Re: [Qemu-devel] top(1) utility implementation in QEMU

[Markus Armbruster]

Fam Zheng <address@hidden> writes:

> On Mon, 09/26 17:28, Daniel P. Berrange wrote:
>> On Mon, Sep 26, 2016 at 07:14:33PM +0530, prashanth sunder wrote:
>> > Hi All,
>> > 
>> > Summary of the discussion and different approaches we had on IRC
>> > regarding a top(1) tool in qemu
>> > 
>> > Implement unique naming for all event loop resources.  Sometimes a
>> > string literal can be used but other times the unique name needs to be
>> > generated at runtime (e.g. filename for an fd).
>> > 
>> > Approach 1)
>> > For a built-in QMP implementation:
>> > We have callbacks from fds, BHs and Timers
>> > So everytime one of them is registered - we add them to the list(what
>> > we see through QMP)
>> > and when they are unregistered - we remove them from the list.
>> > Ex: aio_set_fd_handler(fd, NULL, NULL, NULL) - unregistering an fd -
>> > will remove the fd from the list.
>> > 
>> > QMP API:
>> > set-event-loop-profiling enable=on/off
>> > [interval=seconds] [iothread=name] and it emits a QMP event with
>> > [{name, counter, time_elapsed}]
>> > 
>> > Pros:
>> > It works on all systems.
>> > Cons:
>> > Information present inside glib is exposed only via systemtap tracing
>> > - these will not be available via QMP.
>> > For example - I/O in chardevs, network IO etc
>> 
>> 
>> There's other downsides to QMP approach
>> 
>>  - Emitting data via QMP will change the behaviour of the system
>>    itself, since QMP will trigger usage of the main event loop
>>    which is the thing being traced. The degree of disturbance
>>    will depend on the interval for emitting events
>
> Yes, but compared to a guest that is busy enough to be analyzed with qemu-top,
> I don't think this can be a high degree, even it's at a few events per second.
>
>> 
>>  - If the interval is small and you're monitoring more than one
>>    guest at a time, then the overhead of QMP could start to get
>>    quite significant across the host as a whole. This was
>>    mentioned at the summit wrt existing I/O stats expose by
>>    QEMU for block / net device backends.
>
> qemu-top is supposed to run only in foreground when human attends. So I'm not
> concerned about the system wide overall overhead.
>
>> 
>>  - The 'top' tool does not actually have direct access to
>>    QMP for any libvirt guests and we've unlikely to want to
>>    expose such QMP events via libvirt in any kind of supported
>>    API, as they're very use-case specific in design. So at best
>>    the app would have to use libvirt QMP passthrough which is
>>    acceptable for developer / test environments, but not
>>    something that's satisfactory for production deployments.
>
> Just another idea: my original though on how to send statistics to 'qemu-top',
> was a specialized channel like a socket with a minimized protocol (e.g. a
> mini-QMP, with only whitelisted commands, or an event-only QMP, or simply in 
> an
> ad-hoc format).

What's the advantage over simply using another QMP monitor?  Naturally,
injecting arbitrary QMP commands behind libvirt's back isn't going to
end well, but "don't do that then".  Information queries and listening
to events should be safe.

Note that we could have a QMP command to spawn monitors.  Fun!

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