Re: [Starpu-devel] Device OpenCL monopolisant 1 cœur CPU + usage CPU

[Samuel Thibault <samuel.thibault@inria.fr>]

Bonjour, désolé pour la latence, je croyais que quelqu'un avait déjà
répondu

Paul Godard, on Wed 21 Dec 2016 16:41:05 +0100, wrote:
> Je fais fasse à un comportement étrange de StarPU auquel je n'ai trouvé
> aucune mention dans la documentation.
> Il semblerait que StarPU attribue 1 cœur CPU pour chaque device OpenCL
> présent sur ma machine. Et retire ainsi ces cœurs CPU des workers
> utilisables.

Ah, je pensais que c'était documenté. J'ai rajouté ceci dans la faq:

When running with CUDA or OpenCL devices, I am seeing less CPU cores

Yes, this is on purpose.

Since GPU devices are way faster than CPUs, StarPU needs to react quickly when
a task is finished, to feed the GPU with another task (StarPU actually submits
a couple of tasks in advance so as to pipeline this, but filling the pipeline
still has to be happening often enough), and thus it has to dedicate threads 
for
this, and this is a very CPU-consuming duty. StarPU thus dedicates one CPU 
core
for driving each GPU.

Such dedication is also useful when a codelet is hybrid, i.e. while kernels 
are
running on the GPU, the codelet can run some computation, which thus be run by
the CPU core instead of driving the GPU.

Samuel