Re: Cache relation sizes? - Mailing list pgsql-hackers

On Tue, Aug 4, 2020 at 2:21 PM Thomas Munro <thomas.munro@gmail.com> wrote:
> On Tue, Aug 4, 2020 at 3:54 AM Konstantin Knizhnik
> <k.knizhnik@postgrespro.ru> wrote:
> > This shared relation cache can easily store relation size as well.
> > In addition it will solve a lot of other problems:
> > - noticeable overhead of local relcache warming
> > - large memory consumption in case of larger number of relations
> > O(max_connections*n_relations)
> > - sophisticated invalidation protocol and related performance issues
> > Certainly access to shared cache requires extra synchronization.But DDL
> > operations are relatively rare.
> > So in most cases we will have only shared locks. May be overhead of
> > locking will not be too large?
>
> Yeah, I would be very happy if we get a high performance shared
> sys/rel/plan/... caches in the future, and separately, having the
> relation size available in shmem is something that has come up in
> discussions about other topics too (tree-based buffer mapping,
> multi-relation data files, ...).  ...

After recent discussions about the limitations of relying on SEEK_END
in a nearby thread[1], I decided to try to prototype a system for
tracking relation sizes properly in shared memory.  Earlier in this
thread I was talking about invalidation schemes for backend-local
caches, because I only cared about performance.  In contrast, this new
system has SMgrRelation objects that point to SMgrSharedRelation
objects (better names welcome) that live in a pool in shared memory,
so that all backends agree on the size.  The scheme is described in
the commit message and comments.  The short version is that smgr.c
tracks the "authoritative" size of any relation that has recently been
extended or truncated, until it has been fsync'd.  By authoritative, I
mean that there may be dirty buffers in that range in our buffer pool,
even if the filesystem has vaporised the allocation of disk blocks and
shrunk the file.

That is, it's not really a "cache".  It's also not like a shared
catalog, which Konstantin was talking about... it's more like the pool
of inodes in a kernel's memory.  It holds all currently dirty SRs
(SMgrSharedRelations), plus as many clean ones as it can fit, with
some kind of reclamation scheme, much like buffers.  Here, "dirty"
means the size changed.

Attached is an early sketch, not debugged much yet (check undir
contrib/postgres_fdw fails right now for a reason I didn't look into),
and there are clearly many architectural choices one could make
differently, and more things to be done... but it seemed like enough
of a prototype to demonstrate the concept and fuel some discussion
about this and whatever better ideas people might have...

Thoughts?

[1]
https://www.postgresql.org/message-id/flat/OSBPR01MB3207DCA7EC725FDD661B3EDAEF660%40OSBPR01MB3207.jpnprd01.prod.outlook.com