Re: [PATCH] Identify LWLocks in tracepoints

On Tue, Apr 13, 2021 at 4:46 PM Andres Freund <andres@anarazel.de> wrote:
> I still don't like the two bytes, fwiw ;). Especially because it's 4
> bytes due to padding right now.

I'm not surprised by that disclosure. But I think it's entirely worth
it. Making wait states visible in pg_stat_activity isn't the most
useful thing I've ever done to PostgreSQL, but it's far from the least
useful. If we can get the same benefit with less overhead, that's
great.

> I'd like to move the LWLock->waiters list to outside of the lwlock
> itself - at most TotalProcs LWLocks can be waited for, so we don't need
> millions of empty proclist_heads. That way we could also remove the
> proclist indirection - which shows up a fair bit in contended workloads.
>
> And if we had a separate "lwlocks being waited for" structure, we could
> also add more information to it if we wanted to...
>
> The difficulty of course is having space to indicate which of these
> "waiting for" lists are being used - there's not enough space in ->state
> right now to represent that. Two possibile approaches:
>
> - We could make it work if we restricted MAX_BACKENDS to be 2**14 - but
>   while I personally think that's a sane upper limit, I already had a
>   hard time getting consensus to lower the limit to 2^18-1.
>
> - Use a 64bit integer. Then we can easily fit MAX_BACKENDS lockers, as
>   well as an offset to one of MAX_BACKENDS "wait lists" into LWLock.

I'd rather not further reduce MAX_BACKENDS. I still think some day
we're going to want to make that bigger again. Maybe not for a while,
admittedly. But, do you need to fit this into "state"? If you just
replaced "waiters" with a 32-bit integer, you'd save 4 bytes and have
bits left over (and maybe restrict the tranche ID to 2^14 and squeeze
that in too, as you mention).

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