linux-ext4 - Re: [PATCH 3/3] mm: slub: Default slub_max

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Date:	Thu, 12 May 2011 19:46:41 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Dave Jones <davej@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Colin King <colin.king@...onical.com>,
	Raghavendra D Prabhu <raghu.prabhu13@...il.com>,
	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-ext4 <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 3/3] mm: slub: Default slub_max_order to 0

On Thu, May 12, 2011 at 11:48:19AM -0500, Christoph Lameter wrote:
> Try order = 1 which gives you SLAB like interaction with the page
> allocator. Then we at least know that it is the order 2 and 3 allocs that
> are the problem and not something else.

order 1 should work better, because it's less likely we end up here
(which leaves RECLAIM_MODE_LUMPYRECLAIM on and then see what happens
at the top of page_check_references())

   else if (sc->order && priority < DEF_PRIORITY - 2)
   	sc->reclaim_mode |= syncmode;

with order 1 more likely we end up here as enough pages are freed for
order 1 and we're safe:

     else
	sc->reclaim_mode = RECLAIM_MODE_SINGLE | RECLAIM_MODE_ASYNC;

None of these issue should materialize with COMPACTION=n. Even
__GFP_WAIT can be left enabled to run compaction without expecting
adverse behavior, but running compaction may still not be worth it for
small systems where the benefit of having order 1/2/3 allocation may
not outweight the cost of compaction itself.
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