linux-kernel - Re: [patch 5/9] Convert anon_vma lock to rw

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Date:	Thu, 3 Apr 2008 00:12:28 +0200
From:	Andrea Arcangeli <andrea@...ranet.com>
To:	Christoph Lameter <clameter@....com>
Cc:	Hugh Dickins <hugh@...itas.com>, Robin Holt <holt@....com>,
	Avi Kivity <avi@...ranet.com>, Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>, steiner@....com,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com
Subject: Re: [patch 5/9] Convert anon_vma lock to rw_sem and refcount

On Wed, Apr 02, 2008 at 02:56:25PM -0700, Christoph Lameter wrote:
> I am a bit surprised that brk performance is that important. There may be 

I think it's not brk but fork that is being slowed down, did you
oprofile? AIM forks a lot... The write side fast path generating the
overscheduling I guess is when the new vmas are created for the child
and queued in the parent anon-vma in O(1), so immediate, even
preempt-rt would be ok with it spinning and not scheduling, it's just
a list_add (much faster than schedule() indeed). Every time there's a
collision when multiple child forks simultaneously and they all try to
queue in the same anon-vma, things will slowdown.
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