Re: Reducing relcache memory usage: deduping index shapes

> Performance, and memory overhead?
> Right now, we spend 88 of 200 bytes of the AllocSetContext itself on
> 11 freelists, and finer grained classes would mean larger freelists
> and larger overheads (assuming we don't want to shrink the current
> 8B-8kB range of freelist-supported chunk sizes).
> Additionally, powers of two are cheap to calculate vs arbitrary
> numbers, and this reduces the computational overhead and improves the
> branch-predictability of aset.

I hadn't realized that we're creating tons of memory contexts in
RelationData, e.g. one per index. Beyond wasting memory this is also
cluttering pg_backend_memory_contexts considerably.

I had a quick look if we can easily get rid of rd_indexcxt but there's
some memory from the function manager that is not directly referenced
and can only be implicitly freed by destroying rd_indexcxt.

Is your patch improving on that by also deduplicating the context? (I
haven't had the time to closely look at your patch.)
>> If that's the case we could alternatively do it manually and provide
>> some getter macros.
>
> Possible, yes, but at the cost of (possibly) large code changes to
> migrate to the new macros or inline functions, from direct field
> accesses.
> Whilst I do think saving bytes is worth something, I don't think we
> should be shaving bytes down at the cost of readability and/or
> backpatchability, especially when it's "just" a few bytes per
> relation. Even with 100s of 1000s of relations that'll "just" be a few
> MBs.

Agreed.

--
David Geier