Re: Calculating vm.nr_hugepages

Hello,

I'm writing an Ansible play which is to set the correct value for
vm.nr_hugepages on Linux servers where I hope to make Postgres make use
of huge pages.

However, I'm struggling to find the right formula.

I assume I need to find the same value as I get from running "postgres
-C shared_memory_size_in_huge_pages". I call that my target value.
Note: I cannot simply run "postgres -C ...", because I need my Ansible
play to work against a server where Postgres is running.

I've tried using the formula described at
https://www.cybertec-postgresql.com/en/huge-pages-postgresql/, but it
produces a different value than my target:

Using a shared_buffers value of 21965570048, like in Cybertec
Postgresql's example:
"postgres ... -C 21965570048B" yields: 10719
The formula from Cybertec Postgresql says: 10475

I've also tried doing what ChatGPG suggested:
Number of Huge Pages when shared_buffers is set to 1 GiB =
shared_buffers / huge_page_size
                     = 1073741824 bytes / 2097152 bytes
                     = 512
But that's also wrong compared to "postgres -C ..." (which said 542).

Which formula can I use? It's OK for me for it to be slightly wrong
compared to "postgres -C", but if it's wrong, it needs to be slightly
higher than what "postgres -C" outputs, so that I'm sure there's enough
huge pages for Postgres to be able to use them properly.