Re: [Qemu-devel] [RFC PATCH 0/6] virtio-trace: Support virtio-trace

[Yoshihiro YUNOMAE]

Hi Amit,

Thank you for commenting on our work.

(2012/07/26 20:35), Amit Shah wrote:
> On (Tue) 24 Jul 2012 [11:36:57], Yoshihiro YUNOMAE wrote:

[...]

> > Therefore, we propose a new system "virtio-trace", which uses enhanced
> > virtio-serial and existing ring-buffer of ftrace, for collecting guest kernel
> > tracing data. In this system, there are 5 main components:
> >   (1) Ring-buffer of ftrace in a guest
> >       - When trace agent reads ring-buffer, a page is removed from ring-buffer.
> >   (2) Trace agent in the guest
> >       - Splice the page of ring-buffer to read_pipe using splice() without
> >         memory copying. Then, the page is spliced from write_pipe to virtio
> >         without memory copying.
>
> I really like the splicing idea.

Thanks. We will improve this patch set.

> >   (3) Virtio-console driver in the guest
> >       - Pass the page to virtio-ring
> >   (4) Virtio-serial bus in QEMU
> >       - Copy the page to kernel pipe
> >   (5) Reader in the host
> >       - Read guest tracing data via FIFO(named pipe)
>
> So will this be useful only if guest and host run the same kernel?
>
> I'd like to see the host kernel not being used at all -- collect all
> relevant info from the guest and send it out to qemu, where it can be
> consumed directly by apps driving the tracing.

No, this patch set is used only for guest kernels, so guest and host
don't need to run the same kernel.

> > ***Evaluation***
> > When a host collects tracing data of a guest, the performance of using
> > virtio-trace is compared with that of using native(just running ftrace),
> > IVRing, and virtio-serial(normal method of read/write).
>
> Why is tracing performance-sensitive?  i.e. why try to optimise this
> at all?

To minimize effects for applications on guests when a host collects
tracing data of guests.
For example, we assume the situation where guests A and B are running
on a host sharing I/O device. An I/O delay problem occur in guest A,
but it doesn't for the requirement in guest B. In this case, we need to
collect tracing data of guests A and B, but a usual method using
network takes high load for applications of guest B even if guest B is
normally running. Therefore, we try to decrease the load on guests.
We also use this feature for performance analysis on production
virtualization systems.

[...]

> > ***Just enhancement ideas***
> >   - Support for trace-cmd
> >   - Support for 9pfs protocol
> >   - Support for non-blocking mode in QEMU
>
> There were patches long back (by me) to make chardevs non-blocking but
> they didn't make it upstream.  Fedora carries them, if you want to try
> out.  Though we want to converge on a reasonable solution that's
> acceptable upstream as well.  Just that no one's working on it
> currently.  Any help here will be appreciated.

Thanks! In this case, since a guest will stop to run when host reads
trace data of the guest, char device is needed to add a non-blocking
mode. I'll read your patch series. Is the latest version 8?
http://lists.gnu.org/archive/html/qemu-devel/2010-12/msg00035.html

> >   - Make "vhost-serial"
>
> I need to understand a) why it's perf-critical, and b) why should the
> host be involved at all, to comment on these.

a) To make collecting overhead decrease for application on a guest.
   (see above)
b) Trace data of host kernel is not involved even if we introduce this
   patch set.

Thank you,

--
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: address@hidden