for multiple instance env, it is recommended to have instance_memory to hard value and database_memory to automatic and STMM ON.
Coz recently we faced a issue with multiple instance, server was crashing with swapping and it used to happen very quick and it was not captured in db2pd coz of reboot.
System admin said db2sysc of one of instance causing this. Both instance_memory was set to automatic. So if db2 only consider free memory it shld not go to swap right ? . may be wrong assumption. Please suggest
You may be exposed to an APAR related to leaking of memory. It may be unrelated to STMM. Post relevant facts (operating system details, DB2-server edition and fixpack details, whether TSA/hadr is involved or other cluster manager, the backup/logarchive solution), and the ulimits of the instance-owners (if Unix).
I've seen a number of times some "running out" application which suddenly started to consume enormous application heap size. Since APPL_MEMORY was automatic it led to OS swapping or even hang as in your case.
To catch a problematic situation you can configure to fire db2 FODC with db2fodc - DB2 first occurrence data collection command with -detect option.
It should help IBM support to investigate the root cause of the problem.