Re: Increase Vacuum ring buffer.

On 2017-07-26 19:46, Claudio Freire wrote:
> On Wed, Jul 26, 2017 at 1:39 PM, Sokolov Yura
> <funny(dot)falcon(at)postgrespro(dot)ru> wrote:
>> On 2017-07-24 12:41, Sokolov Yura wrote:
>> test_master_1/pretty.log
> ...
>> time activity tps latency stddev min max
>> 11130 av+ch 198 198ms 374ms 7ms 1956ms
>> 11160 av+ch 248 163ms 401ms 7ms 2601ms
>> 11190 av+ch 321 125ms 363ms 7ms 2722ms
>> 11220 av+ch 1155 35ms 123ms 7ms 2668ms
>> 11250 av+ch 1390 29ms 79ms 7ms 1422ms
>
> vs
>
>> test_master_ring16_1/pretty.log
>> time activity tps latency stddev min max
>> 11130 av+ch 26 1575ms 635ms 101ms 2536ms
>> 11160 av+ch 25 1552ms 648ms 58ms 2376ms
>> 11190 av+ch 32 1275ms 726ms 16ms 2493ms
>> 11220 av+ch 23 1584ms 674ms 48ms 2454ms
>> 11250 av+ch 35 1235ms 777ms 22ms 3627ms
>
> That's a very huge change in latency for the worse
>
> Are you sure that's the ring buffer's doing and not some methodology
> snafu?

Well, I tuned postgresql.conf so that there is no such
catastrophic slows down on master branch. (with default
settings such slowdown happens quite frequently).
bgwriter_lru_maxpages = 10 (instead of default 200) were one
of such tuning.

Probably there were some magic "border" that triggers this
behavior. Tuning postgresql.conf shifted master branch on
"good side" of this border, and faster autovacuum crossed it
to "bad side" again.

Probably, backend_flush_after = 2MB (instead of default 0) is
also part of this border. I didn't try to bench without this
option yet.

Any way, given checkpoint and autovacuum interference could be
such noticeable, checkpoint clearly should affect autovacuum
cost mechanism, imho.

With regards,
--
Sokolov Yura aka funny_falcon
Postgres Professional: https://postgrespro.ru
The Russian Postgres Company