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

[Kevin Wolf]

Am 27.06.2014 um 14:01 hat Stefan Hajnoczi geschrieben:
> 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.

Shouldn't it be easy enough to take the old code, remove the batching
there and then measure if you get the same 40%?

Kevin

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