> 15 марта 2016 г., в 19:57, Oleg Bartunov <[email protected]> написал(а):
>
>
>
> On Tue, Mar 15, 2016 at 7:43 PM, Alexander Korotkov
> <[email protected] <mailto:[email protected]>> wrote:
> On Tue, Mar 15, 2016 at 12:57 AM, Robert Haas <[email protected]
> <mailto:[email protected]>> wrote:
> On Mon, Mar 14, 2016 at 4:42 PM, Andres Freund <[email protected]
> <mailto:[email protected]>> wrote:
> > On 2016-03-14 16:16:43 -0400, Robert Haas wrote:
> >> > I have already shown [0, 1] the overhead of measuring timings in linux on
> >> > representative workload. AFAIK, these tests were the only one that showed
> >> > any numbers. All other statements about terrible performance have been
> >> > and
> >> > remain unconfirmed.
> >>
> >> Of course, those numbers are substantial regressions which would
> >> likely make it impractical to turn this on on a heavily-loaded
> >> production system.
> >
> > A lot of people operating production systems are fine with trading a <=
> > 10% impact for more insight into the system; especially if that
> > configuration can be changed without a restart. I know a lot of systems
> > that use pg_stat_statements, track_io_timing = on, etc; just to get
> > that. In fact there's people running perf more or less continuously in
> > production environments; just to get more insight.
> >
> > I think it's important to get as much information out there without
> > performance overhead, so it can be enabled by default. But I don't think
> > it makes sense to not allow features in that cannot be enabled by
> > default, *if* we tried to make them cheap enough beforehand.
>
> Hmm, OK. I would have expected you to be on the other side of this
> question, so maybe I'm all wet. One point I am concerned about is
> that, right now, we have only a handful of types of wait events. I'm
> very interested in seeing us add more, like I/O and client wait. So
> any overhead we pay here is likely to eventually be paid in a lot of
> places - thus it had better be extremely small.
>
> OK. Let's start to produce light, not heat.
>
> As I get we have two features which we suspect to introduce overhead:
> 1) Recording parameters of wait events which requires some kind of
> synchronization protocol.
> 2) Recording time of wait events because time measurements might be expensive
> on some platforms.
>
> Simultaneously there are machines and workloads where both of these features
> doesn't produce measurable overhead. And, we're talking not about toy
> databases. Vladimir is DBA from Yandex which is in TOP-20 (by traffic)
> internet companies in the world. They do run both of this features in
> production highload database without noticing any overhead of them.
>
> It would be great progress, if we decide that we could add both of these
> features controlled by GUC (off by default).
>
> enable_waits_statistics ?
>
>
> If we decide so, then let's start working on this. At first, we should
> construct list of machines and workloads for testing. Each list of machines
> and workloads would be not comprehensive. But let's find something that would
> be enough for testing of GUC controlled, off by default features. Then we
> can turn our conversation from theoretical thoughts to particular benchmarks
> which would be objective and convincing to everybody.
>
> Vladimir, could you provide a test suite, so other people could measure
> overhead on their machines ?
I have somehow described it here [0]. Since the majority of concerns were
around LWLocks, the plan was to reconstruct a workload under heavy LWLocks
pressure. This can easily be done even with pgbench in two following scenarios:
1. Put all the data in shared buffers and on tmpfs and run read/write
test. Contention would be around ProcArrayLock.
2. Put all the data in RAM but not all in shared buffers and run
read-only test. Contention would be around buffer manager.
IMHO, these two tests are good to be representative and not depend much on
hardware.
[0]
http://www.postgresql.org/message-id/[email protected]
>
>
>
>
> Otherwise, let's just add these features to the list of unwanted
> functionality and close this question.
>
> ------
> Alexander Korotkov
> Postgres Professional: http://www.postgrespro.com
> <http://www.postgrespro.com/>
> The Russian Postgres Company
--
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