lkml.org 
[lkml]   [2011]   [Jul]   [29]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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


\
 
 \ /
  Last update: 2011-07-29 16:53    [W:0.091 / U:0.324 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site