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Date: Thu, 26 Jun 2014 17:43:10 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit
 580b6b2aa2
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To: Ming Lei <tom.leiming@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

Il 26/06/2014 17:37, Ming Lei ha scritto:
> On Thu, Jun 26, 2014 at 11:29 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> Il 26/06/2014 17:14, Ming Lei ha scritto:
>>
>>> Hi Stefan,
>>>
>>> I found VM block I/O thoughput is decreased by more than 40%
>>> on my laptop, and looks much worsen in my server environment,
>>> and it is caused by your commit 580b6b2aa2:
>>>
>>>           dataplane: use the QEMU block layer for I/O
>>>
>>> I run fio with below config to test random read:
>>>
>>> [global]
>>> direct=1
>>> size=4G
>>> bsrange=4k-4k
>>> timeout=20
>>> numjobs=4
>>> ioengine=libaio
>>> iodepth=64
>>> filename=/dev/vdc
>>> group_reporting=1
>>>
>>> [f]
>>> rw=randread
>>>
>>> Together with throughput drop, the latency is improved a little.
>>>
>>> With this commit, I/O block submitted to fs becomes much smaller
>>> than before, and more io_submit() need to be called to kernel, that
>>> means iodepth may become much less.
>>>
>>> I am not surprised with the result since I did compare VM I/O
>>> performance between qemu and lkvm before, which has no big qemu
>>> lock problem and handle I/O in a dedicated thread, but lkvm's block
>>> IO is still much worse than qemu from view of throughput, because
>>> lkvm doesn't submit block I/O at batch like the way of previous
>>> dataplane, IMO.
>>
>>
>> What is your elevator setting in both the host and the guest?  Usually
>> deadline gives the best performance.
>
> The test is based on cfq, but I just run a quick test with deadline, looks
> no obvious difference.

Can you give us your QEMU command line?

Paolo