Re: MultiXact\SLRU buffers configuration

Le 15/02/2021 à 18:17, Andrey Borodin a écrit :
>
>> 23 дек. 2020 г., в 21:31, Gilles Darold <gilles(at)darold(dot)net> написал(а):
>>
>> Sorry for the response delay, we have run several others tests trying to figure out the performances gain per patch but unfortunately we have very heratic results. With the same parameters and patches the test doesn't returns the same results following the day or the hour of the day. This is very frustrating and I suppose that this is related to the Azure architecture. The only thing that I am sure is that we had the best performances results with all patches and
>>
>> multixact_offsets_slru_buffers = 256
>> multixact_members_slru_buffers = 512
>> multixact_local_cache_entries = 4096
>>
>>
>> but I can not say if all or part of the patches are improving the performances. My feeling is that performances gain related to patches 1 (shared lock) and 3 (conditional variable) do not have much to do with the performances gain compared to just tuning the multixact buffers. This is when the multixact contention is observed but perhaps they are delaying the contention. It's all the more frustrating that we had a test case to reproduce the contention but not the architecture apparently.
> Hi! Thanks for the input.
> I think we have a consensus here that configuring SLRU size is beneficial for MultiXacts.
> There is proposal in nearby thread [0] on changing default value of commit_ts SLRU buffers.
> In my experience from time to time there can be problems with subtransactions cured by extending subtrans SLRU.
>
> Let's make all SLRUs configurable?
> PFA patch with draft of these changes.
>
> Best regards, Andrey Borodin.
>
>
> [0] https://www.postgresql.org/message-id/flat/20210115220744.GA24457%40alvherre.pgsql
>

The patch doesn't apply anymore in master cause of error: patch failed:
src/backend/utils/init/globals.c:150

An other remark about this patch is that it should be mentionned in the
documentation (doc/src/sgml/config.sgml) that the new configuration
variables need a server restart, for example by adding "This parameter
can only be set at server start." like for shared_buffers. Patch on
postgresql.conf mention it.

And some typo to be fixed:

s/Tipically/Typically/

s/asincronous/asyncronous/

s/confugured/configured/

s/substrnsactions/substransactions/

--
Gilles Darold
LzLabs GmbH