On Wed, Aug 30, 2023 at 8:12 AM Troels Arvin <troels@xxxxxxxx> wrote:
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.
Good morning Troels,
I had a similar thread a couple of years ago, you may want to read:
In it, Justin Przby provides the detailed code for exactly what factors into HugePages, if you require that level of precision.
I hadn't seen that Cybertec blog post before. I ended up using my own equation that I derived in that thread after Justin shared his info. The chef/ruby code involved is:
if shared_buffers_size > 40000
padding = 500
end
shared_buffers_usage = shared_buffers_size + 200 + (25 * shared_buffers_size / 1024)
max_connections_usage = (max_connections - 100) / 20
wal_buffers_usage = (wal_buffers_size - 16) / 2
vm.nr_hugepages = ((shared_buffers_usage + max_connections_usage + wal_buffers_usage + padding) / 2).ceil()
max_connections_usage = (max_connections - 100) / 20
wal_buffers_usage = (wal_buffers_size - 16) / 2
vm.nr_hugepages = ((shared_buffers_usage + max_connections_usage + wal_buffers_usage + padding) / 2).ceil()
wal_buffers_size is usually 16MB so wal_buffers_usage ends up being zeroed out. This has worked out for our various postgres VM sizes. There's obviously going to be a little extra HugePages that goes unused, but these VMs are dedicated for postgresql usage and shared_buffers_size defaults to 25% of VM memory so there's still plenty to spare. But we use this so we can configure vm.nr_hugepages at deployment time via Chef.
Don.
Don Seiler
www.seiler.us
www.seiler.us
