RE: [RFC 0/2] kvm: Transcendent Memory (tmem) on KVM

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Avi Kivity [mailto:avi@redhat.com]
> Subject: Re: [RFC 0/2] kvm: Transcendent Memory (tmem) on KVM
> 
> On 03/08/2012 06:29 PM, Akshay Karle wrote:
> > Hi,
> >
> > We are undergraduate engineering students of Maharashtra Academy of
> > Engineering, Pune, India and we are working on a project entitled
> > 'Transcendent Memory on KVM' as a part of our academics.
> >
> > Since kvm is one of the most popular hypervisors available,
> > we decided to implement this technique for kvm.
> >
> > Any comments/feedback would be appreciated and will help us a lot with our work.
> 
> One of the potential problems with tmem is reduction in performance when
> the cache hit rate is low, for example when streaming.
> 
> Can you test this by creating a large file, for example with
> 
>   dd < /dev/urandom > file bs=1M count=100000
> 
> and then measuring the time to stream it, using
> 
>   time dd < file > /dev/null
> 
> with and without the patch?
> 
> Should be done on a cleancache enabled guest filesystem backed by a
> virtio disk with cache=none.
> 
> It would be interesting to compare kvm_stat during the streaming, with
> and without the patch.

Hi Avi --

The "WasActive" patch (https://lkml.org/lkml/2012/1/25/300) 
is intended to avoid the streaming situation you are creating here.
It increases the "quality" of cached pages placed into zcache
and should probably also be used on the guest-side stubs (and/or maybe
the host-side zcache... I don't know KVM well enough to determine
if that would work).

As Dave Hansen pointed out, the WasActive patch is not yet correct
and, as akpm points out, pageflag bits are scarce on 32-bit systems,
so it remains to be seen if the WasActive patch can be upstreamed.
Or maybe there is a different way to achieve the same goal.
But I wanted to let you know that the streaming issue is understood
and needs to be resolved for some cleancache backends just as it was
resolved in the core mm code.

The measurement you suggest would still be interesting even
without the WasActive patch as it measures a "worst case".

Dan