linux-kernel - Re: Regression from 2.6.36

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Date:	Tue, 12 Apr 2011 18:31:32 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Changli Gao <xiaosuo@...il.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Américo Wang <xiyou.wangcong@...il.com>,
	Jiri Slaby <jslaby@...e.cz>, azurIt <azurit@...ox.sk>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org, Jiri Slaby <jirislaby@...il.com>
Subject: Re: Regression from 2.6.36

On Wed, 13 Apr 2011 09:23:11 +0800 Changli Gao <xiaosuo@...il.com> wrote:

> On Wed, Apr 13, 2011 at 6:49 AM, Andrew Morton
> <akpm@...ux-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().

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