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

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Date:	Fri, 29 Jul 2011 22:45:46 +0800
From:	Liu Yuan <namei.unix@...il.com>
To:	Stefan Hajnoczi <stefanha@...il.com>
CC:	"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 07/29/2011 08:50 PM, Stefan Hajnoczi wrote:
> I hit a weirdness yesterday, just want to mention it in case you notice it too.
>
> When running vanilla qemu-kvm I forgot to use aio=native.  When I
> compared the results against virtio-blk-data-plane (which *always*
> uses Linux AIO) I was surprised to find average 4k read latency was
> lower and the standard deviation was also lower.
>
> So from now on I will run tests both with and without aio=native.
> aio=native should be faster and if I can reproduce the reverse I'll
> try to figure out why.
>
> Stefan
On my laptop, I don't meet this weirdo. the emulated POSIX AIO is much 
worse than the Linux AIO as expected. If iodepth goes deeper, the gap 
gets wider.

If not set aio=none, qemu uses emulated posix aio interface to do the 
IO. I peek at the posix-aio-compat.c,it uses thread pool and sync 
preadv/pwritev to emulate the AIO behaviour. The sync IO interface would 
even cause much poorer performance for random rw, since io-scheduler 
would possibly never get a chance to merge the requests stream. 
(blk_finish_plug->queue_unplugged->__blk_run_queue)

Yuan
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