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

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Message-ID: <20110729105540.GA16250@infradead.org>
Date:	Fri, 29 Jul 2011 06:55:40 -0400
From:	Christoph Hellwig <hch@...radead.org>
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>
Subject: Re: [RFC PATCH]vhost-blk: In-kernel accelerator for virtio block
 device

On Fri, Jul 29, 2011 at 03:59:53PM +0800, Liu Yuan wrote:
> I noted bdrv_aio_multiwrite() do the murging job, but  I am not sure

Just like I/O schedulers it's actually fairly harmful on high IOPS,
low latency devices.  I've just started doing a lot of qemu bencharks,
and disabling that multiwrite mess alone gives fairly nice speedups.

The major issue seems to be additional memory allocations and cache
lines - a problem that actually is fairly inherent all over the qemu
code.

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