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

[Avi Kivity <avi@redhat.com>]

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).

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

It can provide the justification for that patch, yes.

-- 
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