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

Avi Kivity
2012-03-20T10:53:18Z

On 03/20/2012 12:48 PM, Peter Zijlstra wrote:
> 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.

You use the dma engine for eager copying, not on demand.

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