Why should I start a parallel pool manually?
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Felix Müller
on 10 Feb 2023
Edited: Felix Müller
on 11 Feb 2023
I want to run a parfor-loop. I can start a parallel pool "manually" with the command
parpool('local', 5);
but I can also directly start the loop with
parfor (i = 1:10, 5)
It is less code (and just as understandable) to use parfor directly. Are there advantages to starting (and then closing) the pool manually? If so, which?
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Walter Roberson
on 10 Feb 2023
Are you trying to measure how long the parallel portion of the code takes? Is it fair that the timing for the same code might be very different if the user just happens to run the code within half an hour of the previous parallel run, before the pool timed out?
When you create the pool manually (or use gcp to find a current pool if it exists) then you can be consistent about what you are measuring.
You also need the pool handle for some parallel constructs, such as parfeval to run a future on a cluster or to use background pool
Now suppose you want to retest with 6 pool members instead of 5. If you specified the size of the parfor call then you have to edit every such place.
3 Comments
Raymond Norris
on 10 Feb 2023
The way you're calling parfor, parpool doesn't honor the value 5. For example, I have a 4 core laptop. Notice, in both cases a pool of 4 workers start, regardless of the size of workers I want dedicated to the parfor-loop.
>> parfor (i = 1:10,2), rand; end
Starting parallel pool (parpool) using the 'Processes' profile ...
Connected to the parallel pool (number of workers: 4).
>> delete(gcp)
Parallel pool using the 'Processes' profile is shutting down.
>> parfor (i = 1:10,5), rand; end
Starting parallel pool (parpool) using the 'Processes' profile ...
Connected to the parallel pool (number of workers: 4).
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