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

[Avi Kivity <avi@xxxxxxxxxx>]

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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