Licensing: Parallel Computing Toolbox labs vs independent MATLAB sessions

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Richard Brown 2012-5-2

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From a licensing perspective, what's the difference between running n independent MATLAB sessions and running n Parallel Computing Toolbox labs? I sometimes wish I could have more labs on my local machine to test algorithms, but of course run into the limit of 12. But I can have unlimited (well, hardware limited) independent sessions running.

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Geoff 2012-5-2

Here's a thought: Run the parallel toolbox with multiple instances of MatLab =)

I'm gonna try this on my forecasting server that has 16 cores. Two instances of MatLab with parallel toolbox should utilise that quite nicely!

If you write your parallel code effectively, then you should be maxing out all your cores (up to 12, anyway) most of the time. If not, then adding an extra instance of MatLab would do the trick. I wouldn't add too many though, or the OS scheduler is going to struggle.

Generally speaking, having more workers than CPU cores reduces performance (IF all workers are actually doing work).

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Answer 1

Ken Atwell 2012-5-2

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Independent MATLAB sessions are just that, independent. You could create more than 12 if you wanted to (and your system can handle it), but they would not be able to partition a single task with PARFOR, for example.

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Geoff 2012-5-2

@Walter: Yes, but we're talking about MatLab still, right? I commented earlier that you only reduces performance if you expect them all to be doing work (and I meant 100% load)... But threaded GUI with the parallel toolbox is not typically what you would do in MatLab is it? The whole point of multiple workers in MatLab, as I see it, is to do processing. The only time I would want more workers than the number of cores is if I know that some workers are going to be blocked on I/O. Until MatLab starts providing real threading utilities like semaphores, I don't see its parallel toolbox as particularly useful for anything other than a performance boost.

@Richard: In that case if you need to simulate more than 12 threads you might need to do it manually, but then you'll be testing the correctness of your threading simulation as much as testing your algorithms. You may as well prototype the whole thing in C to begin with. Do you use any linear algebra libraries to handle all the mundane stuff? In my opinion, proving that an algorithm works correctly with 12 threads is sufficient excuse to throw it at a machine with 1000 threads, or however well-hung your supercomputer is =) But there are some who would call me a cowboy.

Geoff 2012-5-3

No I don't have any experience with those. I've used some pretty raw BLAS stuff in the past but dunno if it was from an official library. I wanted to know what you used, then maybe check it out! ;-) I do have some code that uses Blitz++ which is a heavily templated C++ monster, but I haven't played round with it. Makes your C++ look almost like MatLab. Needs a real compiler though (ie gcc) - not sure if the latest Microsoft Visual C++ can handle the necessary optimisations.

Kozlov Alexandre 2013-8-23

@Ken: is there any limit for a number of independent MatLab sessions I can run on a single computer? I don't talk about hardware limitation, but a legal one. I have an NNU licence, and I've been told that one cannot run more than 2 MatLab sessions simultaneously. But I never seen any confirmation of this limitation...

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Answer 2

Geoff 2012-5-2

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Coming from a different programming background I would intuitively say that with parallelisation you can work concurrently with a single copy of a large dataset in memory. With multiple processes you must load that dataset into each process.

If you had a fairly sizable dataset and a moderate number of workers, the inability to share memory across MatLab processes could be a show-stopper.

However, I don't know for certain if this is really what happens in MatLab. If it doesn't share data across worker threads I'd be extremely surprised, and wonder why I paid $2000 for the toolbox.