Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2

[Ming Lei]

On Thu, Jun 26, 2014 at 11:29 PM, Paolo Bonzini <address@hidden> 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.


Thanks,
-- 
Ming Lei