Re: [ANNOUNCE] Native Linux KVM tool v2

[Sasha Levin]

On Fri, 2011-06-17 at 06:00 +0100, Stefan Hajnoczi wrote:
> On Fri, Jun 17, 2011 at 2:03 AM, Sasha Levin <levinsasha928@xxxxxxxxx> wrote:
> > On Thu, 2011-06-16 at 17:50 -0500, Anthony Liguori wrote:
> >> On 06/16/2011 09:48 AM, Pekka Enberg wrote:
> >> > On Wed, Jun 15, 2011 at 6:53 PM, Pekka Enberg<penberg@xxxxxxxxxx>  wrote:
> >> >> - Fast QCOW2 image read-write support beating Qemu in fio benchmarks. See the
> >> >>   following URL for test result details: https://gist.github.com/1026888
> >> >
> >> > It turns out we were benchmarking the wrong guest kernel version for
> >> > qemu-kvm which is why it performed so much worse. Here's a summary of
> >> > qemu-kvm beating tools/kvm:
> >> >
> >> > https://raw.github.com/gist/1029359/9f9a714ecee64802c08a3455971e410d5029370b/gistfile1.txt
> >> >
> >> > I'd ask for a brown paper bag if I wasn't so busy eating my hat at the moment.
> >>
> >> np, it happens.
> >>
> >> Is that still with QEMU with IDE emulation, cache=writethrough, and
> >> 128MB of guest memory?
> >>
> >> Does your raw driver support multiple parallel requests?  It doesn't
> >> look like it does from how I read the code.  At some point, I'd be happy
> >> to help ya'll do some benchmarking against QEMU.
> >>
> >
> > Each virtio-blk device can process requests regardless of other
> > virtio-blk devices, which means that we can do parallel requests for
> > devices.
> >
> > Within each device, we support parallel requests in the sense that we do
> > vectored IO for each head (which may contain multiple blocks) in the
> > vring, we don't do multiple heads because when I've tried adding AIO
> > I've noticed that at most there are 2-3 possible heads - and since it
> > points to the same device it doesn't really help running them in
> > parallel.
> 
> One thing that QEMU does but I'm a little suspicious of is request
> merging.  virtio-blk will submit those 2-3 heads using
> bdrv_aio_multiwrite() if they become available in the same virtqueue
> notify.  The requests will be merged if possible.
> 
> My feeling is that we should already have merged requests coming
> through virtio-blk and there should be no need to do any merging -
> which could be a workaround for a poor virtio-blk vring configuration
> that prevented the guest from sending large requests.  However, this
> feature did yield performance improvements with qcow2 image files when
> it was introduced, so that would be interesting to look at.
> 
> Are you enabling indirect descriptors on the virtio-blk vring?  That
> should allow more requests to be made available because you don't run
> out of vring descriptors so easily.

No, but we're usually not getting close to running out of vring
descriptors either.

-- 

Sasha.

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