Re: Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule?
От | Fabien COELHO |
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Тема | Re: Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule? |
Дата | |
Msg-id | alpine.DEB.2.10.1507050704550.6474@sto обсуждение исходный текст |
Ответ на | Re: Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule? (Robert Haas <robertmhaas@gmail.com>) |
Ответы |
Re: Let PostgreSQL's On Schedule checkpoint write buffer
smooth spread cycle by tuning IsCheckpointOnSchedule?
Re: Let PostgreSQL's On Schedule checkpoint write buffer smooth spread cycle by tuning IsCheckpointOnSchedule? |
Список | pgsql-hackers |
Hello Robert, >> I've looked at the maths. >> >> I think that the load is distributed as the derivative of this >> function, that is (1.5 * x ** 0.5): It starts at 0 but very quicky >> reaches 0.5, it pass the 1.0 (average load) around 40% progress, and >> ends up at 1.5, that is the finishing load is 1.5 the average load, >> just before fsyncing files. This looks like a recipee for a bad time: I >> would say this is too large an overload. I would suggest a much lower >> value, say around 1.1... >> The other issue with this function is that it should only degrade >> performance by disrupting the write distribution if someone has WAL on >> a different disk. As I understand it this thing does only make sense if >> the WAL & the data are on the samee disk. This really suggest a guc. > > I am a bit skeptical about this. We need test scenarios that clearly > show the benefit of having and of not having this behavior. It might be > that doing this always is fine for everyone. Do you mean I have to proove that there is an actual problem induced from this patch? The logic fails me: I thought the patch submitter would have to show that his/her patch did not harm performance in various reasonable cases. At least this is what I'm told in another thread:-) Currently this patch changes heavily the checkpoint write load distribution in many cases with a proof which consist in showing that it may improve tps *briefly* on *one* example, as far as I understood the issue and the tests. If this is enough proof to apply the patch, then the minimum is that it should be possible to desactivate it, hence a guc. Having a guc would also help to test the feature with different values than 1.5, which really seems harmful from a math point of view. I'm not sure at all that a power formula is the right approach. The potential impact I see would be to aggravate significantly the write stall issues I'm working on, but the measures provided in these tests do not even look at that or measure that. -- Fabien.
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