Re: [Qemu-devel] Performance about x-data-plane

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Tue, Jan 03, 2017 at 12:02:14PM -0500, Weiwei Jia wrote:
> > The expensive part is the virtqueue kick.  Recently we tried polling the
> > virtqueue instead of waiting for the ioeventfd file descriptor and got
> > double-digit performance improvements:
> > https://lists.gnu.org/archive/html/qemu-devel/2016-12/msg00148.html
> >
> > If you want to understand the performance of your benchmark you'll have
> > compare host/guest disk stats (e.g. request lifetime, disk utilization,
> > queue depth, average request size) to check that the bare metal and
> > guest workloads are really sending comparable I/O patterns to the
> > physical disk.
> >
> > Then you using Linux and/or QEMU tracing to analyze the request latency
> > by looking at interesting points in the request lifecycle like virtqueue
> > kick, host Linux AIO io_submit(2), etc.
> >
> 
> Thank you. I will look into "polling the virtqueue" as you said above.
> Currently, I just use blktrace to see disk stats and add logs in the
> I/O workload to see the time latency for each request. What kind of
> tools are you using to analyze request lifecycle like virtqueue kick,
> host Linux AIO iosubmit, etc.
> 
> Do you trace the lifecycle like this
> (http://www.linux-kvm.org/page/Virtio/Block/Latency#Performance_data)
> but it seems to be out of date. Does it
> (http://repo.or.cz/qemu-kvm/stefanha.git/shortlog/refs/heads/tracing-dev-0.12.4)
> still work on QEMU 2.4.1?

The details are out of date but the general approach to tracing the I/O
request lifecycle still apply.

There are multiple tracing tools that can do what you need.  I've CCed
Karl Rister who did the latest virtio-blk dataplane tracing.

"perf record -a -e kvm:\*" is a good start.  You can use "perf probe" to
trace QEMU's trace events (recent versions have sdt support, which means
SystemTap tracepoints work) and also trace any function in QEMU:
http://blog.vmsplice.net/2011/03/how-to-use-perf-probe.html

Stefan

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