* Daniel P. Berrange (berra...@redhat.com) wrote: > On Thu, Sep 07, 2017 at 04:13:41PM +0800, Peter Xu wrote: > > On Wed, Sep 06, 2017 at 12:54:28PM +0100, Daniel P. Berrange wrote: > > > On Wed, Sep 06, 2017 at 12:31:58PM +0100, Dr. David Alan Gilbert wrote: > > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > This does imply that you need a separate monitor I/O processing, from > > > > > the > > > > > command execution thread, but I see no need for all commands to > > > > > suddenly > > > > > become async. Just allowing interleaved replies is sufficient from the > > > > > POV of the protocol definition. This interleaving is easy to handle > > > > > from > > > > > the client POV - just requires a unique 'serial' in the request by the > > > > > client, that is copied into the reply by QEMU. > > > > > > > > OK, so for that we can just take Marc-André's syntax and call it 'id': > > > > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.html > > > > > > > > then it's upto the caller to ensure those id's are unique. > > > > > > Libvirt has in fact generated a unique 'id' for every monitor command > > > since day 1 of supporting QMP. > > > > > > > I do worry about two things: > > > > a) With this the caller doesn't really know which commands could be > > > > in parallel - for example if we've got a recovery command that's > > > > executed by this non-locking thread that's OK, we expect that > > > > to be doable in parallel. If in the future though we do > > > > what you initially suggested and have a bunch of commands get > > > > routed to the migration thread (say) then those would suddenly > > > > operate in parallel with other commands that we're previously > > > > synchronous. > > > > > > We could still have an opt-in for async commands. eg default to executing > > > all commands in the main thread, unless the client issues an explicit > > > "make it async" command, to switch to allowing the migration thread to > > > process it async. > > > > > > { "execute": "qmp_allow_async", > > > "data": { "commands": [ > > > "migrate_cancel", > > > ] } } > > > > > > > > > { "return": { "commands": [ > > > "migrate_cancel", > > > ] } } > > > > > > The server response contains the subset of commands from the request > > > for which async is supported. > > > > > > That gives good negotiation ability going forward as we incrementally > > > support async on more commands. > > > > I think this goes back to the discussion on which design we'd like to > > choose. IMHO the whole async idea plus the per-command-id is indeed > > cleaner and nicer, and I believe that can benefit not only libvirt, > > but also other QMP users. The problem is, I have no idea how long > > it'll take to let us have such a feature - I believe that will include > > QEMU and Libvirt to both support that. And it'll be a pity if the > > postcopy recovery cannot work only because we cannot guarantee a > > stable monitor. > > This is not a blocker for having postcopy recovery feature merged. > It merely means that in a situation where the mainloop is blocked, > then we can't recover, in other situations we'll be able to recover > fine. Sure it would be nice to fix that problem too, but I don't > see it as a block.
It's probably OK to merge the recovery code before the monitor code; but I don't think it's something you'd want to tell users about - a 'postcopy recovery that only works rarely' isn't much use. Dave > I don't think the hacks proposed are a good tradeoff, compared to > fixing the fundamental problem with the monitor impl in QEMU. We > have discussed this monitor problem for years pretty much since > day 1 of QMP being designed, but it never gets serious attention. > IMHO it is well overdue to change that and focus attention on the > root problem and not just punt it down the road yet again by adding > short term hacks. > > Adding an extra monitor channel, even as a short term hack, is > *not* short term from libvirt's POV - we'll have to carry that > code for many years into the future, even after QEMU provides > a real fix. So even if QEMU provides such a short term hack, I > would none the less be strongly against libvirt using it. > > > 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 :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
