Re: MultiXact\SLRU buffers configuration

> Andrey Borodin wrote 2022-07-23 11:39:
> 
> Yura, thank you for your benchmarks!
> We already knew that patch can save the day on pathological workloads,
> now we have a proof of this.
> Also there's the evidence that user can blindly increase size of SLRU
> if they want (with the second patch). So there's no need for hard
> explanations on how to tune the buffers size.

Hi @Andrey.Borodin, With some considerations and performance checks from 
@Yura.Sokolov we simplified your approach by the following:

1. Preamble. We feel free to increase any SLRU's, since there's no 
performance degradation on large Buffers count using your SLRU buckets 
solution.
2. `slru_buffers_size_scale` is only one config param introduced for all 
SLRUs. It scales SLRUs upper cap by power 2.
3. All SLRU buffers count are capped by both `MBuffers (shared_buffers)` 
and `slru_buffers_size_scale`. see
4. Magic initial constants `NUM_*_BUFFERS << slru_buffers_size_scale` 
are applied for every SLRU.
5. All SLRU buffers are always sized as power of 2, their hash bucket 
size is always 8.

There's attached patch for your consideration. It does gather and 
simplify both `v21-0001-Make-all-SLRU-buffer-sizes-configurable.patch` 
and `v21-0002-Divide-SLRU-buffers-into-8-associative-banks.patch` to 
much simpler approach.

Thank you, Yours,
- Ivan