Re: [libvirt] Re: [PATCH 2/3] Introduce monitor 'wait' command
Gerd Hoffmann wrote:
On 04/08/09 16:33, Daniel P. Berrange wrote:
> On Wed, Apr 08, 2009 at 09:16:43AM -0500, Anthony Liguori wrote:
>> The wait command will pause the monitor the command was issued in
>> until a new
>> event becomes available. Events are queued if there isn't a waiter
>> present.
>> The wait command completes after a single event is available.
>>
>> Today, we queue events indefinitely but in the future, I suspect
>> we'll drop
>> events that are older than a certain amount of time to avoid infinitely
>> allocating memory for long running VMs.
>>
>> To make use of the new notification mechanism, this patch introduces a
>> qemu_notify_event() API. This API takes three parameters: a class
>> which is
>> meant to classify the type of event being generated, a name which is
>> meant to
>> distinguish which event in the class that has been generated, and a
>> details
>> parameters which is meant to allow events to send arbitrary data with
>> a given
>> event.
>
> Perhaps we should have the ability to turn on/off events, via a
> 'notify EVENT'
> command, and a way turn off the prompt on the channel used for receiving
> events.
That would nicely solve the "queue events indefinitely" issue. By
default no events are generated. Apps which want receive them (and thus
receive them) can enable them as needed.
Sounds reasonable to me as well.
> And then in the 2nd monitor channel, a single 'wait' command would turn
> off the monitor prompt and make the channel dedicated for just events,
> one per line
>
> (qemu) wait
> rtc-change UTC+0100
> vnc-client connect 192.46.12.4:9353
> vnc-client disconnect 192.46.12.4:9353
> vnc-client connect 192.46.12.2:9353
> vnc-client disconnect 192.46.12.2:9353
IMHO this is more useful than having "wait" just get one event. You'll
need a dedicated monitor channel for events anyway, so with
one-event-per-wait the management app would have to issue wait in a loop.
But doesn't it have to _loop_ anyway? If wait returned multiple events,
the management app would have to loop over the results and then anyway
over the actual wait to get the next chunk - thus twice. To me, one
event per wait invocation looks simpler to handle.
BTW: "wait" is quite generic. Maybe we should name the commands
notify-*, i.e. have
notify-enable $class
notify-disable $class
notify-getevents
My 2 cents:
event_enable $class1[,...]
event_disable $class1[,...]
with a special class 'all' and
event_wait
to finally collect the queued and enabled events. There is just the
question what to do with queued events of a certain class that gets
disabled before the events were dequeued. Purge them selectively or let
the user do this via event_event? I'm not a fan of cleanup via magic
timeouts / event aging.
Jan
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