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

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Message-ID: <CAJSP0QWgmXns89se+xdGgM6i1_hsfVWPQ8caHua9d-dDA4CTDQ@mail.gmail.com>
Date:	Mon, 16 Jul 2012 12:58:21 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Asias He <asias@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	kvm@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>,
	virtualization@...ts.linux-foundation.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Add vhost-blk support

On Thu, Jul 12, 2012 at 4:35 PM, Asias He <asias@...hat.com> 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.

Stefan
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