Re: Perf Benchmarking and regression.

Hi,

Thanks for benchmarking!

On 2016-05-06 19:43:52 +0530, Mithun Cy wrote:
> 1. # first bad commit: [ac1d7945f866b1928c2554c0f80fd52d7f977772] Make idle
> backends exit if the postmaster dies.
> this made performance to drop from
>
> 15947.21546 (15K +) to 13409.758510 (arround 13K+).

Let's debug this one first, it's a lot more local. I'm rather surprised
that you're seing a big effect with that "few" TPS/socket operations;
and even more that our efforts to address that problem haven't been
fruitful (given we've verified the fix on a number of machines).

Can you verify that removing
AddWaitEventToSet(FeBeWaitSet, WL_POSTMASTER_DEATH, -1, NULL, NULL);
in src/backend/libpq/pqcomm.c : pq_init() restores performance?

I think it'd be best to test the back/forth on master with
bgwriter_flush_after = 0
checkpointer_flush_after = 0
backend_flush_after = 0
to isolate the issue.

Also, do you see read-only workloads to be affected too?

> 2. # first bad commit: [428b1d6b29ca599c5700d4bc4f4ce4c5880369bf] Allow to
> trigger kernel writeback after a configurable number of writes.

FWIW, it'd be very interesting to test again with a bigger
backend_flush_after setting.

Greetings,

Andres Freund