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

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Date:	Wed, 2 Nov 2011 13:27:01 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Avi Kivity <avi@...hat.com>, Andrea Arcangeli <aarcange@...hat.com>
Cc:	James Bottomley <James.Bottomley@...senPartnership.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)

> From: Avi Kivity [mailto:avi@...hat.com]
> Subject: Re: [GIT PULL] mm: frontswap (for 3.2 window)
> 
> On 11/02/2011 06:02 PM, Andrea Arcangeli wrote:
> > Hi Avi,
> >
> > On Wed, Nov 02, 2011 at 05:44:50PM +0200, Avi Kivity wrote:
> > > 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.
> >
> > I also think it's bad design for Virt usage, but hey, without this
> > they can't even run with cache=writeback/writethrough and they're
> > forced to cache=off, and then they claim specvirt is marketing, so for
> > Xen it's better than nothing I guess.
> 
> Surely Xen can use the pagecache, it uses Linux for I/O just like kvm.
> 
> > I'm trying right now to evaluate it as a pure zcache host side
> > optimization.
> 
> zcache style usage is fine.  It's purely internal so no ABI constraints,
> and no hypercalls either.  It's still synchronous though so RAMster like
> approaches will not work well.

Still experimental, but only the initial local put must be synchronous.
RAMster uses a separate thread to "remotify" pre-compressed pages.
The "get" still needs to be synchronous, but (if I ever have time to
get back to coding it) I've got some ideas on how to fix that.  If
I manage to get that working, perhaps it could be used for Andrea's
write-precompressed-zcache-pages-to-disk.

Dan
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