cputime seems to count serial computation time, while tic-toc does not

The codes below will show the difference. But why? Does that mean, cputime calculates the time for serial computation and tic--toc for parallel or SPMD(Single Process, Multiple Data)? When using SPMD, is the computation in parallel? And if you can use "top" in your server to monitor the cpu usage, you will see usage is more than 100%, that means you are using more than 1 cpu to calculate. Mine cpu usage was 1147%.

clc;
clear;

tic

t=cputime;

N=4000;

all=magic(N);

b=all*all*all*all*all*all*all;

cpu_time=cputime-t

toc_time=toc

============================

My running result is:

cpu_time =

      167.3800

toc_time =

    14.8073

Because the cpu usage is 1147% for me, so toc_time*1147% is almost the cpu_time result. So I think the cputime calculates the serial computation time.

Answer 1

[Jan]

See also:

• http://www.mathworks.com/help/techdoc/ref/cputime.html
• http://blogs.mathworks.com/loren/2006/07/12/what-are-you-really-measuring/
• http://www.mathworks.com/matlabcentral/newsreader/view_thread/296850
• http://newsgroups.derkeiler.com/Archive/Comp/comp.soft-sys.matlab/2005-08/msg00874.html
• http://www.mathkb.com/Uwe/Forum.aspx/matlab/40003/Difference-cputime-tic-toc

EDITED: For measuring timings I suggest Steve Eddins' FEX: timeit

[xin]

I am building some my own functions to beat the matlab built-in functions. So I think the best way to compare is using cputime.

[Matt Fig]

That conclusion apparently doesn't follow from your single premise...

Could you flesh out your reasoning a bit more?