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

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Date:	Sun, 12 Jul 2009 20:16:01 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Anthony Liguori <anthony@...emonkey.ws>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	npiggin@...e.de, akpm@...l.org, jeremy@...p.org,
	xen-devel@...ts.xensource.com, tmem-devel@....oracle.com,
	alan@...rguk.ukuu.org.uk, linux-mm@...ck.org,
	kurt.hackel@...cle.com, Rusty Russell <rusty@...tcorp.com.au>,
	dave.mccracken@...cle.com, Marcelo Tosatti <mtosatti@...hat.com>,
	sunil.mushran@...cle.com, Schwidefsky <schwidefsky@...ibm.com>,
	chris.mason@...cle.com, Balbir Singh <balbir@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/4] (Take 2): transcendent memory ("tmem") for Linux

On 07/12/2009 07:20 PM, Dan Magenheimer wrote:
>>> that information; but tmem is trying to go a step further by making
>>> the cooperation between the OS and hypervisor more explicit
>>> and directly beneficial to the OS.
>>>        
>> KVM definitely falls into the camp of trying to minimize
>> modification to the guest.
>>      
>
> No argument there.  Well, maybe one :-) Yes, but KVM
> also heavily encourages unmodified guests.  Tmem is
> philosophically in favor of finding a balance between
> things that work well with no changes to any OS (and
> thus work just fine regardless of whether the OS is
> running in a virtual environment or not), and things
> that could work better if the OS is knowledgable that
> it is running in a virtual environment.
>    


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 virtualization is a flash-in-
> the-pan, no modifications to the OS is the right answer.
> 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.

> Is it the tmem API or the precache/preswap API layered on
> top of it that is problematic?  Both currently assume copying
> but perhaps the precache/preswap API could, with minor
> modifications, meet KVM's needs better?
>
>    

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.Well, first, tmem's very name means memory that is "beyond the
> range of normal perception".  This is certainly not the first class
> of memory in use in data centers that can't be accounted at
> process granularity.  I'm thinking disk array caches as the
> primary example.  Also lots of tools that work great in a
> non-virtualized OS are worthless or misleading in a virtual
> environment.
>
>    

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.


-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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