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matlabparallel-processingparfor
AlexandrosB
AlexandrosB

Reputation: 133

Number of workers in Matlab's parfor

I am running a for loop using MATLAB's parfor function. My CPU's specs are

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I set preferred number of workers to 24. However, MATLAB sets this number to 6. Is number of workers bounded by the number of cores or by (number of cores)x(number of processors=6x12?

Upvotes: 2

Views: 1865

Answers (1)

Max
Max

Reputation: 4045

Matlab prefers to limit the number of workers to the number of cores (six in your case). Your CPU (intel i7-9750H) has hyperthreading, i.e. you can run multiple (here 2) threads per core. However, this is of no use if you want to run them under full-load, which means that there is simply no resources available to switch to a different task (what the additional threads effectively are).

See the documentation.

Restricting to one worker per physical core ensures that each worker has exclusive access to a floating point unit, which generally optimizes performance of computational code. If your code is not computationally intensive, for example, it is input/output (I/O) intensive, then consider using up to two workers per physical core. Running too many workers on too few resources may impact performance and stability of your machine.

Note that Matlab needs to stream data to every core in order to run the distributed code. This is some kind of initialization effort and the reason why you won't be able to cut the runtime in half if you double the number of cores/workers. And that is also the explanation why there is no use for Matlab to make use of hyperthreading. It would just mean to increase the initial streaming effort without any speed-up -- in fact, the core would probably force matlab to save intermediate results and switch to the other task from time to time... which is the same task as before;)

Upvotes: 4

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