Re: postgresql latency & bgwriter not doing its job

[oops, wrong from, resent...]

Hello Jeff,

>> The culprit I found is "bgwriter", which is basically doing nothing to
>> prevent the coming checkpoint IO storm, even though there would be ample
>> time to write the accumulating dirty pages so that checkpoint would find a
>> clean field and pass in a blink. Indeed, at the end of the 500 seconds
>> throttled test, "pg_stat_bgwriter" says:
>
> Are you doing pg_stat_reset_shared('bgwriter') after running pgbench -i?

Yes, I did.

> You don't want your steady state stats polluted by the bulk load.

Sure!

>> buffers_checkpoint = 19046
>> buffers_clean = 2995
>
> Out of curiosity, what does buffers_backend show?

buffers_backend = 157

> In any event, this almost certainly is a red herring.

Possibly. It is pretty easy to reproduce, though.

> Whichever of the three ways are being used to write out the buffers, it
> is the checkpointer that is responsible for fsyncing them, and that is
> where your drama is almost certainly occurring. Writing out with one
> path rather than a different isn't going to change things, unless you
> change the fsync.

Well, I agree partially. ISTM that the OS does not need to wait for fsync
to start writing pages if it is receiving one minute of buffer writes at
50 writes per second, I would have thought that some scheduler should
start handling the flow before fsync... So I thought that if bgwriter was
to write the buffers is would help, but maybe there is a better way.

> Also, are you familiar with checkpoint_completion_target, and what is it
> set to?

The default 0.5. Moving to 0.9 seems to worsen the situation.

--
Fabien.