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