Re: Background Processes and reporting

On Tue, Mar 15, 2016 at 7:43 PM, Alexander Korotkov <
a(dot)korotkov(at)postgrespro(dot)ru> wrote:

> On Tue, Mar 15, 2016 at 12:57 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
>
>> On Mon, Mar 14, 2016 at 4:42 PM, Andres Freund <andres(at)anarazel(dot)de>
>> 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 ?

>
> 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
> The Russian Postgres Company
>