Re: Bgwriter strategies

Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
>> imola-336 imola-337 imola-340
>> writes by checkpoint 38302 30410 39529
>> writes by bgwriter 350113 2205782 1418672
>> writes by backends 1834333 265755 787633
>> writes total 2222748 2501947 2245834
>> allocations 2683170 2657896 2699974
>
>> It looks like Tom's idea is not a winner; it leads to more writes than
>> necessary.
>
> The incremental number of writes is not that large; only about 10% more.
> The interesting thing is that those "extra" writes must represent
> buffers that were re-touched after their usage_count went to zero, but
> before they could be recycled by the clock sweep. While you'd certainly
> expect some of that, I'm surprised it is as much as 10%. Maybe we need
> to play with the buffer allocation strategy some more.
>
> The very small difference in NOTPM among the three runs says that either
> this whole area is unimportant, or DBT2 isn't a good test case for it;
> or maybe that there's something wrong with the patches?

The small difference in NOTPM is because the I/O still wasn't saturated
even with 10% extra writes.

I ran more tests with a higher number of warehouses, and the extra
writes start to show in the response times. See tests 341-344:
http://community.enterprisedb.com/bgwriter/.

I scheduled a test with the moving average method as well, we'll see how
that fares.

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com