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SubjectRe: [RFC][PATCH 00/26] sched/numa
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On Tue, 2012-03-20 at 12:18 +0200, Avi Kivity wrote:
> On 03/19/2012 10:03 PM, Peter Zijlstra wrote:
> > On Mon, 2012-03-19 at 14:16 +0200, Avi Kivity wrote:
> > > > Afaik we do not use dma engines for memory migration.
> > >
> > > We don't, but I think we should.
> >
> > ISTR we (the community) had this discussion once. I also seem to
> > remember the general consensus being that DMA engines would mostly
> > likely not be worth the effort, although I can't really recall the
> > specifics.
> >
> > Esp. for 4k pages the setup of the offload will likely be more expensive
> > than actually doing the memcpy.
>
> If you're copying a page, yes. If you're copying a large vma, the
> per-page setup cost is likely to be very low.
>
> Especially if you're copying across nodes.

But wouldn't you then have to wait for the entire copy to complete
before accessing any of the memory? That sounds like a way worse latency
hit than the per-page lazy-migrate.


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