Re: Restructured Shared Buffer Hash Table

On Mon, Oct 12, 2026 at 9:02 PM Dhruv Aron <dhruv(dot)aron(at)gmail(dot)com> wrote:

> On Mon, Oct 12, 2026 at 1:40 PM Alexandre Felipe <
> o(dot)alexandre(dot)felipe(at)gmail(dot)com> wrote:
>
>> 1. mod vs bitwise and
>> Instead of computing [hashcode % num_buckets] you could save
>> bucket_mask = num_bucket - 1, and do hashcode & bucket_mask.
>>
>> 2. the 1.5 factor
>> On my first review I pointed out that in one specific test for
>> shared_buffers
>> as a power of two the number of buckets in your patch ended up being half
>> as many as the previous version (maybe that is the correct way to go).
>> Or maybe you want at least to rewrite that line using integer arithmetic.
>>
>
> Regarding 1: Yes, the bitwise-AND should work and would most likely be
> more performant than using modulo.
> Regarding 2: Since the patched table does not allow inserting a new entry
> before deleting the stale entry for the same slot, there was no need to
> additionally allocate NUM_BUFFER_PARTITIONS buckets (the reason why the
> number of buckets for the master code was rounded up to 0x8000 instead of
> 0x4000 for 128MB SB). Could you elaborate more on what you mean by
> '[rewriting] that line using integer arithmetic?'
>

1.5 * num_bucket implicitly casts to floating point, and later back to
integer, that was not meant to be permanent.
A safer option would be (num_buckets | (num_buckets >> 1)), no wrap-around
if some day
this gets increased to UINT_MAX. Or simply num_buckets, without
multiplication at all.

Regards,
Alexandre

>