Re: reducing the overhead of frequent table locks - now, with WIP patch

On Mon, Jun 6, 2011 at 2:54 AM, Heikki Linnakangas
<heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> Ah, I remember I saw that vxid lock pop up quite high in an oprofile profile
> recently. I think it was the case of executing a lot of very simple prepared
> queries. So it would be nice to address that, even from a single CPU point
> of view.

It doesn't seem too hard to do, although I have to think about the
details. Even though the VXID locks involved are Exclusive locks,
they are actually very much like the "weak" locks that the current
patch accelerates, because the Exclusive lock is taken only by the
VXID owner, and it can therefore be safely assumed that the initial
lock acquisition won't block anything. Therefore, it's really
unnecessary to touch the primary lock table at transaction start (and
to only touch it at the end if someone's waiting). However, there's a
fly in the ointment: when someone tries to ShareLock a VXID, we need
to determine whether that VXID is still around and, if so, make an
Exclusive lock entry for it in the primary lock table. And, unlike
what I'm doing for strong relation locks, it's probably NOT acceptable
for that to acquire and release every per-backend LWLock, because
every place that waits for VXID locks waits for a list of locks in
sequence, so we could end up with O(n^2) behavior. Now, in theory
that's not a huge problem: the VXID includes the backend ID, so we
ought to be able to figure out which single per-backend LWLock is of
interest and just acquire/release that one. Unfortunately, it appears
that there's no easy way to go from a backend ID to a PGPROC. The
backend IDs are offsets into the "ProcState" array, so they give us a
pointer to the backend's sinval state, not its PGPROC. And while the
PGPROC has a pointer to the sinval info, there's no pointer in the
opposite direction. Even if there were, we'd probably need to hold
SInvalWriteLock in shared mode to follow it.

That might not be the end of the world, since VXID locks are fairly
infrequently used, but it's certainly a little grotty. I do rather
wonder if we should be trying to reduce the number of separate places
where we list the running processes. We have arrays of PGPROC
structures, and then we have one set of pointers to PGPROCs in the
ProcArray, and then we have the ProcState structures for sinval. I
wonder if there's some way to rearrange all this to simplify the
bookkeeping.

BTW, how do you identify from oprofile that *vxid* locks were the
problem? I didn't think it could produce that level of detail.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company