PGPROC alignment (was Re: pgsql: Separate RecoveryConflictReasons from procsignals)

(moving to pgsql-hackers)

On 10/02/2026 18:41, Andres Freund wrote:
> On 2026-02-10 17:52:16 +0200, Heikki Linnakangas wrote:
>> If there's a performance reason to keep have it be aligned - and maybe there
>> is - we should pad it explicitly.
>
> We should make it a power of two or such. There are some workloads where the
> indexing from GetPGProcByNumber() shows up, because it ends up having to be
> implemented as a 64 bit multiplication, which has a reasonably high latency
> (3-5 cycles). Whereas a shift has a latency of 1 and typically higher
> throughput too.

Power of two means going to 1024 bytes. That's a lot of padding. Where
have you seen that show up?

Attached is a patch to align to cache line boundary. That's
straightforward if that's what we want to do.

> Re false sharing: We should really separate stuff that changes (like
> e.g. pendingRecoveryConflicts) and never changing stuff (backendType). You
> don't need overlapping structs to have false sharing issues if you mix
> different access patterns inside a struct that's accessed across processes...

Makes sense, although I don't want to optimize too hard for performance,
at the expense of readability. The current order is pretty random
anyway, though.

It'd probably be good to move the subxids cache to the end of the
struct. That'd act as natural padding, as it's not very frequently used,
especially the tail end of the cache. Or come to think of it, it might
be good to move the subxids cache out of PGPROC altogether. It's mostly
frequently accessed in GetSnapshotData(), and for that it'd actually be
better if it was in a separate "mirrored" array, similar to the main xid
and subxidStates. That would eliminate the pgprocnos[pgxactoff] lookup
from GetSnapshotdata() altogether.

I'm a little reluctant to mess with this without a concrete benchmark
though. Got one in mind?

- Heikki