A rather odd performance issue happened at work a few days ago. All of a sudden, one of the databases started to work very slowly, and a quick look in ASH data showed that it was spending over 70% of its time waiting on “row cache lock” (not to be confused with “latch: row cache lock”).
It was a test database (220.127.116.11) on an overloaded server with non-standard configuratioin, so my initial reaction was to bounce the database and hope that the symptoms would go away, but it didn’t work. So there was no other choice but to investigate the root cause. The next simplest thing to “turning it off and then back on” was finding the blocking session in ASH (the V$ACTIVE_SESSION_HISTORY view has columns BLOCKING_SESSION and BLOCKING_SESSION_SERIAL#). Most of the entries for this wait event had NULLs in these columns, but among those NULLs there were sid’s and serial#’s of two sessions. Unfortunately, both sessions were from background processes, MMON and SMON, so killing them obviously wasn’t a tempting choice. MMON is responsible for collecting AWR data and such, so the instance could probably survive if it gets killed, but SMON must always be running or the instace would crash.