On Tue, Feb 22, 2011 at 12:55 PM, Robert Haas <robertmh...@gmail.com> wrote: > A little OT, but ISTM that the buffer pin mechanism by its nature is > prone to lock upgrade hazards. A cleanup lock is essentially an > access exclusive lock on the buffer, while a buffer pin is an access > share lock. In the middle, we have the usual share and exclusive > (content) locks. We regularly try to upgrade buffer pins to any of > the higher locking levels, which is quite unlike what we do with > regular (heavyweight) locks, where we take some pains to avoid lock > upgrades.
Except that pins don't block exclusive locks so there's no deadlock risk. The oddity here is on Vacuums super-exclusive "lock" which is the real equivalent of an "exclusive lock". However there's the added bonus that there can only be one vacuum on a table at a time. That makes it safe It might be interesting to have autovacuum skip a block it finds pinned for too long. Since it's willing to abort entirely if someone gets a conflicting lock skipping a block doesn't seem like a big deal. It won't let us advance the frozenxid but it'll speed up a subsequent vacuum. At some point I wonder if we'll find ourselves wanting to keep track of frozenxid per block and keep a kind of priority queue of blocks to vacuum based on their age. I continue to wonder whether we're reinventing standard garbage collection processes. Incidentally, even if we allowed multiple vacuum processes per table I think it could be coded to be safe as long as each vacuum only needs to acquire the super exclusive lock on a single block at a time and doesn't try to acquire other locks while holding it. -- greg -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
