On Wed, Feb 15, 2023 at 08:16:43AM +0000, wangw.f...@fujitsu.com wrote: > When I refer to the GUC "max_locks_per_transaction", I find that the > description > of the shared lock table size in pg-doc[1] is inconsistent with the code > (guc_table.c). BTW, the GUC "max_predicate_locks_per_xact" has similar > problems. > > I think the descriptions in pg-doc are correct. > - GUC "max_locks_per_transaction" > Please refer to the macro "NLOCKENTS" used to obtain max_table_size in the > function InitLocks. > > - GUC "max_predicate_locks_per_xact" > Please refer to the macro "NPREDICATELOCKTARGETENTS" used to obtain > max_table_size in the function InitPredicateLocks.
The GUC description for max_locks_per_transaction was first added in b700a67 (July 2003). Neither the GUC description nor the documentation was updated when max_prepared_transactions was introduced in d0a8968 (July 2005). However, the documentation was later fixed via 78ef2d3 (August 2005). It looks like the GUC description for max_predicate_locks_per_transaction was wrong from the start. In dafaa3e (February 2011), the GUC description does not include max_prepared_transactions, but the documentation does. It's interesting that the documentation cites max_connections, as the tables are sized using MaxBackends, which includes more than just max_connections (e.g., autovacuum_max_workers, max_worker_processes, max_wal_senders). After some digging, I see that MaxBackends was the original variable used for max_connections (which was called max_backends until 648677c (July 2000)), and it wasn't until autovacuum_max_workers was introduced in e2a186b (April 2007) before max_connections got its own MaxConnections variable and started diverging from MaxBackends. So, even with your patch applied, I don't think the formulas are correct. I don't know if it's worth writing out the exact formula, though. It doesn't seem to be kept up-to-date, and I don't know if users would choose different values for max_locks_per_transaction if it _was_ updated. Perhaps max_connections is a good enough approximation of MaxBackends most of the time... -- Nathan Bossart Amazon Web Services: https://aws.amazon.com
