linux-kernel - Re: [PATCH 00/27] Latest numa/core release, v16

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Date:	Wed, 21 Nov 2012 12:04:25 -1000
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	David Rientjes <rientjes@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Paul Turner <pjt@...gle.com>,
	Lee Schermerhorn <Lee.Schermerhorn@...com>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH 00/27] Latest numa/core release, v16

On Wed, Nov 21, 2012 at 7:10 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> Because scalability slowdowns are often non-linear.

Only if you hold locks or have other non-cpu-private activity.

Which the vsyscall code really shouldn't have.

That said, it might be worth removing the "prefetchw(&mm->mmap_sem)"
from the VM fault path. Partly because software prefetches have never
ever worked on any reasonable hardware, and partly because it could
seriously screw up things like the vsyscall stuff.

I think we only turn prefetchw into an actual prefetch instruction on
3DNOW hardware. Which is the *old* AMD chips. I don't think even the
Athlon does that.

Anyway, it might be interesting to see a instruction-level annotated
profile of do_page_fault() or whatever

> So with CONFIG_NUMA_BALANCING=y we are taking a higher page
> fault rate, in exchange for a speedup.

The thing is, so is autonuma.

And autonuma doesn't show any of these problems. Autonuma didn't need
vsyscall hacks, autonuma didn't need TLB flushing optimizations,
autonuma just *worked*, and in fact got big speedups when Mel did the
exact same loads on that same machine, presumably with all the same
issues..

Why are you ignoring that fact?

                  Linus
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