Subject: Re: Frontswap [PATCH 0/4] (was Transcendent Memory): overview
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Avi Kivity <avi@redhat.com>
Cc: Pavel Machek <pavel@ucw.cz>, Dan Magenheimer <dan.magenheimer@oracle.com>,
       linux-kernel@vger.kernel.org, linux-mm@kvack.org, jeremy@goop.org,
       hugh.dickins@tiscali.co.uk, ngupta@vflare.org, JBeulich@novell.com,
       chris.mason@oracle.com, kurt.hackel@oracle.com,
       dave.mccracken@oracle.com, npiggin@suse.de, akpm@linux-foundation.org,
       riel@redhat.com
In-Reply-To: <4BDA8324.7090409@redhat.com>
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Date: Fri, 30 Apr 2010 09:04:26 -0700
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On Fri, 2010-04-30 at 10:13 +0300, Avi Kivity wrote:
> On 04/30/2010 04:45 AM, Dave Hansen wrote:
> >
> > A large portion of CMM2's gain came from the fact that you could take
> > memory away from guests without _them_ doing any work.  If the system is
> > experiencing a load spike, you increase load even more by making the
> > guests swap.  If you can just take some of their memory away, you can
> > smooth that spike out.  CMM2 and frontswap do that.  The guests
> > explicitly give up page contents that the hypervisor does not have to
> > first consult with the guest before discarding.
> >    
> 
> Frontswap does not do this.  Once a page has been frontswapped, the host 
> is committed to retaining it until the guest releases it.  It's really 
> not very different from a synchronous swap device.
> 
> I think cleancache allows the hypervisor to drop pages without the 
> guest's immediate knowledge, but I'm not sure.

Gah.  You're right.  I'm  reading the two threads and confusing the
concepts.  I'm a bit less mystified why the discussion is revolving
around the swap device so much. :)

-- Dave

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