linux-kernel - Re: [RFC][PATCH 1/6] count transparent hugepage splits

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Date:	Fri, 4 Feb 2011 22:18:25 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	David Rientjes <rientjes@...gle.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael J Wolf <mjwolf@...ibm.com>
Subject: Re: [RFC][PATCH 1/6] count transparent hugepage splits

On Thu, Feb 03, 2011 at 01:22:14PM -0800, David Rientjes wrote:
> i.e. no global locking, but we've accepted the occassional off-by-one 
> error (even though splitting of hugepages isn't by any means lightning 
> fast and the overhead of atomic ops would be negligible).

Agreed losing an increment is not a problem, but in very large systems
it will become a bottleneck. It's not super urgent, but I think it
needs to become a per-cpu counter sooner than later (not needed
immediately but I would appreciate an incremental patch soon to
address that). split_huge_page is already fully SMP scalable if the
rmap isn't shared (i.e. fully SMP scalable across different execve)
and I'd like it to stay that way because split_huge_page can run at
high frequency at times from different processes, so in very large
systems it may be measurable, with that cacheline bouncing around 1024
cpus. pages_collapsed is not a problem because it's only used by one
kernel thread so it can't be contended. Again not super urgent but
better to optimize it ;).
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