cputime with kmeans return 0

Question

Phu Lai 2016-12-20

1
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https://ww2.mathworks.cn/matlabcentral/answers/317545-cputime-with-kmeans-return-0

回答： Walter Roberson 2016-12-20

I am measuring the cputime taken by kmeans algorithm for each iteration using cputime feature. However, some iteration return cputime = 0. Here's my implementation:

   load fisheriris;
   [~,C] = kmeans(meas, 3, 'options',statset('MaxIter', 1),'Display', 'off');
   results=[];
   for i = 1:15
          t=cputime;
          [~,C] = kmeans(meas, 3, 'options',statset('MaxIter', 1),'Start',C, 'Display', 'off');
          elapsedCPUTime=cputime-t;
          results=[results;elapsedCPUTime];
   end

This is the results I got for 15 iterations: 0, 0, 0.046875, 0, 0, 0, 0, 0, 0.03125, 0, 0, 0, 0, 0 ,0.03125. My first thought is that the computational time was too quick, hence 0 second. Is it true? If so, how can we achieve more precise cputime?

Thanks a lot.

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Answer 1

Walter Roberson 2016-12-20

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https://ww2.mathworks.cn/matlabcentral/answers/317545-cputime-with-kmeans-return-0#answer_247893

cpu time is difficult to measure, especially in a multi-processor machine that is running multiple programs (your system very likely has daemons or system processes running while you are running MATLAB.)

You should consider using timeit() or tic() and toc()