On 21/12/15 13:53, Tomas Vondra wrote:
> On 12/21/2015 12:03 PM, Heikki Linnakangas wrote:
>> On 17/12/15 19:07, Robert Haas wrote:
>>> If it works well empirically, does it really matter that it's
>>> arbitrary? I mean, the entire planner is full of fairly arbitrary
>>> assumptions about which things to consider in the cost model and
>>> which to ignore. The proof that we have made good decisions there
>>> is in the query plans it generates. (The proof that we have made
>>> bad decisions in some cases in the query plans, too.)
>>
>> Agreed.
>
> What if it only seems to work well because it was tested on cases it was
> designed for? What about the workloads that behave differently?
>
> Whenever we do changes to costing and query planning, we carefully
> consider counter-examples and cases where it might fail. I see nothing
> like that in this thread - all I see is a bunch of pgbench tests, which
> seems rather insufficient to me.
Agreed on that too.
> I'm ready to spend some time on this, assuming we can agree on what
> tests to run. Can we come up with realistic workloads where we expect
> the patch might actually work poorly?
I think the worst case scenario would be the case where there is no
FPW-related WAL burst at all, and checkpoints are always triggered by
max_wal_size rather than checkpoint_timeout. In that scenario, the
compensation formula will cause the checkpoint to be too lazy in the
beginning, and it will have to catch up more aggressively towards the
end of the checkpoint cycle.
One such scenario might be to do only COPYs into a table with no
indexes. Or hack pgbench to do concentrate all the updates on only a few
very rows. There will be a FPW on those few pages initially, but the
spike will be much shorter. Or turn full_page_writes=off, and hack the
patch to do compensation even when fullpage_writes=off, and then just
run pgbench.
- Heikki