Why does my Matlab 2014a only start 12 workers when I ask for more?

First of all I'm running Matlab 2014a. I have access to multiple machines running 32 hardware cores each with 2 threads. I know that matlab might not be able to utilize the threads for 64 workers, but when I try to start a parallel pool using the 'local' profile with NumWorkers = 32, only 12 workers are spawned (verified by inspecting the process list with "top"). I've read that the limit on the number of local workers in previous versions was 12, but this should be removed in matlab 2014a according to the release notes. I have verified that the system indeed has 32 cores using lscpu giving me 32 physical cores each with 2 threads (it says 64 cpus).

Is anybody able to clarify whether I should be able to start 32 workers or not? The 'local' profile was set to 32 workers by default.

Answer 1

[Edric Ellis]

I think the problem here is that you either need to explicitly state when calling parpool how many workers you want, like so:

parpool('local', 32)

or, you need to change your parallel preferences to increase the "preferred pool size".

[Malte Nissen]

It works. Great! My next headache is then how to modify the default property without any gui available since I'm doing this on a machine with command-line interface only? I have been able to track down the property to parallel.Settings.Pool.PreferredNumWorkers, but it's a class property and hence not just a static variable.

[Malte Nissen]

Okay so finally I found the answer to this question myself: In your home directory there exists a .matlab/R2014a/parallel.settings file. In this file you should insert a value line at the preferredNumWorkers within the "double" tag with the preferred number of workers:

 <value>32.0</value>

[Edric Ellis]

Sorry I didn't reply - your other option is to do the following which completely bypasses preferences etc.:

c = parcluster('local');
c.NumWorkers = 32;
parpool(c, c.NumWorkers);

[Debejyo Chakraborty]

Thats the way to go Edric. Thanks. I hate tinkering with GUIs.