Enabling OpenMP under WSL

[walkersw]

So I have Firedrake working under WSL, but it seems to only want OMP_NUM_THREADS = 1. I ran lscpu, and it clearly knows I have more cores/threads. Is there something else to do when installing Firedrake to enable more threads?

[rckirby]

If you want more cores, use mpiexec. That is easier to support than a hybrid parallel model and actually helps with data locality

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On Nov 3, 2022, at 8:16 AM, Shawn Walker ***@***.***> wrote:  So I have Firedrake working under WSL, but it seems to only want OMP_NUM_THREADS = 1. I ran lscpu, and it clearly knows I have more cores/threads. Is there something else to do when installing Firedrake to enable more threads? — Reply to this email directly, view it on GitHub, or unsubscribe. You are receiving this because you are subscribed to this thread.

[walkersw]

Ok, I did that, and it does run in parallel, but it just runs the same code in parallel. It also still says I have OMP_NUM_THREADS set greater than 1. I assume I need to modify the Firedrake script to have a single simulation run, where the solving is done in parallel.

[connorjward]

I don't understand what you mean here. The way to use Firedrake in parallel is to write a single script as though it would be run in serial and then run it with the command mpiexec -n <# processors> python mycode.py. This will automatically run your code in parallel and do things like distributing the mesh between processors. There's more information here.

The reason you are seeing the warning about OMP_NUM_THREADS is because Firedrake only supports distributed memory parallelism, not shared memory parallelism (e.g. via OpenMP). In the past we've found that having OMP_NUM_THREADS greater than one can actually slow your code down which is why we emit this warning. The easiest way to make it go away is include the line export OMP_NUM_THREADS=1 inside your ~/.bashrc file.

[walkersw]

Ok, I was mistaken because I saw a lot of repeat print statements; I thought this meant it was running the same code multiple times (I also found the demo on printing in parallel).

But I see that the number of SNES iterations was not duplicated, so it is working as it should.