On Wed, Feb 22, 2023 at 8:37 AM Nathan Bossart <nathandboss...@gmail.com> wrote: > 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...
Thanks very much for your careful review.
Yes, you are right. I think the formulas in the v1 patch are all approximations.
I think the exact formula (see function InitializeMaxBackends) is:
```
max_locks_per_transaction * (max_connections + autovacuum_max_workers +
1 +
max_worker_processes + max_wal_senders +
max_prepared_transactions)
```
After some rethinking, I think users can easily get exact value according to
exact formula, and I think using accurate formula can help users adjust
max_locks_per_transaction or max_predicate_locks_per_transaction if needed. So,
I used the exact formulas in the attached v2 patch.
Regards,
Wang Wei
v2-0001-Fix-the-description-of-shared-lock-table-size-and.patch
Description: v2-0001-Fix-the-description-of-shared-lock-table-size-and.patch
