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

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Message-ID: <CAJSP0QX4VqP+_YdfrqHtK5GrB+c8xmziBr=k9PE1WeyWWH_cgQ@mail.gmail.com>
Date:	Tue, 17 Jul 2012 12:36:32 +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 Tue, Jul 17, 2012 at 9:29 AM, Asias He <asias@...hat.com> wrote:
> On 07/16/2012 07:58 PM, Stefan Hajnoczi wrote:
>> Does the vhost-blk implementation do anything fundamentally different
>> from userspace?  Where is the overhead that userspace virtio-blk has?
>
>
>
> Currently, no. But we could play with bio directly in vhost-blk as Christoph
> suggested which could make the IO path from guest to host's real storage
> even shorter in vhost-blk.

Wait :).  My point is that writing new code without systematically
investigating performance means that we're essentially throwing random
things and seeing what sticks.

Adding bio mode would make vhost-blk and kvmtool more different.
It'll probably make vhost-blk slightly faster but harder to compare
against kvmtool.  It's easier to start profiling before making that
change.

The reason I said "special-purpose kernel module" is because kvmtool
could be suffering from a bottleneck that can be fixed.  Other
userspace applications would also benefit from that fix - it would be
generally useful.  Adding a vhost-blk kernel module works around this
but only benefits KVM specifically.

Stefan
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