Re: [RFC][PATCH 00/26] sched/numa

Avi Kivity
2012-03-19T15:46:14Z

On 03/19/2012 02:24 PM, Avi Kivity wrote:
> On 03/19/2012 02:20 PM, Peter Zijlstra wrote:
> > On Mon, 2012-03-19 at 13:42 +0200, Avi Kivity wrote:
> > > It's the standard space/time tradeoff. Once solution wants more
> > > storage, the other wants more faults.
> > >
> > > Note scanners can use A/D bits which are cheaper than faults.
> >
> > I'm not convinced.. the scanner will still consume time even if the
> > system is perfectly balanced -- it has to in order to determine this.
> >
> > So sure, A/D/other page table magic can make scanners faster than faults
> > however you only need faults when you're actually going to migrate a
> > task. Whereas you always need to scan, even in the stable state.
> >
> > So while the per-instance times might be in favour of scanning, I'm
> > thinking the accumulated time is in favour of faults.
>
> When you migrate a vnode, you don't need the faults at all. You know
> exactly which pages need to be migrated, you can just queue them
> immediately when you make that decision.
>
> The scanning therefore only needs to pick up the stragglers and can be
> set to a very low frequency.

Running the numbers, 4GB = 1Mpages, at 2us per page migration that's 2
seconds to migrate an entire process, perhaps 2x-3x that for kvm. So as
long numa balancing happens at a lower frequency than once every few
minutes, the gains should be higher than the loss. If those numbers are
not too wrong then migrate on fault should be a win.

--
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