Re: postgresql latency & bgwriter not doing its job

Hello Heikki,

> For the kicks, I wrote a quick & dirty patch for interleaving the fsyncs, see 
> attached. It works by repeatedly scanning the buffer pool, writing buffers 
> belonging to a single relation segment at a time.

I tried this patch on the same host I used with the same "-R 25 -L 200 -T 
5000", alas without significant positive effects, about 6% of transactions 
were "lost", including stretches of several seconds of unresponsiveness.

Maybe this is because "pgbench -N" only basically touches 2 tables 
(accounts & history), so there are few file syncs involved, thus it does 
not help much with spreading a lot of writes.
  2014-08-30 23:52:24.167 CEST:   LOG:  checkpoint starting: xlog  2014-08-30 23:54:09.239 CEST:   LOG:  checkpoint
sync:number=1 file=base/16384/11902 time=24.529 msec  2014-08-30 23:54:23.812 CEST:   LOG:  checkpoint sync: number=1
file=base/16384/16397_vmtime=32.547 msec  2014-08-30 23:54:24.771 CEST:   LOG:  checkpoint sync: number=1
file=base/16384/11873time=557.470 msec  2014-08-30 23:54:37.419 CEST:   LOG:  checkpoint complete: wrote 4931 buffers
(30.1%);    0 transaction log file(s) added, 0 removed, 3 recycled;     write=122.946 s, sync=10.129 s, total=133.252
s;    sync files=6, longest=9.854 s, average=1.790 s
 

Note that given the small load and table size, pgbench implies random 
I/Os and basically nothing else.

-- 
Fabien.