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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Thu, Jun 26, 2014 at 11:14:16PM +0800, Ming Lei wrote:
> 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.
> 
> But now you change the way of submitting I/O, could you share
> the motivation about the change? Is the throughput drop you expect?

Thanks for reporting this.  40% is a serious regression.

We were expecting a regression since the custom Linux AIO codepath has
been replaced with the QEMU block layer (which offers features like
image formats, snapshots, I/O throttling).

Let me know if you get stuck working on a patch.  Implementing batching
sounds like a good idea.  I never measured the impact when I wrote the
ioq code, it just seemed like a natural way to structure the code.

Hopefully this 40% number is purely due to batching and we can get most
of the performance back.

Stefan

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