Re: [GENERAL] Up to date conventional wisdom re max shared_buffer size?

Andres Freund <andres@anarazel.de> writes:

> On 2017-09-20 13:00:34 -0500, Jerry Sievers wrote:
>
>> >> Pg 9.3 on monster 2T/192 CPU Xenial thrashing
>> >
>> > Not sure what the word "thrashing" in that sentence means.
>> 
>> Cases of dozens or hundreds of sessions running typical statements for
>> this system but running 100% on their CPUs.  Seems to be triggered by
>> certain heavy weight batch jobs kicking off on this generally OLTP
>> system.
>> 
>> ISTM there might be LW lock contention happening around some sort of
>> shared resource where the lock wait implementation is a CPU spinner.
>
> Yes, we improved that a lot in 9.5, 9.6 and 10.  The really bad
> scenarios - I've seen 95% cpu time spent in locking - should all be
> fixed.

Yup, as I suspected from studying the rel notes but some shops including
ours are slow getting enough app dev and QA resources aligned for DB
system major upgrades thus we have ended up doing them 2 versions at a
time prior and this time 3 versions up :-(

>
> I'd try to make sure that both transparent hugepages and zone reclaim
> mode are disabled - the latter probably is already, but the former might
> still cause some problems.

Ack.  We'll research this.

>
>
>> > Things have improved a lot since 9.3 WRT to scalability, so I'd not
>> > infer too much from 9.3 performance on a larger box.
>> 
>> Understood.  The situation got worse when we moved to the even bigger
>> box also running a 4.x kernel which I presume was no where near existent
>> when 9.3 was our current Pg version.
>
> I suspect it's more the bigger box than the newer kernel. The more
> sockets and cores you have, the more lock contention bites you. That's
> because inter-socket / cpu transfers get more expensive with more cores.
>

Ack

>
>> >> Upgrade pending but we recently started having $interesting performance
>> >> issues at times looking like I/O slowness and other times apparently
>> >> causing CPU spins.
>> >
>> > That's not something we can really usefully comment on given the amount
>> > of information.
>> 
>> Ack'd.
>> 
>> I'd like to strace some of the spinning backends when/if we get another
>> opportunity to observe the problem to see if by syscall or libfunc name
>> we can learn more about what's the cause.
>
> I think the causes are known, and fixed - don't think there's much you
> can do besides upgrading, unless you want to backport a number of
> complex patches yourself.
>
> FWIW, usually perf gives better answers than strace in this type of
> scenario.

>
>
>> >> Anyway, shared_buffer coherency generally high but does take big dips
>> >> that are sometimes sustained for seconds or even minutes.
>> >
>> > "shared_buffer coherency"?
>> 
>> As measured querying pg_stat_databases and comparing total reads to read
>> hits.  Run frequently such as once /5-seconds and factored into a hit
>> percentage.  May stay up around 100% for several ticks but then go way
>> down which may or not sustain.
>> 
>> This is an OLTP app using Rails with hundreds of tables both trivial
>> n structure as well as having partitions, large payloads... TOAST and
>> the like.
>> 
>> TPS can measure in the ~5-10k range.
>
> That's cache hit rate, not coherency ;)

My bad

Thanks again.

>
> - Andres

-- 
Jerry Sievers
Postgres DBA/Development Consulting
e: postgres.consulting@comcast.net
p: 312.241.7800


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