linux-kernel - Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block device

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Date:	Mon, 01 Aug 2011 11:17:51 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	Liu Yuan <namei.unix@...il.com>,
	Stefan Hajnoczi <stefanha@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Khoa Huynh <khoa@...ibm.com>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block
 device

On 07/29/2011 06:25 PM, Sasha Levin wrote:
> On Fri, 2011-07-29 at 20:01 +0800, Liu Yuan wrote:
> >  Looking at this long list,most are function pointers that can not be
> >  inlined, and the internal data structures used by these functions are
> >  dozons. Leave aside code complexity, this long code path would really
> >  need retrofit. As Christoph simply put, this kind of mess is inherent
> >  all over the qemu code. So I am afraid, the 'retrofit'  would end up to
> >  be a re-write the entire (sub)system. I have to admit that, I am
> >  inclined to the MST's vhost approach, that write a new subsystem other
> >  than tedious profiling and fixing, that would possibly goes as far as
> >  actually re-writing it.
>
> I don't think the fix for problematic userspace is to write more kernel
> code.
>
> vhost-net improved throughput and latency by several factors, allowing
> to achieve much more than was possible at userspace alone.
>
> With vhost-blk we see an improvement of ~15% - which I assume by your
> and Christoph's comments can be mostly attributed to QEMU. Merging a
> module which won't improve performance dramatically compared to what is
> possible to achieve in userspace (even if it would require a code
> rewrite) sounds a bit wrong to me

Agree.  vhost-net works around the lack of async zero copy networking 
interface.  Block I/O on the other hand does have such an interface, and 
in addition transaction rates are usually lower.  All we're saving is 
the syscall overhead.

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

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