linux-kernel - Re: [tip:sched/numa] sched/numa: Introduce sys_numa

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Date:	Fri, 18 May 2012 18:04:01 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Rik van Riel <riel@...hat.com>, mingo@...nel.org, hpa@...or.com,
	linux-kernel@...r.kernel.org, torvalds@...ux-foundation.org,
	pjt@...gle.com, bharata.rao@...il.com, akpm@...ux-foundation.org,
	Lee.Schermerhorn@...com, aarcange@...hat.com, danms@...ibm.com,
	suresh.b.siddha@...el.com, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:sched/numa] sched/numa: Introduce sys_numa_{t,m}bind()

On Fri, 2012-05-18 at 11:00 -0500, Christoph Lameter wrote:

> Having the page local is a win if there are a sufficient number of
> accesses to amortize the effort to move the page. Given the expensive
> nature of page migration there would need to be a large number of accesses
> to a page to justify the effort.

Right, but this is true of any migration scheme and not specific to MoF.

> > How does it matter how you migrate?
> 
> Migrate on fault incurs two types of costs:
> 
> 1. Unmapping. This results in additional faults to reestablish the ptes.
> 
> 2. Actual lazy migrate. More faults. Now the page needs to be copied to
>    the new node and the actual migration work is done. 

Nah, only the 1 fault is extra. Regular migration already needs to unmap
and copy and reinstate, so the only extra work is the fault to trigger
it.




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