Multi threaded FFTW 3.1.2 on a shared memory computer?

This looks like it could be a synchronisation problem. You can get this type of behaviour if all threads except one are locked out e.g. By a semaphore to a library call.

Thank you for your answer. I am not sur to understand everything. I am a student in Atmospheric Sciences and it is the first time I hear about a semaphore.

I've tried to read and learn about that but I still don't understand what I could be doing wrong. Here is the piece of code where I create the plan. I follow this page.

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