Re: Configurable FP_LOCK_SLOTS_PER_BACKEND

Hi,

On 2023-08-08 16:44:37 -0400, Robert Haas wrote:
> On Mon, Aug 7, 2023 at 6:05 PM Andres Freund <andres@anarazel.de> wrote:
> > I think the biggest flaw of the locking scheme is that the LockHash locks
> > protect two, somewhat independent, things:
> > 1) the set of currently lockable objects, i.e. the entries in the hash table [partition]
> > 2) the state of all the locks [in a partition]
> >
> > It'd not be that hard to avoid the shared hashtable lookup in a number of
> > cases, e.g. by keeping LOCALLOCK entries around for longer, as I suggest
> > above.  But we can't, in general, avoid the lock on the partition anyway, as
> > the each lock's state is also protected by the partition lock.
> 
> Yes, and that's a huge problem. The main selling point of the whole
> fast-path mechanism is to ease the pressure on the lock manager
> partition locks, and if we did something like what you described in
> the previous email without changing the locking regimen, we'd bring
> all of that contention back. I'm pretty sure that would suck.

Yea - I tried to outline how I think we could implement the fastpath locking
scheme in a less limited way in the earlier email, that I had quoted above
this bit.  Here I was pontificating on what we possibly should do in addition
to that. I think even if we had "unlimited" fastpath locking, there's still
enough pressure on the lock manager locks that it's worth improving the
overall locking scheme.

Greetings,

Andres Freund