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Date: Tue, 5 Jan 2010 19:22:43 -0800
From: Arjan van de Ven <arjan@infradead.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
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To: Christoph Lameter <cl@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, Peter Zijlstra <peterz@infradead.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "minchan.kim@gmail.com" <minchan.kim@gmail.com>, "hugh.dickins" <hugh.dickins@tiscali.co.uk>, Nick Piggin <nickpiggin@yahoo.com.au>, Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@linux-foundation.org>
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On Tue, 5 Jan 2010 09:17:11 -0600 (CST)
Christoph Lameter <cl@linux-foundation.org> wrote:

> On Tue, 5 Jan 2010, Arjan van de Ven wrote:
> 
> > while I appreciate the goal of reducing contention on this lock...
> > wouldn't step one be to remove the page zeroing from under this
> > lock? that's by far (easily by 10x I would guess) the most
> > expensive thing that's done under the lock, and I would expect a
> > first order of contention reduction just by having the zeroing of a
> > page not done under the lock...
> 
> The main issue is cacheline bouncing. mmap sem is a rw semaphore and
> only held for read during a fault.

depends on the workload; on a many-threads-java workload, you also get
it for write quite a bit (lots of malloc/frees in userspace in addition
to pagefaults).. at which point you do end up serializing on the
zeroing.

There's some real life real big workloads that show this pretty badly;
so far the workaround is to have glibc batch up a lot of the free()s..
but that's just pushing it a little further out.

> 


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org

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