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Date: Thu, 26 Jun 2014 17:29:54 +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>, Stefan Hajnoczi <stefanha@redhat.com>, qemu-devel@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <famz@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

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.

Paolo