On Fri, Sep 15, 2017 at 01:06:44PM +0100, Dr. David Alan Gilbert wrote: > * Daniel P. Berrange (berra...@redhat.com) wrote: > > On Fri, Sep 15, 2017 at 11:49:26AM +0100, Stefan Hajnoczi wrote: > > > On Fri, Sep 15, 2017 at 11:50:57AM +0800, Peter Xu wrote: > > > > On Thu, Sep 14, 2017 at 04:19:11PM +0100, Stefan Hajnoczi wrote: > > > > > On Thu, Sep 14, 2017 at 01:15:09PM +0200, Marc-André Lureau wrote: > > > > > > There should be a limit in the number of requests the thread can > > > > > > queue. Before the patch, the limit was enforced by system socket > > > > > > buffering I think. Now, should oob commands still be processed even > > > > > > if > > > > > > the queue is full? If so, the thread can't be suspended. > > > > > > > > > > I agree. > > > > > > > > > > Memory usage must be bounded. The number of requests is less > > > > > important > > > > > than the amount of memory consumed by them. > > > > > > > > > > Existing QMP clients that send multiple QMP commands without waiting > > > > > for > > > > > replies need to rethink their strategy because OOB commands cannot be > > > > > processed if queued non-OOB commands consume too much memory. > > > > > > > > Thanks for pointing out this. Yes the memory usage problem is valid, > > > > as Markus pointed out as well in previous discussions (in "Flow > > > > Control" section of that long reply). Hopefully this series basically > > > > can work from design prospective, then I'll add this flow control in > > > > next version. > > > > > > > > Regarding to what we should do if the limit is reached: Markus > > > > provided a few options, but the one I prefer most is that we don't > > > > respond, but send an event showing that a command is dropped. > > > > However, I would like it not queued, but a direct reply (after all, > > > > it's an event, and we should not need to care much on ordering of it). > > > > Then we can get rid of the babysitting of those "to be failed" > > > > requests asap, meanwhile we don't lose anything IMHO. > > > > > > > > I think I also missed at least a unit test for this new interface. > > > > Again, I'll add it after the whole idea is proved solid. Thanks, > > > > > > Another solution: the server reports available receive buffer space to > > > the client. The server only guarantees immediate OOB processing when > > > the client stays within the receive buffer size. > > > > > > Clients wishing to take advantage of OOB must query the receive buffer > > > size and make sure to leave enough room. > > > > I don't think having to query it ahead of time is particularly nice, > > and of course it is inherantly racy. > > > > I would just have QEMU emit an event when it pausing processing of the > > incoming commands due to a full queue. If the event includes the ID > > of the last queued command, the client will know which (if any) of > > its outstanding commands are delayed. Another even can be sent when > > it restarts reading. > > Hmm and now we're implementing flow control! > > a) What exactly is the current semantics/buffer sizes? > b) When do clients send multiple QMP commands on one channel without > waiting for the response to the previous command? > c) Would one queue entry for each class of commands/channel work > (Where a class of commands is currently 'normal' and 'oob')
I do wonder if we need to worry about request limiting at all from the client side. For non-OOB commands clients will wait for a reply before sending a 2nd non-OOB command, so you'll never get a deep queue for. OOB commands are supposed to be things which can be handled quickly without blocking, so even if a client sent several commands at once without waiting for replies, they're going to be processed quickly, so whether we temporarily block reading off the wire is a minor detail. IOW, I think we could just have a fixed 10 command queue and apps just pretend that there's an infinite queue and nothing bad would happen from the app's POV. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
