Re: LWLock optimization for multicore Power machines

On 02/13/2017 03:16 PM, Bernd Helmle wrote:
> Am Samstag, den 11.02.2017, 15:42 +0300 schrieb Alexander Korotkov:
>> Thus, I see reasons why in your tests absolute results are lower than
>> in my
>> previous tests.
>> 1. You use 28 physical cores while I was using 32 physical cores.
>> 2. You run tests in PowerVM while I was running test on bare metal.
>> PowerVM could have some overhead.
>> 3. I guess you run pgbench on the same machine. While in my tests
>> pgbench
>> was running on another node of IBM E880.
>>
>
> Yeah, pgbench was running locally. Maybe we can get some resources to
> run them remotely. Interesting side note: If you run a second postgres
> instance with the same pgbench in parallel, you get nearly the same
> transaction throughput as a single instance.
>
> Short side note:
>
> If you run two Postgres instances concurrently with the same pgbench
> parameters, you get nearly the same transaction throughput for both
> instances each as when running against a single instance, e.g.
>

That strongly suggests you're hitting some kind of lock. It'd be good to
know which one. I see you're doing "pgbench -S" which also updates
branches and other tiny tables - it's possible the sessions are trying
to update the same row in those tiny tables. You're running with scale
1000, but with 100 it's still possible thanks to the birthday paradox.

Otherwise it might be interesting to look at sampling wait events, which
might tell us more about the locks.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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