Re: Patch to address creation of PgStat* contexts with null parent context

Hi,

On 2022-08-05 17:22:38 +0900, Kyotaro Horiguchi wrote:
> I think it a bit different. Previously that memory (but for a bit
> different use, precisely) was required only when stats data is read so
> almost all server processes didn't need it. Now, every server process
> that uses pgstats requires the two memory if it is going to write
> stats. Even if that didn't happen until process termination, that
> memory eventually required to flush possibly remaining data. That
> final write might be avoidable but I'm not sure it's worth the
> trouble. As the result, calling pgstat_initialize() is effectively
> the declaration that the process requires the memory.

I don't think every process will end up calling pgstat_setup_memcxt() -
e.g. walsender, bgwriter, checkpointer probably don't? What do we gain by
creating the contexts eagerly?

> Thus I thought that we may let pgstat_initialize() promptly allocate
> the memory.

That makes some sense - but pgstat_attach_shmem() seems like a very strange
place for the call to CreateCacheMemoryContext().

I wonder if we shouldn't just use TopMemoryContext as the parent for most of
these contexts instead. CacheMemoryContext isn't actually a particularly good
fit anymore.

Greetings,

Andres Freund