Re: [RFC PATCH 0/4]: affinity-on-next-touch

[Samuel Thibault]

Andi writes:
> > With this patch, the kernel reduces the overhead of page distribution via
> > "affinity-on-next-touch" from 2518ms to 366ms compared to the user-level
> 
> The interesting part is less how much faster it is compared to an user
> space implementation, but how much this migrate on touch approach
> helps in general compared to already existing policies. Some hard
> numbers on that would appreciated.

That is described in the papers that Stefan mentioned.  The problem is
that quite often it is very hard or even impossible to know which data
should get which migration, because you have a sparse matrix which gets
accessed by threads according to intermediate results, for instance.

> Note that for the OpenMP case old kernels sometimes had trouble because
> the threads tended to be not scheduled to the final target CPU
> on the first time slice so the memory was often first-touched
> on the wrong node.

It's not only that kind of issue, but in a lot of applications memory
is initialized sequentially by the main thread (not only zeroing, but
also reading files etc).  Setting "next-touch" right after sequential
initialization would just work fine.

Samuel
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