linux-kernel - Re: Defrag in shrinkers

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Message-ID: <20100517002444.GJ8120@dastard>
Date:	Mon, 17 May 2010 10:24:44 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Ed Tomlinson <edt@....ca>
Cc:	Andi Kleen <andi@...stfloor.org>, Christoph Lameter <cl@...ux.com>,
	linux-kernel@...r.kernel.org, xfs@....sgi.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	Pekka Enberg <penberg@...helsinki.fi>, npiggin@...e.de
Subject: Re: Defrag in shrinkers

On Sat, May 15, 2010 at 01:08:17PM -0400, Ed Tomlinson wrote:
> On Friday 14 May 2010 16:36:03 Andi Kleen wrote:
> > Christoph Lameter <cl@...ux.com> writes:
> > 
> > > Would it also be possible to add some defragmentation logic when you
> > > revise the shrinkers? Here is a prototype patch that would allow you to
> > > determine the other objects sitting in the same page as a given object.
> > >
> > > With that I hope that you have enough information to determine if its
> > > worth to evict the other objects as well to reclaim the slab page.
> > 
> > I like the idea, it would be useful for the hwpoison code too,
> > when it tries to clean a page.
> 
> If this is done generally we probably want to retune the 'pressure' put on the slab.  The
> whole reason for the callbacks was to keep the 'pressure on the slab proportional to the
> memory pressure (scan rate).  

I don't see that defrag based reclaim changes the concept of
pressure at all. As long as reclaim follows the nr_to_scan
guideline, then it doesn't matter if we do reclaim from the LRU or
reclaim from a list provided by the slab cache....

FWIW, one thing that would be necessary, I think, is to avoid defrag
until a certain level of fragmentation has occurred - we should do
LRU-based reclaim as much as possible, and only trigger defrag-style
reclaim once we hit a trigger (e.g. once the slab is 25% partial
pages).

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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