linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Message-ID: <alpine.DEB.2.00.1001071007210.901@router.home>
Date:	Thu, 7 Jan 2010 10:11:07 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Arjan van de Ven <arjan@...radead.org>
cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"minchan.kim@...il.com" <minchan.kim@...il.com>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Tue, 5 Jan 2010, Arjan van de Ven wrote:

> On Tue, 5 Jan 2010 09:17:11 -0600 (CST)
> Christoph Lameter <cl@...ux-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.

Again mmap_sem is a rwsem and only a read lock is held. Zeroing in
do_anonymous_page can occur concurrently on multiple processors in the
same address space. The pte lock is intentionally taken *after* zeroing to
allow concurrent zeroing to occur.


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