linux-ext4 - Re: [PATCH 2/4] mm: slub: Do not wake kswapd for SLUBs speculative high-order allocations

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Message-ID: <alpine.DEB.2.00.1105181218520.12423@router.home>
Date:	Wed, 18 May 2011 12:21:08 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
cc:	David Rientjes <rientjes@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Colin King <colin.king@...onical.com>,
	Raghavendra D Prabhu <raghu.prabhu13@...il.com>,
	Jan Kara <jack@...e.cz>, Chris Mason <chris.mason@...cle.com>,
	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 2/4] mm: slub: Do not wake kswapd for SLUBs speculative
 high-order allocations

On Wed, 18 May 2011, Pekka Enberg wrote:

> On 5/17/11 12:10 AM, David Rientjes wrote:
> > On Fri, 13 May 2011, Mel Gorman wrote:
> >
> > > To avoid locking and per-cpu overhead, SLUB optimisically uses
> > > high-order allocations and falls back to lower allocations if they
> > > fail.  However, by simply trying to allocate, kswapd is woken up to
> > > start reclaiming at that order. On a desktop system, two users report
> > > that the system is getting locked up with kswapd using large amounts
> > > of CPU.  Using SLAB instead of SLUB made this problem go away.
> > >
> > > This patch prevents kswapd being woken up for high-order allocations.
> > > Testing indicated that with this patch applied, the system was much
> > > harder to hang and even when it did, it eventually recovered.
> > >
> > > Signed-off-by: Mel Gorman<mgorman@...e.de>
> > Acked-by: David Rientjes<rientjes@...gle.com>
>
> Christoph? I think this patch is sane although the original rationale was to
> workaround kswapd problems.

I am mostly fine with it. The concerns that I have is if there is a
large series of high order allocs then at some point you would want
kswapd to be triggered instead of high order allocs constantly failing.

Can we have a "trigger once in a while" functionality?

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