Re: pg_lock_status() performance

Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> I have a unloaded development server running 8.4b1 that is returning
> from a 'select * from pg_locks' in around 5 ms. While the time itself
> is not a big deal, I was curious and tested querying locks on a fairly
> busy (200-500 tps sustained) running 8.2 on inferior hardware. This
> returned (after an initial slower time) in well under 1 ms most of the
> time. Is this noteworthy? What factors slow down best case
> pg_lock_status() performance?

> edit: I bet it's the max_locks_per_transaction parameter. I really
> cranked it on the dev box during an experiment, to 16384.
> testing...yup that's it. Are there any negative performance
> side-effects that could result from (perhaps overly) cranked
> max_locks_per_transaction?

[squint...] AFAICS the only *direct* cost component in pg_lock_status
is the number of locks actually held or awaited. If there's a
noticeable component that depends on max_locks_per_transaction, it must
be from hash_seq_search() iterating over empty hash buckets. Which is
a mighty tight loop. What did you have max_connections set to?

regards, tom lane