Re: [PATCH 0/5] Add vhost-blk support

From: Stefan Hajnoczi
Date: Mon Jul 16 2012 - 07:59:09 EST

On Thu, Jul 12, 2012 at 4:35 PM, Asias He <asias@xxxxxxxxxx> wrote:
> This patchset adds vhost-blk support. vhost-blk is a in kernel virito-blk
> device accelerator. Compared to userspace virtio-blk implementation, vhost-blk
> gives about 5% to 15% performance improvement.

Why is it 5-15% faster? vhost-blk and the userspace virtio-blk you
benchmarked should be doing basically the same thing:

1. An eventfd file descriptor is signalled when the vring has new
requests available from the guest.
2. A thread wakes up and processes the virtqueue.
3. Linux AIO is used to issue host I/O.
4. An interrupt is injected into the guest.

Does the vhost-blk implementation do anything fundamentally different
from userspace? Where is the overhead that userspace virtio-blk has?

I'm asking because it would be beneficial to fix the overhead
(especially it that could speed up all userspace applications) instead
of adding a special-purpose kernel module to work around the overhead.

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