linux-kernel - Re: [GIT PULL] mm: frontswap (for 3.2 window)

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Date:	Wed, 02 Nov 2011 17:44:50 +0200
From:	Avi Kivity <avi@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Cyclonus J <cyclonusj@...il.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>, ngupta@...are.org,
	Chris Mason <chris.mason@...cle.com>, JBeulich@...ell.com,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [GIT PULL] mm: frontswap (for 3.2 window)

On 11/01/2011 12:16 PM, James Bottomley wrote:
> Actually, I think there's an unexpressed fifth requirement:
>
> 5. The optimised use case should be for non-paging situations.
>
> The problem here is that almost every data centre person tries very hard
> to make sure their systems never tip into the swap zone.  A lot of
> hosting datacentres use tons of cgroup controllers for this and
> deliberately never configure swap which makes transcendent memory
> useless to them under the current API.  I'm not sure this is fixable,
> but it's the reason why a large swathe of users would never be
> interested in the patches, because they by design never operate in the
> region transcended memory is currently looking to address.
>
> This isn't an inherent design flaw, but it does ask the question "is
> your design scope too narrow?"

If you look at cleancache, then it addresses this concern - it extends
pagecache through host memory.  When dropping a page from the tail of
the LRU it first goes into tmem, and when reading in a page from disk
you first try to read it from tmem.  However in many workloads,
cleancache is actually detrimental.  If you have a lot of cache misses,
then every one of them causes a pointless vmexit; considering that
servers today can chew hundreds of megabytes per second, this adds up. 
On the other side, if you have a use-once workload, then every page that
falls of the tail of the LRU causes a vmexit and a pointless page copy.

-- 
error compiling committee.c: too many arguments to function

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