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Subject: Re: Regression from 2.6.36
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Changli Gao <xiaosuo@gmail.com>,
        =?ISO-8859-1?Q?Am=E9rico?= Wang <xiyou.wangcong@gmail.com>,
        Jiri Slaby <jslaby@suse.cz>, azurIt <azurit@pobox.sk>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org, Jiri Slaby <jirislaby@gmail.com>
In-Reply-To: <20110412183132.a854bffc.akpm@linux-foundation.org>
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Content-Type: text/plain; charset="UTF-8"
Date: Wed, 13 Apr 2011 04:37:36 +0200
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Le mardi 12 avril 2011 à 18:31 -0700, Andrew Morton a écrit :
> On Wed, 13 Apr 2011 09:23:11 +0800 Changli Gao <xiaosuo@gmail.com> wrote:
> 
> > On Wed, Apr 13, 2011 at 6:49 AM, Andrew Morton
> > <akpm@linux-foundation.org> wrote:
> > >
> > > It's somewhat unclear (to me) what caused this regression.
> > >
> > > Is it because the kernel is now doing large kmalloc()s for the fdtable,
> > > and this makes the page allocator go nuts trying to satisfy high-order
> > > page allocation requests?
> > >
> > > Is it because the kernel now will usually free the fdtable
> > > synchronously within the rcu callback, rather than deferring this to a
> > > workqueue?
> > >
> > > The latter seems unlikely, so I'm thinking this was a case of
> > > high-order-allocations-considered-harmful?
> > >
> > 
> > Maybe, but I am not sure. Maybe my patch causes too many inner
> > fragments. For example, when asking for 5 pages, get 8 pages, and 3
> > pages are wasted, then memory thrash happens finally.
> 
> That theory sounds less likely, but could be tested by using
> alloc_pages_exact().
> 

Very unlikely, since fdtable sizes are powers of two, unless you hit
sysctl_nr_open and it was changed (default value being 2^20)


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