Re: [Starpu-devel] Performance decline from StarPU 1.2.3 to 1.2.4

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

Mirko Myllykoski, le jeu. 27 sept. 2018 20:21:14 +0200, a ecrit:
> On 2018-09-27 18:32, Samuel Thibault wrote:
> > I'm afraid the issue you are getting is that main memory registration
> > for CUDA DMA transfers is quite costly, and actually not worth the
> > cost for the asynchronism benefit. Setting --disable-cuda-memcpy-peer
> > by hand allows to disable asynchronism optimization and thus the data
> > registration. Ideally registration would be cheap, or cached, to avoid
> > the issue. If your temporary data pieces are small (less than 4MB), you
> > could try the attached patch which will make StarPU use a suballocator
> > not only for the CUDA memory but also for the main memory, which we
> > didn't feel would be useful, but perhaps would be.
> 
> Compiling StarPU 1.2.4 with this patch (without --disable-cuda-memcpy-peer)
> fixes the problem.

Ok, that's the best option: you keep memcpy-peer which avoids spurious
synchronization, but avoid paying the corresponding cost all the
time. I have commited a more elaborated version of the the patch to all
branches.

Thanks!
Samuel