Waits, Enqueues, and Latches - Expert Oracle RAC Performance Diagnostics and Tuning - page 593

Figure 17-3 illustrates four sections of the wait event. Section 1 is a basic identifier, which shows if the wait event
is RAC related. RAC related wait events start with an identifier of gc . The second section differentiates what kind
buffer the wait event is related to: current or consistent read ( cr ). The third section provides an indication on the type
of message the wait event will cover: if the wait event is block activity related or privilege and access related such as
grants. The fourth section of the event name indicates/reflects on the type of operation, such as network-related hops
(two way and three way), busy or congested.
As discussed earlier, wait event information can be obtained by querying three primary views: GV$SYSTEM_EVENT ,
GV$SESSION_EVENT , and GV$SESSION_WAIT . The V$SESSION_WAIT event has a few important columns, described in
Table 17-3 .

Table 17-3. Structure of V$SESSION_WAIT View

Column Name Description

WAIT_CLASS

Name of class the event is grouped under

EVENT Name of event

P1 File #

P2 Block #

P3 Mode requested/mode held/block class

SECONDS_IN_WAIT

Amount of time waited for the current event

STATE Current wait state

The following query will extract the preceding information for the current wait events:

SELECT inst_id,
sid,
event,
wait_class,
p1,
p2,
p3,
seconds_in_wait
FROM gv$session_wait
WHERE event LIKE 'gc%'

INST_ID SID EVENT WAIT_CLASS P1 P2 P3 SWAIT
---------- ----- ------------------------ -------------- ---------- ---------- ---------- -----
2 37 gc current request Cluster 7 306 33554433 0
2 240 gcs log flush sync Other 30 0 138 0
2 269 gc buffer busy acquire Cluster 7 306 1 0
2 280 gc buffer busy release Cluster 7 159 1 0
1 27 gc buffer busy acquire Cluster 7 159 1 0
1 244 gcs log flush sync Other 30 0 138 0
1 265 gc current request Cluster 7 159 33554433 0