linux-kernel - Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures occur

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Date:	Fri, 20 Mar 2015 09:56:06 +0000
From:	Mel Gorman <mgorman@...e.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Dave Chinner <david@...morbit.com>, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Aneesh Kumar <aneesh.kumar@...ux.vnet.ibm.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>, xfs@....sgi.com,
	ppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH 4/4] mm: numa: Slow PTE scan rate if migration failures
 occur

On Thu, Mar 19, 2015 at 04:05:46PM -0700, Linus Torvalds wrote:
> On Thu, Mar 19, 2015 at 3:41 PM, Dave Chinner <david@...morbit.com> wrote:
> >
> > My recollection wasn't faulty - I pulled it from an earlier email.
> > That said, the original measurement might have been faulty. I ran
> > the numbers again on the 3.19 kernel I saved away from the original
> > testing. That came up at 235k, which is pretty much the same as
> > yesterday's test. The runtime,however, is unchanged from my original
> > measurements of 4m54s (pte_hack came in at 5m20s).
> 
> Ok. Good. So the "more than an order of magnitude difference" was
> really about measurement differences, not quite as real. Looks like
> more a "factor of two" than a factor of 20.
> 
> Did you do the profiles the same way? Because that would explain the
> differences in the TLB flush percentages too (the "1.4% from
> tlb_invalidate_range()" vs "pretty much everything from migration").
> 
> The runtime variation does show that there's some *big* subtle
> difference for the numa balancing in the exact TNF_NO_GROUP details.

TNF_NO_GROUP affects whether the scheduler tries to group related processes
together. Whether migration occurs depends on what node a process is
scheduled on. If processes are aggressively grouped inappropriately then it
is possible there is a bug that causes the load balancer to move processes
off a node (possible migration) with NUMA balancing trying to pull it back
(another possible migration). Small bugs there can result in excessive
migration.

-- 
Mel Gorman
SUSE Labs
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