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

[Yoshihiro YUNOMAE <yoshihiro.yunomae.ez@hitachi.com>]

Hi Amit,

Sorry for the late reply.

(2012/07/27 18:43), Amit Shah wrote:
> On (Fri) 27 Jul 2012 [17:55:11], Yoshihiro YUNOMAE wrote:
>> 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:
>>

[...]

>>>>
>>>> ***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
>
> I suppose the latest version on-list is what you quote above.  The
> objections to the patch series are mentioned in Anthony's mails.

I'll check the mails.

> Hans maintains a rebased version of the patches in his tree at
>
> http://cgit.freedesktop.org/~jwrdegoede/qemu/
>
> those patches are included in Fedora's qemu-kvm, so you can try that
> out if it improves performance for you.

Thanks. I'll check those patches.

>>>>   - 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.
>
> I see, so you suggested vhost-serial only because you saw the guest
> stopping problem due to the absence of non-blocking code?  If so, it
> now makes sense.  I don't think we need vhost-serial in any way yet.

I understood. We suggested vhost-serial as one of the ideas for
improving performances. Other features(trace-cmd, 9pfs, and
non-blocking chardev) should be supported first, I think.

> BTW where do you parse the trace data obtained from guests?  On a
> remote host?

It is the best that we can parse the data on a remote host in this
tracing system. Existing trace-cmd can already parse it on a remote
site. If we add the feature collecting event-format data(guest's
debugfs has that) from guests, we can parse tracing data on a remote
host as well as on a host running guests.

Thank you,

-- 
Yoshihiro YUNOMAE
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: yoshihiro.yunomae.ez@hitachi.com