

On Apr 1, 2009, at 7:18 PM, r wrote:
BTW, low "for"
loop performance is commonly blamed on the lack of a JIT compiler, but
I wouldn't be surprised if at least some of the test cases were
actually suffering from issues like memory allocation. After all, JIT
compilers don't typically speed up evaluation by more than a constant

Right, but it's a big factor.  Compare your test2

function retval = test2(n)
 retval = zeros(1, n);
 for n = [1:n]
   retval(n) = n;

has a complexity of O(n):

octave:29> tic;test2(10000);toc
Elapsed time is 0.16 seconds.
octave:30> tic;test2(100000);toc
Elapsed time is 1.9 seconds.

to the equivalent

octave:29> tic; n = 1e5; retval = 1:n; toc
Elapsed time is 0.000756025 seconds.
octave:30> tic; n = 1e5; retval = (1:n)(1:n); toc
Elapsed time is 0.00757694 seconds.
octave:31> tic; n = 1e5; retval = [1:n]; toc
Elapsed time is 0.0125589 seconds.

(These apparently take different paths through Octave's lazy-copying logic.)


Rob Mahurin
Department of Physics and Astronomy
University of Tennessee                 865 207 2594
Knoxville, TN 37996                     address@hidden

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