Re: 8.3.9 - latency spikes with Linux (and tuning for consistently low latency)

Marinos Yannikos <mjy(at)geizhals(dot)at> writes:
> we are seeing latency spikes in the 2-3 second range (sometimes 8-10s) for
> queries that usually take 3-4ms on our systems and I am running out of things to
> try to get rid of them.

Have you checked whether the spikes correlate with checkpoints? Turn
on log_checkpoints and watch for awhile. If so, fooling with the
checkpoint parameters might give some relief. However, 8.3 already has
the spread-checkpoint code so I'm not sure how much more win can be had
there.

More generally, you should watch vmstat/iostat output and see if you
can correlate the spikes with I/O activity, CPU peaks, etc.

A different line of thought is that maybe the delays have to do with lock
contention --- log_lock_waits might help you identify that.

> fsync=off

That's pretty scary.

> work_mem=500MB

Yipes. I don't think you have enough RAM for that to be safe.

> commit_delay=100000 (*)

This is probably not a good idea.

> autovacuum=off (*)

Nor this.

regards, tom lane