Re: SLRU optimization - configurable buffer pool and partitioning the SLRU lock

> On 23 Feb 2024, at 12:36, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
>> Another thing I've been thinking is that perhaps it would be useful to
>> make the banks smaller, when the total number of buffers is small. For
>> example, if you have 16 or 32 buffers, it's not really clear to me that
>> it makes sense to have just 1 bank or 2 banks. It might be more
>> sensible to have 4 banks with 4 or 8 buffers instead. That should make
>> the algorithm scale down as well as up ...
>
> It might be helpful to have small-size banks when SLRU buffers are set
> to a very low value and we are only accessing a couple of pages at a
> time (i.e. no buffer replacement) because in such cases most of the
> contention will be on SLRU Bank lock. Although I am not sure how
> practical such a use case would be, I mean if someone is using
> multi-xact very heavily or creating frequent subtransaction overflow
> then wouldn't they should set this buffer limit to some big enough
> value? By doing this we would lose some simplicity of the patch I
> mean instead of using the simple macro i.e. SLRU_BANK_SIZE we would
> need to compute this and store it in SlruShared. Maybe that's not that
> bad.

I'm sure anyone with multiple CPUs should increase, not decrease previous default of 128 buffers (with 512MB shared buffers). Having more CPUs (the only way to benefit from more locks) implies bigger transaction buffers.
IMO making bank size variable adds unneeded computation overhead, bank search loops should be unrollable by compiler etc.
Originally there was a patch set step, that packed bank's page addresses together in one array. It was done to make bank search a SIMD instruction.

Best regards, Andrey Borodin.