Why parfor loop so efficient?

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C.Hu
C.Hu 2016-5-30
回答： Walter Roberson 2016-5-30
Hi I tested two script in the following
%1. Parall_try
parfor i = 1:2000
for j = 1:10000
b(i,j) = i*j+i/j;
end
end
and
%2. Serial_try
for i = 1:2000
for j = 1:10000
a(i,j) = i*j+i/j;
end
end
I set two workers totally. The profile report shows 32s for running serial_try, and about 2s for parallel_try. I noticed the parallel running does not output any i/j index to workspace, which may explain the speed up of parallel running. Is this really true? It motivates me to find a more efficient way to run my script. In script, it output every variable to workspace, which can be very time-consuming. Can I calculate them without output them to workspace? It may sounds absurd because I have not figured out why parfor loop can be that efficient.
  2 个评论
jgg
jgg 2016-5-30
Do you pre-allocate those arrays? That could be the difference.
C.Hu
C.Hu 2016-5-30
No, I clear the workspace everytime before running the script.

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采纳的回答

Walter Roberson
Walter Roberson 2016-5-30
parfor effectively pre-allocates the arrays before use whereas your non-parfor solution does not. That makes a big difference in efficiency.

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