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Date: Wed, 1 Dec 2010 21:34:25 +0000
Message-ID: <AANLkTik7h_4ywLuq2rf+6ZHts18rACNdMMWgrSg-owcs@mail.gmail.com>
Subject: Re: [Qemu-devel] Re: [PATCH 2/3] virtio-pci: Use ioeventfd for
	virtqueue notify
From: Stefan Hajnoczi <stefanha@gmail.com>
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To: Avi Kivity <avi@redhat.com>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, qemu-devel@nongnu.org, Christoph Hellwig <hch@infradead.org>, Khoa Huynh <khoa@us.ibm.com>

On Wed, Dec 1, 2010 at 12:30 PM, Avi Kivity <avi@redhat.com> wrote:
> On 12/01/2010 01:44 PM, Stefan Hajnoczi wrote:
>>
>> >>
>> >> =A0And, what about efficiency? =A0As in bits/cycle?
>> >
>> > =A0We are running benchmarks with this latest patch and will report
>> > results.
>>
>> Full results here (thanks to Khoa Huynh):
>>
>> http://wiki.qemu.org/Features/VirtioIoeventfd
>>
>> The host CPU utilization is scaled to 16 CPUs so a 2-3% reduction is
>> actually in the 32-48% range for a single CPU.
>>
>> The guest CPU utilization numbers include an efficiency metric: %vcpu
>> per MB/sec. =A0Here we see significant improvements too. =A0Guests that
>> previously couldn't get more CPU work done now have regained some
>> breathing space.
>
> Thanks for those numbers. =A0The guest improvements were expected, but th=
e
> host numbers surprised me. =A0Do you have an explanation as to why total =
host
> load should decrease?

The first vcpu does virtqueue kick - it holds the guest driver
vblk->lock across kick.  Before this kick completes a second vcpu
tries to acquire vblk->lock, finds it is contended, and spins.  So
we're burning CPU due to the long vblk->lock hold times.

With virtio-ioeventfd those kick times are reduced an there is less
contention on vblk->lock.

Stefan