[libvirt] [PATCH 4/5] Introcude VIR_CONNECT_GET_ALL_DOMAINS_STATS_BEST_EFFORT

[Daniel P. Berrangé]

On Thu, Jun 14, 2018 at 11:07:43AM +0200, Michal Privoznik wrote:
> On 06/13/2018 05:34 PM, John Ferlan wrote:
> > 
> > $SUBJ: "Introduce" and "NO_WAIT"
> > 
> > 
> > On 06/07/2018 07:59 AM, Michal Privoznik wrote:
> >> https://bugzilla.redhat.com/show_bug.cgi?id=1552092
> >>
> >> If there's a long running job it might cause us to wait 30
> >> seconds before we give up acquiring job. This may cause trouble
> > 
> > s/job/the job/
> > 
> > s/may cause trouble/is problematic/
> > 
> >> to interactive applications that fetch stats repeatedly every few
> >> seconds.
> >>
> >> Solution is to introduce
> > 
> > The solution is...
> > 
> >> VIR_CONNECT_GET_ALL_DOMAINS_STATS_BEST_EFFORT flag which tries to
> >> acquire job but does not wait if acquiring failed.
> >>
> >> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> >> ---
> >>  include/libvirt/libvirt-domain.h |  1 +
> >>  src/libvirt-domain.c             | 10 ++++++++++
> >>  src/qemu/qemu_driver.c           | 15 ++++++++++++---
> >>  3 files changed, 23 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/include/libvirt/libvirt-domain.h b/include/libvirt/libvirt-domain.h
> >> index da773b76cb..1a1d34620d 100644
> >> --- a/include/libvirt/libvirt-domain.h
> >> +++ b/include/libvirt/libvirt-domain.h
> >> @@ -2055,6 +2055,7 @@ typedef enum {
> >>      VIR_CONNECT_GET_ALL_DOMAINS_STATS_SHUTOFF = VIR_CONNECT_LIST_DOMAINS_SHUTOFF,
> >>      VIR_CONNECT_GET_ALL_DOMAINS_STATS_OTHER = VIR_CONNECT_LIST_DOMAINS_OTHER,
> >>  
> >> +    VIR_CONNECT_GET_ALL_DOMAINS_STATS_BEST_EFFORT = 1 << 29, /* ignore stalled domains */
> > 
> > "stalled"?  How about "don't wait on other jobs"
> 
> Well, my hidden idea was also that we can "misuse" this flag to not wait
> on other places too. For instance, if we find out (somehow) that a
> domain is in D state, we would consider it stale even without any job
> running on it. Okay, we have no way of detecting if qemu is in D state
> right now, but you get my point. If we don't lock this flag down to just
> domain jobs (that not all drivers have btw), we can use it more widely.

I would suggest we call it  "NOWAIT" with explanation that we will only
report statistics that can be obtained immediately without any blocking,
whatever may be the cause.


On a tangent, I think this problem really calls for a significantly
different design approach, medium term.

The bulk stats query APIs were a good step forward on what we had
before where users must call many libvirt APIs, but it is still
not very scalable. With huge numbers of guests, we're still having
to serialize stats query calls into 1000's of QEMU processes.

I think we must work with QEMU to define a better interface, taking
advantage of fact we're colocated on the same host. ie we tell QEMU
we we want stats exported in memory page <blah>, and QEMU will keep
that updated at all times.

When libvirt needs the info it can then just read it straight out of
the shared memory page, no blocking on any jobs, no QMP serialization,
etc.

For that matter, we can do a similar thing in libvirt API too. We can
export a shared memory region for applications to use, which we keep
updated on some regular interval that app requests. They can then always
access updated stats without calling libvirt APIs at all.

Regards,
Daniel
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