Re: MultiXact\SLRU buffers configuration

> 1 апр. 2021 г., в 06:40, Thomas Munro <thomas.munro@gmail.com> написал(а):
> 
> 2.  Remove the cap of 128 buffers for xact_buffers as agreed.  We
> still need a cap though, to avoid a couple of kinds of overflow inside
> slru.c, both when computing the default value and accepting a
> user-provided number.  I introduced SLRU_MAX_ALLOWED_BUFFERS to keep
> it <= 1GB, and tested this on a 32 bit build with extreme block sizes.
BTW we do not document maximum values right now.
I was toying around with big values. For example if we set different big xact_buffers we can get something like
FATAL:  not enough shared memory for data structure "Notify" (72768 bytes requested)
FATAL:  not enough shared memory for data structure "Async Queue Control" (2492 bytes requested)
FATAL:  not enough shared memory for data structure "Checkpointer Data" (393280 bytes requested)

But never anything about xact_buffers. I don't think it's important, though.

> 
> Likewise, I removed the cap of 16 buffers for commit_ts_buffers, but
> only if you have track_commit_timestamp enabled.  
Is there a reason to leave 16 pages if commit_ts is disabled? They might be useful for some artefacts of previously enabled commit_ts?

> 4.  Change the default for commit_ts_buffers back to shared_buffers /
> 1024 (with a minimum of 4), because I think you might have changed it
> by a copy and paste error -- or did you intend to make the default
> higher?
I changed default due to some experiments with https://www.postgresql.org/message-id/flat/20210115220744.GA24457%40alvherre.pgsql
In fact most important part of that thread was removing the cap, which is done by the patchset now.

Thanks!

Best regards, Andrey Borodin.