Open Stack

On Fri, 2014-03-21 at 00:32 +0200, Alexei Kornienko wrote:
> On 03/21/2014 12:15 AM, Jay Pipes wrote:
> > On Fri, 2014-03-21 at 00:03 +0200, Alexei Kornienko wrote:
> >> Hello,
> >>
> >> We've done some profiling and results are quite interesting:
> >> during 1,5 hour ceilometer inserted 59755 events (59755 calls to
> >> record_metering_data)
> >> this calls resulted in total 2591573 SQL queries.
> > Yes, this matches my own experience with Ceilo+MySQL. But do not assume
> > that there are 2591573/59755 or around 43 queries per record meter
> > event. That is misleading. In fact, the number of queries per record
> > meter event increases over time, as the number of retries climbs due to
> > contention between readers and writers.
> >
> >> And the most interesting part is that 291569 queries were ROLLBACK
> >> queries.
> > Yep, I noted that as well. But, this is not unique to Ceilometer by any
> > means. Just take a look at any database serving Nova, Cinder, Glance, or
> > anything that uses the common SQLAlchemy code. You will see a huge
> > percentage of entire number of queries taken up by ROLLBACK statements.
> > The problem in Ceilometer is just that the write:read ratio is much
> > higher than any of the other projects.
> >
> > I had a suspicion that the rollbacks have to do with the way that the
> > oslo.db retry logic works, but I never had a chance to investigate it
> > further. Would be really interested to see similar stats against
> > PostgreSQL and see if the rollback issue is isolated to MySQL (I suspect
> > it is).
> Rollbacks are caused not by retry logic but by create_or_update logic:
> We first try to do INSERT in sub-transaction when it fails we rollback 
> this transaction and do update instead.

No, that isn't correct, AFAIK. We first do a SELECT into the table and
then if no result, try an insert:

https://github.com/openstack/ceilometer/blob/master/ceilometer/storage/impl_sqlalchemy.py#L286-L292

The problem, IMO, is twofold. There does not need to be nested
transactional containers around these create_or_update lookups -- i.e.
the lookups can be done outside of the main transaction begin here:

https://github.com/openstack/ceilometer/blob/master/ceilometer/storage/impl_sqlalchemy.py#L335

Secondly, given the volume of inserts (that also generate selects), a
simple memcache lookup cache would be highly beneficial in cutting down
on writer/reader contention in MySQL.

These are things that can be done without changing the schema (which has
other issues that can be looked at of course).

Best,
-jay

> This is caused by poorly designed schema that requires such hacks.
> Cause of this I suspect that we'll have similar results for PostgreSQL.
> 
> Tomorrow we'll do the same tests with PostgreSQL and MongoDB to see if 
> there is any difference.
> 
> >
> > Best,
> > -jay
> >
> >> We do around 5 rollbacks to record a single event!
> >>
> >> I guess it means that MySQL backend is currently totally unusable in
> >> production environment.
> >>
> >> Please find a full profiling graph attached.
> >>
> >> Regards,
> >>
> >> On 03/20/2014 10:31 PM, Sean Dague wrote:
> >>
> >>> On 03/20/2014 01:01 PM, David Kranz wrote:
> >>>> On 03/20/2014 12:31 PM, Sean Dague wrote:
> >>>>> On 03/20/2014 11:35 AM, David Kranz wrote:
> >>>>>> On 03/20/2014 06:15 AM, Sean Dague wrote:
> >>>>>>> On 03/20/2014 05:49 AM, Nadya Privalova wrote:
> >>>>>>>> Hi all,
> >>>>>>>> First of all, thanks for your suggestions!
> >>>>>>>>
> >>>>>>>> To summarize the discussions here:
> >>>>>>>> 1. We are not going to install Mongo (because "is's wrong" ?)
> >>>>>>> We are not going to install Mongo "not from base distribution", because
> >>>>>>> we don't do that for things that aren't python. Our assumption is
> >>>>>>> dependent services come from the base OS.
> >>>>>>>
> >>>>>>> That being said, being an integrated project means you have to be able
> >>>>>>> to function, sanely, on an sqla backend, as that will always be part of
> >>>>>>> your gate.
> >>>>>> This is a claim I think needs a bit more scrutiny if by "sanely" you
> >>>>>> mean "performant". It seems we have an integrated project that no one
> >>>>>> would deploy using the sql db driver we have in the gate. Is any one
> >>>>>> doing that?  Is having a scalable sql back end a goal of ceilometer?
> >>>>>>
> >>>>>> More generally, if there is functionality that is of great importance to
> >>>>>> any cloud deployment (and we would not integrate it if we didn't think
> >>>>>> it was) that cannot be deployed at scale using sqla, are we really going
> >>>>>> to say it should not be a part of OpenStack because we refuse, for
> >>>>>> whatever reason, to run it in our gate using a driver that would
> >>>>>> actually be used? And if we do demand an sqla backend, how much time
> >>>>>> should we spend trying to optimize it if no one will really use it?
> >>>>>> Though the slow heat job is a little different because the slowness
> >>>>>> comes directly from running real use cases, perhaps we should just set
> >>>>>> up a "slow ceilometer" job if the sql version is too slow for its budget
> >>>>>> in the main job.
> >>>>>>
> >>>>>> It seems like there is a similar thread, at least in part, about this
> >>>>>> around marconi.
> >>>>> We required a non mongo backend to graduate ceilometer. So I don't think
> >>>>> it's too much to ask that it actually works.
> >>>>>
> >>>>> If the answer is that it will never work and it was a checkbox with no
> >>>>> intent to make it work, then it should be deprecated and removed from
> >>>>> the tree in Juno, with a big WARNING that you shouldn't ever use that
> >>>>> backend. Like Nova now does with all the virt drivers that aren't tested
> >>>>> upstream.
> >>>>>
> >>>>> Shipping in tree code that you don't want people to use is bad for
> >>>>> users. Either commit to making it work, or deprecate it and remove it.
> >>>>>
> >>>>> I don't see this as the same issue as the slow heat job. Heat,
> >>>>> architecturally, is going to be slow. It spins up real OSes and does
> >>>>> real thinks to them. There is no way that's ever going to be fast, and
> >>>>> the dedicated job was a recognition that to support this level of
> >>>>> services in OpenStack we need to give them more breathing room.
> >>>> Peace. I specifically noted that difference in my original comment. And
> >>>> for that reason the heat slow job may not be temporary.
> >>>>> Architecturally Ceilometer should not be this expensive. We've got some
> >>>>> data showing it to be aberrant from where we believe it should be. We
> >>>>> should fix that.
> >>>> There are plenty of cases where we have had code that passes gate tests
> >>>> with acceptable performance but falls over in real deployment. I'm just
> >>>> saying that having a driver that works ok in the gate but does not work
> >>>> for real deployments is of no more value that not having it at all.
> >>>> Maybe less value.
> >>>> How do you propose to solve the problem of getting more ceilometer tests
> >>>> into the gate in the short-run? As a practical measure l don't see why
> >>>> it is so bad to have a separate job until the complex issue of whether
> >>>> it is possible to have a real-world performant sqla backend is resolved.
> >>>> Or did I miss something and it has already been determined that sqla
> >>>> could be used for large-scale deployments if we just fixed our code?
> >>> I think right now the ball is back in the ceilometer court to do some
> >>> performance profiling, and lets see what comes of that. I don't think
> >>> we're getting more test before the release in any real way.
> >>>
> >>>>> Once we get a base OS in the gate that lets us direct install mongo from
> >>>>> base packages, we can also do that. Or someone can 3rd party it today.
> >>>>> Then we'll even have comparative results to understand the differences.
> >>>> Yes. Do you know which base OS's are candidates for that?
> >>> Ubuntu 14.04 will have a sufficient level of Mongo, so some time in the
> >>> Juno cycle we should have it in the gate.
> >>>
> >>> 	-Sean
> >>>
> >>>
> >>>
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