Running multiprocess applications from MATLAB

Question

I\'ve written a multitprocess application in VC++ and tried to execute it with command line arguments with the system command from MATLAB. It runs, but only on one core ---

Answer 1

you have to execute in MATLAB command-line:

setenv OMP_NUM_THREADS 4

if you want to use 4 threads.

Answer 2

Run the command outside of Matlab and see how many cores its using. There should be no difference running it from within Matlab because its just a call down to the operating system. IE. equivalent from running on command line.

EDIT

Ok odd, what do you get when you call feature('NumCores') ? What version of Matlab are you using?

Does it help to enable this?

Answer 3

The question is how Matlab is affecting your app's behavior, since it's a separate process. I suspect Matlab is modifying environment variables in a manner that affects OMP, maybe because it uses OMP internally, and the process you are spawning from Matlab is inheriting this modified environment.

Do a "set > plain.txt" from the command window where you're launching you app plain, and "system('set > from_matlab.txt')" from within Matlab, and diff the outputs. This will show you the differences in environment variables that Matlab is introducing. When I do this, this appears in the environment inherited from Matlab, but not in the plain command window's environment.

OMP_NUM_THREADS=1 

That looks like an OpenMP setting related to the function calls in your question. I'll bet your spawned app is seeing that and respecting it.

I don't know why Matlab is setting it. But as a workaround, when you launch the app from Matlab, instead of calling it directly, call a wrapper .bat file that clears the OMP_NUM_THREADS environment variable, or sets it to a higher number.