linux-kernel - Re: ppc: RECLAIM

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Message-ID: <alpine.DEB.2.02.1402181424490.20772@chino.kir.corp.google.com>
Date:	Tue, 18 Feb 2014 14:27:11 -0800 (PST)
From:	David Rientjes <rientjes@...gle.com>
To:	Michal Hocko <mhocko@...e.cz>
cc:	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: ppc: RECLAIM_DISTANCE 10?

On Tue, 18 Feb 2014, Michal Hocko wrote:

> Hi,
> I have just noticed that ppc has RECLAIM_DISTANCE reduced to 10 set by
> 56608209d34b (powerpc/numa: Set a smaller value for RECLAIM_DISTANCE to
> enable zone reclaim). The commit message suggests that the zone reclaim
> is desirable for all NUMA configurations.
> 
> History has shown that the zone reclaim is more often harmful than
> helpful and leads to performance problems. The default RECLAIM_DISTANCE
> for generic case has been increased from 20 to 30 around 3.0
> (32e45ff43eaf mm: increase RECLAIM_DISTANCE to 30).
> 
> I strongly suspect that the patch is incorrect and it should be
> reverted. Before I will send a revert I would like to understand what
> led to the patch in the first place. I do not see why would PPC use only
> LOCAL_DISTANCE and REMOTE_DISTANCE distances and in fact machines I have
> seen use different values.
> 

I strongly suspect that the patch is correct since powerpc node distances 
are different than the architectures you're talking about and get doubled 
for every NUMA domain that the hardware supports.
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