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Re: [Qemu-devel] Re: [PATCH 2/3] virtio-pci: Use ioeventfd for virtqueue


From: Avi Kivity
Subject: Re: [Qemu-devel] Re: [PATCH 2/3] virtio-pci: Use ioeventfd for virtqueue notify
Date: Mon, 06 Dec 2010 17:09:24 +0200
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.12) Gecko/20101103 Fedora/1.0-0.33.b2pre.fc14 Lightning/1.0b3pre Thunderbird/3.1.6

On 12/01/2010 11:34 PM, Stefan Hajnoczi wrote:
>>  The guest CPU utilization numbers include an efficiency metric: %vcpu
>>  per MB/sec.  Here we see significant improvements too.  Guests that
>>  previously couldn't get more CPU work done now have regained some
>>  breathing space.
>
>  Thanks for those numbers.  The guest improvements were expected, but the
>  host numbers surprised me.  Do 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.

Makes sense.

--
error compiling committee.c: too many arguments to function




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