Re: MultiXact\SLRU buffers configuration

On Sun, Apr 4, 2021 at 7:57 AM Andrey Borodin <x4mmm@yandex-team.ru> wrote:
> 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.

I had added the hash table size in SimpleLruShmemSize(), but then
SimpleLruInit() passed that same value in when allocating the struct, so
the struct was oversized.  Oops.  Fixed.

> > 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?

Alvaro, do you have an opinion on that?

The remaining thing that bothers me about this patch set is that there is
still a linear search in the replacement algorithm, and it runs with an
exclusive lock.  That creates a serious problem for large caches that still
aren't large enough.  I wonder if we can do something to improve that
situation in the time we have.  I considered a bunch of ideas but could
only find one that fits with slru.c's simplistic locking while tracking
recency.  What do you think about a hybrid of SLRU and random replacement,
that retains some characteristics of both?  You could think of it as being
a bit like the tournament selection of the genetic algorithm, with a
tournament size of (say) 8 or 16.  Any ideas on how to evaluate this and
choose the number?  See attached.