How to simulate 'out of shared memory' caused by max_pred_locks_per_transaction?

Question

Im using PostgreSQL 13.3 on x86_64-pc-linux-gnu, and Im trying to find the root cause of :

ERROR:  out of shared memory
HINT:  You might need to increase max_pred_locks_per_transaction.

So I test by using BEGIN ISOLATION LEVEL SERIALIZABLE; then query with conditions, the problem is that even the number of SIReadLock is larger than max_pred_locks_per_transaction*max_connections, I still can query, there is no 'out of shared memory' at all:

smart=*# show max_pred_locks_per_transaction;
 max_pred_locks_per_transaction 
--------------------------------
 10
(1 row)

smart=*# show max_connections;
 max_connections 
-----------------
 3
(1 row)

smart=*# SELECT count(*) FROM pg_locks WHERE mode = 'SIReadLock';
 count 
-------
   250
(1 row)

So please help me to indicate that SIReadLock is limited by which configuration ? and, which configuration and lock relate to 'You might need to increase max_pred_locks_per_transaction' ?

Thank you.

Answer 1

If you look at the code, you will find that the maximum number of predicate locks is calculated as

max_pred_locks_per_transaction * (
    max_connections +
    autovacuum_max_workers +
    1 +
    max_worker_processes +
    max_wal_senders + max_prepared_xacts
)