How to check session waits across all RAC instances in Oracle

GV$ views span every RAC instance. Joining GV$SESSION_WAIT with GV$SESSION gives you every wait across the cluster, ranked by how long each session has been stuck — the fastest way to spot cluster-wide contention.

SET LINESIZE 200
SET PAGESIZE 1000

COL username   FORMAT a20
COL event      FORMAT a30
COL wait_class FORMAT a15

SELECT s.inst_id,
       NVL(s.username, '(oracle)') AS username,
       s.sid,
       s.serial#,
       sw.event,
       sw.wait_class,
       sw.wait_time,
       sw.seconds_in_wait,
       sw.state
FROM   gv$session_wait sw,
       gv$session      s
WHERE  s.sid     = sw.sid
AND    s.inst_id = sw.inst_id
ORDER BY sw.seconds_in_wait DESC;