Re: MultiXact\SLRU buffers configuration

Hi, Anastasia!

> 28 авг. 2020 г., в 23:08, Anastasia Lubennikova <a.lubennikova@postgrespro.ru> написал(а):
> 
> 1) The first patch is sensible and harmless, so I think it is ready for committer. I haven't tested the performance impact, though.
> 
> 2) I like the initial proposal to make various SLRU buffers configurable, however, I doubt if it really solves the problem, or just moves it to another place?
> 
> The previous patch you sent was based on some version that contained changes for other slru buffers numbers: 'multixact_offsets_slru_buffers' and 'multixact_members_slru_buffers'. Have you just forgot to attach them? The patch message "[PATCH v2 2/4]" hints that you had 4 patches)
> Meanwhile, I attach the rebased patch to calm down the CFbot. The changes are trivial.
> 
> 2.1) I think that both min and max values for this parameter are too extreme. Have you tested them?
> 
> +               &multixact_local_cache_entries,
> +               256, 2, INT_MAX / 2,
> 
> 2.2) MAX_CACHE_ENTRIES is not used anymore, so it can be deleted.
> 
> 3) No changes for third patch. I just renamed it for consistency.

Thank you for your review. 

Indeed, I had 4th patch with tests, but these tests didn't work well: I still did not manage to stress SLRUs to reproduce problem from production...

You are absolutely correct in point 2: I did only tests with sane values. And observed extreme performance degradation with values ~ 64 megabytes. I do not know which highest values should we pick? 1Gb? Or highest possible functioning value?

I greatly appreciate your review, sorry for so long delay. Thanks!

Best regards, Andrey Borodin.