Re: postgresql latency & bgwriter not doing its job

On Tue, Sep 2, 2014 at 8:14 AM, Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:

>
> There is scan_whole_pool_milliseconds, which currently forces bgwriter to
>>>> circle the buffer pool at least once every 2 minutes. It is currently
>>>> fixed, but it should be trivial to turn it into an experimental guc that
>>>> you could use to test your hypothesis.
>>>>
>>>
>>> I recompiled with the variable coldly set to 1000 instead of 120000. The
>>> situation is slightly degraded (15% of transactions were above 200 ms
>>> late). However it seems that bgwriter did not write much more pages:
>>>
>>
>>
>> You should probably try it set to 200 rather than 1000, to put it on an
>> equal footing with the checkpoint_timeout of 0.2 seconds you reported on.
>>
>
> As I understand it, the setting makes the bgwriter processe scan all
> shared_buffers every this amount of time... but ITSM that the key point is
> that bgwriter has no insentive to start writing out buffers anyway with its
> current decision rules, and that should not change with the frequency at
> which they are scanned (?)

Ah, I see now. The usage counts are not zero, so it visits the buffer and
then leaves it alone.

Cheers,

Jeff