parallel computing speed question when the body of the parfor loop takes about 2 seconds

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I have a question about parallel:
parfor ii=1:N dosomething%% end
in the parfor loop, the time of dosomething is 2 seconds in each loop, and N is about 100. In that condition, is it efficient to use parfor here???
I feel confused about that as I test the time and find the more core I use, the more time I spend!!!!!! Could someone help to answer this question?!!!!

Accepted Answer

Ken Atwell
Ken Atwell on 13 May 2015
It really depends on what "dosomething" is doing. If it is pure computation, it should be a big win. If there is file or network I/O going on, your mileage will vary depending on the application and hardware.
Also keep an eye on memory usage on your computer -- if you're using all of your physical RAM, your computer may be spending much of its time swapping, more than offsetting and gain you'd get from multiprocessor operation.
Walter Roberson
Walter Roberson on 13 May 2015
Pure computation on a local system is fine. If you were using the Distributed Computing Engine to send to several hosts then there would be network traffic. If you were load()'ing files then there would be file I/O.

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More Answers (1)

Guanfeng Gao
Guanfeng Gao on 13 May 2015
Thanks!!! I do not have several hosts or file I/O in my parallel loop!!! However, I am still not satisfied with the speed. E.G. there are 100 iterations (parfor ii=1:100), and for each iteration it takes 2 seconds; when I use 32 cores it takes 129 seconds, which means that the parallel only speedup to "2*100/129~2" than the regular one when I use 32 cores!!!! That really confused me!!!!!!
  1 Comment
Walter Roberson
Walter Roberson on 13 May 2015
Are the matrices large? If so then you might be encountering a lot of overhead in transmitting the values back. Try experimenting with drange() instead of parfor()

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