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

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Date:	Thu, 19 Mar 2015 16:05:46 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Mel Gorman <mgorman@...e.de>, 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 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.
It must be *very* unstable for it to make that big of a difference.
But I feel at least a *bit* better about "unstable algorithm changes a
small varioation into a factor-of-two" vs that crazy factor-of-20.

Can you try Mel's change to make it use

        if (!(vma->vm_flags & VM_WRITE))

instead of the pte details? Again, on otherwise plain 3.19, just so
that we have a baseline. I'd be *so* much happer with checking the vma
details over per-pte details, especially ones that change over the
lifetime of the pte entry, and the NUMA code explicitly mucks with.

                           Linus
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