[Openmp-dev] OpenMP code and thread priority on OSX

Cownie, James H via Openmp-dev openmp-dev at lists.llvm.org
Thu Nov 26 04:54:06 PST 2015

The OpenMP runtime does not manipulate thread priorities
A first skim through https://developer.apple.com/library/mac/documentation/Darwin/Conceptual/KernelProgramming/scheduler/scheduler.html doesn't make it clear to me whether threads inherit their priority from their creator.

Since OpenMP threads are created using pthread_create, that page suggests you may need to use pthread_setschedparam inside each thread to bump its priority. (Or you may be able to hack the runtime to pass the priority as an attribute at thread creation time).

BEWARE, though, you will almost certainly also want to change the default sleep behaviour of your OpenMP threads. By default they will spin for 200ms before sleeping in the kernel, which is unlikely to be helpful to the overall user experience if they’re at high-priority! (See KMP_BLOCKTIME).

Overall OpenMP is not designed for concurrency, but for parallelism, and you may find this a hard thing to make work.

-- Jim

James Cownie <james.h.cownie at intel.com>
SSG/DPD/TCAR (Technical Computing, Analyzers and Runtimes)
Tel: +44 117 9071438

-----Original Message-----
From: Openmp-dev [mailto:openmp-dev-bounces at lists.llvm.org] On Behalf Of Stéphane Letz via Openmp-dev
Sent: Wednesday, November 25, 2015 4:53 PM
To: openmp-dev at lists.llvm.org
Subject: [Openmp-dev] OpenMP code and thread priority on OSX


We are testing OpenMP aware audio C++ code with clang 3.7 on OSX. Basically this code is later on started in a realtime audio thread, using CoreAudio API on OSX. It seems that master and worker threads do not "inherit" thread priority from the realtime audio thread as we could expect, so the application do not behave reliably (audi drop out…)

Is there any specific set configuration of the libomp library we need to do ? Like environment variable or similar to set up? 


Stéphane Letz

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