linux-kernel - Re: [resend][PATCH] mm: increase RECLAIM

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Message-ID: <20101012064108.GE25875@balbir.in.ibm.com>
Date:	Tue, 12 Oct 2010 12:11:08 +0530
From:	Balbir Singh <balbir@...ux.vnet.ibm.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
Cc:	David Rientjes <rientjes@...gle.com>,
	Christoph Lameter <cl@...ux.com>, Mel Gorman <mel@....ul.ie>,
	Rob Mueller <robm@...tmail.fm>, linux-kernel@...r.kernel.org,
	Bron Gondwana <brong@...tmail.fm>,
	linux-mm <linux-mm@...ck.org>
Subject: Re: [resend][PATCH] mm: increase RECLAIM_DISTANCE to 30

* KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> [2010-10-12 13:07:35]:

> > On Tue, 12 Oct 2010, KOSAKI Motohiro wrote:
> > 
> > > > It doesn't determine what the maximum latency to that memory is, it relies 
> > > > on whatever was defined in the SLIT; the only semantics of that distance 
> > > > comes from the ACPI spec that states those distances are relative to the 
> > > > local distance of 10.
> > > 
> > > Right. but do we need to consider fake SLIT case? I know actually such bogus
> > > slit are there. but I haven't seen such fake SLIT made serious trouble.
> > > 
> > 
> > If we can make the assumption that the SLIT entries are truly 
> > representative of the latencies and are adhering to the semantics 
> > presented in the ACPI spec, then this means the VM prefers to do zone 
> > reclaim rather than from other nodes when the latter is 3x more costly.
> > 
> > That's fine by me, as I've mentioned we've done this for a couple years 
> > because we've had to explicitly disable zone_reclaim_mode for such 
> > configurations.  If that's the policy decision that's been made, though, 
> > we _could_ measure the cost at boot and set zone_reclaim_mode depending on 
> > the measured latency rather than relying on the SLIT at all in this case.
> 
> ok, got it. thanks.
>

Could we please document the change and help people understand why
with newer kernels they may see the value of zone_reclaim_mode change
on their systems and how to set it back if required. 

-- 
	Three Cheers,
	Balbir
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