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Reducing relation locking overhead

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: pgsql-hackers(at)postgreSQL(dot)org
Subject: Reducing relation locking overhead
Date: 2005-12-02 02:37:47
Message-ID: 20841.1133491067@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-hackers
In looking at the current pgbench results, I notice that one
considerable contribution to LWLock contention is access to the
heavyweight-lock manager.  Almost all of that comes from taking
relation-level locks, so we could cut down the contention if we could
reduce the number of distinct locks taken.  (The lmgr code already
avoids touching shared memory again if, say, you request another
AccessShareLock on a relation you've already AccessShareLocked in
the current transaction.)

I see several places where we might possibly make this kind of
savings:

1. In an UPDATE or DELETE query, we take RowExclusiveLock on the target
relation, and also take AccessShareLock in the scan node that reads the
relation.  It wouldn't take much extra code to make the scan nodes avoid
taking AccessShareLock if the relation is already opened as the query
result relation.  AFAICS there is no downside to this; any lock that
would conflict with AccessShareLock will also conflict with
RowExclusiveLock, so there's no real need to hold both lock types.
(Moreover, we already make a similar kind of optimization when accessing
system catalogs: those routines only take one lock not two.)
Does anyone have an objection to it?

2. In the same way, an index that is used to scan the target relation
will be locked in both RowExclusiveLock and AccessShareLock modes
(corresponding to its roles as both source and update target).  We could
avoid taking both lock types, but this would require some restructuring
because the code responsible for locking the indexes doesn't currently
have access to the EState to find out whether an index belongs to a
target relation.  What I'm thinking here is that maybe there's no point
in maintaining the read versus write distinction at all for indexes ---
we could just take AccessShareLock in both cases.  Any thoughts on the
pros and cons there?

3. We could also save some trips to the lock manager if we adopt the
same position for user indexes as we do for user tables, namely that
locks once taken are held till end of transaction.  Any thoughts about
that?

4. The only reason we need to take relation-level locks on indexes
at all is to make the world safe for REINDEX being done concurrently
with read-only accesses to the table (that don't use the index being
reindexed).  If we went back to requiring exclusive lock for reindex we
could forget all about both #2 and #3.  Particularly for updates of
relations with lots of indexes, this could be a pretty significant win.
However we'd definitely be giving up something that was seen as a
feature at one point, so I'm not sold on this idea ... unless someone
can see a way to reduce the overhead without giving up concurrent
REINDEX.

			regards, tom lane

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