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Re: [Qemu-devel] [RFC v2 15/22] monitor: send event when request queue f
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [RFC v2 15/22] monitor: send event when request queue full |
Date: |
Thu, 19 Oct 2017 15:11:50 +0200 |
User-agent: |
Mutt/1.9.1 (2017-09-22) |
On Thu, Oct 19, 2017 at 03:16:11PM +0800, Peter Xu wrote:
> On Wed, Oct 18, 2017 at 05:28:04PM +0200, Stefan Hajnoczi wrote:
> > On Mon, Oct 16, 2017 at 04:11:58PM +0800, Peter Xu wrote:
> > > On Thu, Oct 12, 2017 at 01:56:20PM +0100, Stefan Hajnoczi wrote:
> > > > On Fri, Sep 29, 2017 at 11:38:37AM +0800, Peter Xu wrote:
> > > > > Set maximum QMP request queue length to 8. If queue full, instead of
> > > > > queue the command, we directly return a "request-dropped" event,
> > > > > telling
> > > > > client that specific command is dropped.
> > > > >
> > > > > Signed-off-by: Peter Xu <address@hidden>
> > > > > ---
> > > > > monitor.c | 15 +++++++++++++++
> > > > > 1 file changed, 15 insertions(+)
> > > > >
> > > > > diff --git a/monitor.c b/monitor.c
> > > > > index 1e9a6cb6a5..d9bed31248 100644
> > > > > --- a/monitor.c
> > > > > +++ b/monitor.c
> > > > > @@ -3971,6 +3971,8 @@ static void monitor_qmp_bh_dispatcher(void
> > > > > *data)
> > > > > }
> > > > > }
> > > > >
> > > > > +#define QMP_ASYNC_QUEUE_LEN_MAX (8)
> > > >
> > > > Why 8?
> > >
> > > I proposed this in previous discussion and no one objected, so I just
> > > used it. It's here:
> > >
> > > https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg03989.html
> > > (please don't go over the thread; I'll copy the related paragraphs)
> > >
> > > """
> > > ...
> > > Regarding to queue size: I am afraid max_size=1 may not suffice?
> > > Otherwise a simple batch of:
> > >
> > > {"execute": "query-status"} {"execute": "query-status"}
> > >
> > > Will trigger the failure. But I definitely agree it should not be
> > > something very large. The total memory will be this:
> > >
> > > json limit * queue length limit * monitor count limit
> > > (X) (Y) (Z)
> > >
> > > Now we have (X) already (in form of a few tunables for JSON token
> > > counts, etc.), we don't have (Z), and we definitely need (Y).
> > >
> > > How about we add limits on Y=16 and Z=8?
> > >
> > > We can do some math if we want some more exact number though.
> > > ...
> > > """
> > >
> > > Oops, I proposed "16", but I used "8"; I hope 8 is good enough, but I
> > > am definitely not sure whether "1" is good.
> >
> > I understand the concern about breaking existing clients but choosing an
> > arbitrary magic number isn't a correct solution to that problem because
> > existing clients may exceed the magic number!
>
> I agree.
>
> >
> > Instead I think QMP should only look ahead if the out-of-band feature
> > has been negotatiated. This way existing clients continue to work. New
> > clients will have to avoid sending a batch of requests or they must
> > handle the queue size limit error.
>
> Hmm yes I just noticed that although I broadcasted the "OOB"
> capability but actually I skipped the negociation phase (so OOB is
> always enabled). I think I should have that for sure.
>
> IIUC below new handle_qmp_command() should be always compatible with
> old clients then:
>
> handle_qmp_command ()
> {
> ...
> if (oob_enabled) {
> if (cmd_is_oob (req)) {
> // execute command
> qmp_dispatch (req);
> return;
> }
> if (queue_full (mon)) {
> // drop req
> send_full_event (mon);
> return;
> }
> }
>
> queue (req);
> kick (task);
>
> if (!oob_enabled) {
> // if oob not enabled, we don't process next request before previous
> // one finishes, and queue length will always be either 0 or 1.
> // Note: this means the parsing thread can block now.
> wait_until_req_handled (req);
> }
> }
>
> This will be somehow more complicated than before though, since if
> with this, we need to make sure all the QMP clients have enabled OOB
> feature to make sure OOB command can work. Otherwise even if only one
> QMP client didn't enable OOB, then it may block at waiting for the
> request to finish, and it will block the whole monitor IOThread as
> well (which is currently shared by OOB and non-OOB monitors).
>
> Or, maybe, I should just create one IOThread for each QMP monitor.
Or temporarily stop monitoring a client's chardev while the request is
being processed if OOB isn't negotiated. That way a single IOThread can
still service multiple QMP monitors with differing OOB settings.
Stefan