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[Octave-bug-tracker] [bug #57805] Some array permutations are much slowe

From: Marco Caliari
Subject: [Octave-bug-tracker] [bug #57805] Some array permutations are much slower than others
Date: Thu, 13 Feb 2020 07:51:42 -0500 (EST)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:72.0) Gecko/20100101 Firefox/72.0


                 Summary: Some array permutations are much slower than others
                 Project: GNU Octave
            Submitted by: caliari
            Submitted on: Thu 13 Feb 2020 12:51:40 PM UTC
                Category: None
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: None
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
                 Release: 5.1.0
         Discussion Lock: Any
        Operating System: GNU/Linux



Dear all,

the following code

n = 500;
X = randn (100, 100, 100);
P = perms(1:3);
for i = 1:5
  for k = 1:1000
    permute (X, P(i,:));
  t(i) = toc
average = mean (t)
variance = var (t)

gives me an average elapsed time of 1.5 seconds with a variance of 0.3 seconds
in octave (with the first permutation much slower than the others), while an
average elapsed time of 1.0 with a variance of 0.03 in matlab (with
-singleCompThread, otherwise it is multithreaded and even faster). Is it a
desired feature to have permute much faster for some permutations?


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