Philip Warner <[EMAIL PROTECTED]> writes: > At 11:04 PM 3/05/2004, Tom Lane wrote: >> How confident are you in those "processes"? I don't know of any other >> mechanism for 'tuple concurrently updated' failures in ANALYZE than >> concurrent analyze runs ...
> Fairly. In this particular instance the error was probably caused bu a > manually run VACUUM (part of me stressing it to encourage the error). Yeah, I see a process 14295 in your dump that seems to be trying to ANALYZE (at least, it's got write lock on pg_statistic...). 8631 is the incumbent ANALYZE, and it's got locks all over the place :-( I think what we have here is an evil side-effect of the change a couple versions back to allow standalone ANALYZE to run as a single transaction. A database-wide ANALYZE will therefore accumulate AccessShareLock on each table it touches, and it won't release these locks until commit. So the scenario goes like this: 1. Somewhere relatively early in its run, ANALYZE processes pg_statistic. So it's now holding AccessShareLock on pg_statistic. 2. As the ANALYZE proceeds, it issues sinval messages due to the updates it's making in pg_statistic. This is normal. There will be (at least) one such message per column analyzed, and it sounds like your database has many columns. Plus of course other catalog updates could be occurring in other backends. 3. There is at least one other backend sitting idle and therefore not reading sinval messages. So eventually the sinval queue gets 70% full and the SIGUSR2 escape-hatch is triggered. 4. The idle backends (and eventually non-idle ones, too, whenever they next reach the idle loop) try to do the NOTIFY thing, and get blocked trying to acquire AccessExclusiveLock on pg_listener. They will now be stuck until the ANALYZE completes. As a quick-hack fix, I think it would do to reduce the locks taken on pg_listener in async.c from AccessExclusiveLock to ExclusiveLock. This would serve the purpose of serializing async.c processing without creating a conflict against ANALYZE's AccessShareLock. Some other things we ought to think about for the future: * Is it really a good idea for database-wide ANALYZE to run as a single transaction? Holding all those locks is a recipe for deadlocks, even if they're as inoffensive as AccessShareLocks normally are. * Can we use something less heavyweight than ProcessIncomingNotify to deal with the sinval-clearing problem? Not only is that routine expensive, but it is a serialization bottleneck, which is exactly what we *don't* want in something that all the backends are getting told to do at the same time. I think the original motivation for that hack was because we didn't have a spare signal number available to dedicate to sinval response, but SIGUSR1 has been free for a couple releases now. I'm very tempted to commandeer it for sinval. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 7: don't forget to increase your free space map settings