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

[Konrad Rzeszutek Wilk]

On Thu, Mar 15, 2012 at 08:01:52PM +0200, Avi Kivity wrote:
> On 03/15/2012 07:49 PM, Dan Magenheimer wrote:
> > > 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.
> 
> Nice.  This takes care of the tail-end of the streaming (the more
> important one - since it always involves a cold copy).  What about the
> other side?  Won't the read code invoke cleancache_get_page() for every
> page? (this one is just a null hypercall, so it's cheaper, but still
> expensive).

That is something we should fix - I think it was mentioned in the frontswap
email thread the need for batching and it certainly seems required as those
hypercalls aren't that cheap.
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