linux-kernel - Re: [RFC][PATCH 00/26] sched/numa

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Message-ID: <1332240525.18960.403.camel@twins>
Date:	Tue, 20 Mar 2012 11:48:45 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Avi Kivity <avi@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Mike Galbraith <efault@....de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Dan Smith <danms@...ibm.com>,
	Bharata B Rao <bharata.rao@...il.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [RFC][PATCH 00/26] sched/numa

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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