On Fri, May 22, 2020 at 3:15 PM, Thomas Munro < thomas.mu...@gmail.com > wrote:
> > > > Predicate locks are released by ClearOldPredicateLocks(), which releases > SERIALIZABLEXACTs once they are no longer interesting. It has a > conservative idea of what is no longer interesting: it waits until the > lowest xmin across active serializable snapshots is >= the transaction's > finishedBefore xid, which was the system's next xid (an xid that hasn't > been used yet*) at the time the SERIALIZABLEXACT committed. One > implication of this scheme is that SERIALIZABLEXACTs are cleaned up in > commit order. If you somehow got into a state where a few of them were > being kept around for a long time, but others committed later were being > cleaned up (which I suppose must be the case or your system would be > complaining about running out of SERIALIZABLEXACTs), that might imply that > there is a rare leak somewhere in this scheme. In the past I have wondered > if there might be a problem with wraparound in the xid tracking for > finished transactions, but I haven't worked out the details (transaction > ID wraparound is both figuratively and literally the Ground Hog Day of > PostgreSQL bug surfaces). > > > > Thanks for the detailed reply, Thomas. Is SERIALIZABLEXACT transaction ID wraparound the same as global xid wraparound? The max transaction age in the db is ~197M [1] so I don't think we've gotten close to global wraparound lately. Would it be helpful to cross-post this thread to pgsql-bugs or further investigate on my end -Mike [1] superhuman@ production => select datname, datfrozenxid, age(datfrozenxid) from pg_catalog.pg_database; datname | datfrozenxid | age ---------------+--------------+----------- cloudsqladmin | 4173950091 | 169089900 template0 | 4266855294 | 76184697 postgres | 4173951306 | 169088685 template1 | 4266855860 | 76184131 superhuman | 4145766807 | 197273184
