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

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Date:	Fri, 29 Jul 2011 22:50:04 +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 10:45 PM, Liu Yuan wrote:
> 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
Typo. not merge, I mean *sort* the reqs
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