From: Christoph Lameter <cl@linux.com>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0
Date: Thu, 12 May 2011 10:27:00 -0500 (CDT)
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Cc: Mel Gorman <mgorman@suse.de>, Andrew Morton <akpm@linux-foundation.org>,
  Colin King <colin.king@canonical.com>,
  Raghavendra D Prabhu <raghu.prabhu13@gmail.com>, Jan Kara <jack@suse.cz>,
  Chris Mason <chris.mason@oracle.com>, Pekka Enberg <penberg@kernel.org>,
  Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
  linux-fsdevel <linux-fsdevel@vger.kernel.org>,
  linux-mm <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>,
  linux-ext4 <linux-ext4@vger.kernel.org>
To: James Bottomley <James.Bottomley@HansenPartnership.com>
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On Thu, 12 May 2011, James Bottomley wrote:

> > >   */
> > >  static int slub_min_order;
> > > -static int slub_max_order = PAGE_ALLOC_COSTLY_ORDER;
> > > +static int slub_max_order;
> >
> > If we really need to do this then do not push this down to zero please.
> > SLAB uses order 1 for the meax. Lets at least keep it theere.
>
> 1 is the current value.  Reducing it to zero seems to fix the kswapd
> induced hangs.  The problem does look to be some shrinker/allocator
> interference somewhere in vmscan.c, but the fact is that it's triggered
> by SLUB and not SLAB.  I really think that what's happening is some type
> of feedback loops where one of the shrinkers is issuing a
> wakeup_kswapd() so kswapd never sleeps (and never relinquishes the CPU
> on non-preempt).

The current value is PAGE_ALLOC_COSTLY_ORDER which is 3.

> > We have been using SLUB for a long time. Why is this issue arising now?
> > Due to compaction etc making reclaim less efficient?
>
> This is the snark argument (I've said it thrice the bellman cried and
> what I tell you three times is true).  The fact is that no enterprise
> distribution at all uses SLUB.  It's only recently that the desktop
> distributions started to ... the bugs are showing up under FC15 beta,
> which is the first fedora distribution to enable it.  I'd say we're only
> just beginning widespread SLUB testing.

Debian and Ubuntu have been using SLUB for a long time (and AFAICT from my
archives so has Fedora). I have been running those here for a couple of
years and the issues that I see here seem to be only with the most
recent kernels that now do compaction and other reclaim tricks.





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