Detect if postgres was started without huge pages when huge_page = try

[blogh]

Sometimes we cannot afford to not start if we don't have huge pages.
It would be neat to have a service to detect when we couldn't start with the huge pages when huge_page = try.

[rjuju]

I'm a bit dubious about this one. Is there a way to know if postgres could map its shmem with huge pages at the sql level? If not, it means that it's a check that would only be possible is check_pgactivity is run locally which greatly limits the usefulness.

[Krysztophe]

I suppose this is not something visible from pg_shmem_allocations ?

Anyway: if we have to dig into /proc, that would not be the 1st service that needs to run on the DB server.

[blogh]

I found this : https://access.redhat.com/solutions/320303?sc_cid=cp
It's not usable everywhere for instance RH 5 has no KernelPageSize.

It whould be neat to be able to fetch this info directly from postgres.

[rjuju]

I'm afraid that it's not that reliable. It's probably ok for a postgres dedicated server, but as soon as you have other services (including other postgres instances), you can't use those numbers anymore.

I agree that having a way to have an SQL API for that in postgres would be nice. There was a thread about raising the log level for the messages about failed attempt to use huge pages a few days ago and I already tried to add this point to the discussion.

[blogh]

great ! thanks

[Krysztophe]

In PG15, we now have shared_memory_size and shared_memory_size_in_huge_pages.

Assuming a single instance on a dedicated server, the service could check that most SB are huge pages.
(ie checking that shared_memory_size_in_huge_pages * huge_page_size = shared_buffers,
with a 10% tolerance just in case?)

With Huge pages, I always fear that PG does not find the HP for whatever reason, and use only 4k pages, wasting memory.
Having such an alert would be less radical than huge_pages = on and a failure to start.

Problem: PG does not return the HP size value, huge_page_size is 0 by default.
It could be a parameter of the service, it could be autodetected in /proc/meminfo, or the service could simply fail if huge_page_size is not defined.

[frost242]

I'm not confident that shared_memory_size_in_huge_pages could be used to determine if PostgreSQL was started with all shared memory in huge pages. This GUC is just here to tell how much huge pages a PostgreSQL instance will need to allocate, it's just a helper to tune the system.

[Krysztophe]

You-re right, shared_memory_size_in_huge_pages is the desired size, not what is in HP.

[rjuju]

FTR upstream finally decided to properly report whether huge pages are used or not, see postgres/postgres@a14354c.

We will be able to write a proper check for that starting with pg 17.

[Krysztophe]

That service should be possible with PG17 : https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=a14354cac

[rjuju]

Isn't that the exact same commit I mentioned a year ago?