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

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Message-ID: <1311953104.30835.15.camel@lappy>
Date:	Fri, 29 Jul 2011 18:25:04 +0300
From:	Sasha Levin <levinsasha928@...il.com>
To:	Liu Yuan <namei.unix@...il.com>
Cc:	Stefan Hajnoczi <stefanha@...il.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Avi Kivity <avi@...hat.com>, 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 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.

-- 

Sasha.

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