Re: scalability bottlenecks with (many) partitions (and more)

On Tue, Jun 25, 2024 at 6:04 AM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> Yeah, definitely needs comment explaining this.
>
> I admit those numbers are pretty arbitrary primes, to implement a
> trivial hash function. That was good enough for a PoC patch, but maybe
> for a "proper" version this should use a better hash function. It needs
> to be fast, and maybe it doesn't matter that much if it's not perfect.

Right. My guess is that if we try too hard to make the hash function
good, there will be a performance hit. Unlike, say, strings that come
from the user, we have no reason to believe that relfilenumbers will
have any particular structure or pattern to them, so a low-quality,
fast function seems like a good trade-off to me. But I'm *far* from a
hashing expert, so I'm prepared for someone who is to tell me that I'm
full of garbage.

> I don't think I've heard the term "partially-associative cache" before
> That's an excellent question. I don't know.
>
> I agree 64 groups is pretty arbitrary, and having 1024 may not be enough
> even with a modest number of partitions. When I was thinking about using
> a higher value, my main concern was that it'd made the PGPROC entry
> larger. Each "fast-path" group is ~72B, so 64 groups is ~4.5kB, and that
> felt like quite a bit.
>
> But maybe it's fine and we could make it much larger - L3 caches tend to
> be many MBs these days, although AFAIK it's shared by threads running on
> the CPU.
>
> I'll see if I can do some more testing of this, and see if there's a
> value where it stops improving / starts degrading, etc.

Sounds good.

--
Robert Haas
EDB: http://www.enterprisedb.com