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

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Date:	Mon, 19 Mar 2012 13:09:52 +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 Mon, 2012-03-19 at 13:42 +0200, Avi Kivity wrote:
> > That's intentional, it keeps the work accounted to the tasks that need
> > it.
> 
> The accounting part is good, the extra latency is not.  If you have
> spare resources (processors or dma engines) you can employ for eager
> migration why not make use of them.

Afaik we do not use dma engines for memory migration. 

In any case, if you do cross-node migration frequently enough that the
overhead of copying pages is a significant part of your time then I'm
guessing there's something wrong.

If not, the latency should be armortised enough to not matter.

> > > - doesn't work with dma engines
> >
> > How does that work anyway? You'd have to reprogram your dma engine, so
> > either the ->migratepage() callback does that and we're good either way,
> > or it simply doesn't work at all.
> 
> If it's called from the faulting task's context you have to sleep, and
> the latency gets increased even more, plus you're dependant on the dma
> engine's backlog.  If you do all that from a background thread you don't
> have to block (you might have to cancel or discard a migration if the
> page was changed while being copied). 

The current MoF implementation simply bails and uses the old page. It
will never block.

Its all a best effort approach, a 'few' stray pages is OK as long as the
bulk of the pages are local.

If you're concerned, we can add per mm/vma counters to track this.
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