You can set maxNumCompThreads(1); in toolbox/local/matlabrc.m, thus overwriting any user setting.
You can do this in 2 ways:

a) add the line before the section where user preferences are checked. In this way the preferences of the users are still repected, but the internal default behavior is overwritten.

b) add the line after the section. In this way you overwrite both the user settings and the internal setting, and you force your users to activate multi-threading explicitly in every session. That’s how we do it here.

The new default, i.e. automatic multi-threading, plus the problem with the Java garbage collector threads (1 per core) can turn Matlab into a DoS tool on our big SMPs, with 96-144 cores!

-Bernd

There is no such control over multithreading.

–Loren

Is there some way to change the multithreading default at a system-wide level from “automatic” to “manual” so that we (the system administrators) can keep the aggregate thread count to something manageable?

For a start, MATLAB wasn’t using multi-threaded BLAS in version 5.3. That has come much more recently.

You probably should contact technical support as your issue warrants more attention than I can give it here.

–Loren

Thanks,
Uvernes

You still haven’t shown code that we could look at. Please send a sample to technical support with information on the runs on different computers so they can dive into this with more information.

–Loren

The n-D grid created by the code represents the parameter space of an ODE system. At each point in the parameter grid, a cost function (ODE output vs. data) is evaluated and stored. There is a heuristic to decide if the resolution along any one parameter dimension is high enough. If it isn’t, the number of points along that dimension is doubled and the cost function is evaluated/stored at the new points. The evaluations don’t depend on each other and these evaluations are carried out simultaneously on all cores of my PCs (maxNumCompThreads returns 2 or 4 depending on the PC).

The multi-processor system has been executing the same code on only one processor. (It may be worth noting that my PCs are running Vista. The workstation has basic SPARC architecture and is running Solaris.)

If this is getting outside your area of expertise, I’ll pass it along to Technical Support. I’d still welcome feedback from you or your commenters. Thanks again.

- J. Hengenius

Whether or not MATLAB uses more than 1 thread for an operations depends very much on what operation is being done. You haven’t supplied enough information for any of us to give a concrete answer, including how you know the 2- and 4- core PCs are using the multithreading. Perhaps you can send the details to tech support for more context.

–Loren

I’m a little late to this party, but hopefully you or your readers can provide some feedback. I have some code that iteratively builds an n-D array. On my PCs (2-core and 4-core) R2008a’s multithreading utilizes all cores to run the code. I recently tried the same code on a 24-processor workstation. While maxNumCompThreads returns 24, only one CPU is active. Another commenter asked about this in one of your previous posts, but I didn’t see any responses.

Would anyone care to chime in with insights?

Thanks,
J. Hengenius

As far as I know, currently all the timers run in one shared thread. Since that is something under the hood, it could change in a future release.

–Loren