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Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread
From: |
Peter Xu |
Subject: |
Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread |
Date: |
Thu, 7 Sep 2017 16:13:41 +0800 |
User-agent: |
Mutt/1.5.24 (2015-08-30) |
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 (address@hidden) 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.
I'm curious whether there are other requirements (besides postcopy
recovery) that would want an always-alive monitor to run some
lock-free commands? If there is, I'd be more inclined to first
provide a work-around solution like "-qmp-lockfree", and we can
provide a better solution afterwards until when the whole async QMP
work ready.
>
> > b) I still worry how the various IO channels will behave on another
> > thread. But that's more a general feeling rather than anything
> > specific.
>
> The only complexity will be around making sure the Chardev code uses
> the right GMainContext for any watches on the underlying QIOChannel,
> so that we poll() from the custom thread instead of the main thread.
> IOW, as long as all I/O is done from the single thread everything
> should work fine.
>
> 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 :|
--
Peter Xu
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/06
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread,
Peter Xu <=
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Stefan Hajnoczi, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Stefan Hajnoczi, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Peter Xu, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Daniel P. Berrange, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Markus Armbruster, 2017/09/07
- Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread, Dr. David Alan Gilbert, 2017/09/07