linux-kernel - RE: [Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") for Linux

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Message-ID: <e60ab548-f0be-4a75-a10b-1f2eb89247a7@default>
Date:	Sun, 12 Jul 2009 13:39:07 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	npiggin@...e.de, akpm@...l.org, xen-devel@...ts.xensource.com,
	tmem-devel@....oracle.com, kurt.hackel@...cle.com,
	Rusty Russell <rusty@...tcorp.com.au>, jeremy@...p.org,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sunil.mushran@...cle.com, chris.mason@...cle.com,
	Anthony Liguori <anthony@...emonkey.ws>,
	Schwidefsky <schwidefsky@...ibm.com>, dave.mccracken@...cle.com,
	Marcelo Tosatti <mtosatti@...hat.com>,
	alan@...rguk.ukuu.org.uk, Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: RE: [Xen-devel] Re: [RFC PATCH 0/4] (Take 2): transcendent memory
 ("tmem") for Linux

> CMM2 and tmem are not any different in this regard; both require OS 
> modification, and both make information available to the 
> hypervisor.  In 
> fact CMM2 is much more intrusive (but on the other hand provides much 
> more information).
>
> > For those that believe it will be pervasive in the
> > future, finding the right balance is a critical step
> > in operating system evolution.
> 
> You're arguing for CMM2 here IMO.

I'm arguing that both are a good thing and a step in
the right direction.  In some ways, tmem is a bigger
step and in some ways CMM2 is a bigger step.

> My take on this is that precache (predecache?) / preswap can be 
> implemented even without tmem by using write-through backing for the 
> virtual disk.  For swap this is actually slight;y more efficient than 
> tmem preswap, for preuncache slightly less efficient (since 
> there will 
> be some double caching).  So I'm more interested in other use 
> cases of tmem/CMM2.
> 
> Right, the transient uses of tmem when applied to disk objects 
> (swap/pagecache) are very similar to disk caches.  Which is 
> why you can 
> get a very similar effect when caching your virtual disks; 
> this can be 
> done without any guest modification.

Write-through backing and virtual disk cacheing offer a
similar effect, but it is far from the same.
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