Waits, Enqueues, and Latches - Expert Oracle RAC Performance Diagnostics and Tuning - page 597

Place Holder Events

gc cr request
gc current request
gc cr multi block request
gc current multi block request

Block Oriented Message Oriented Resource Contention Load Congestion

gc cr block busy
gc current block busy
gc buffer busy
gc freelist
gc prepare
gc claim
wait for master scn

gc cr block congested
gc current block congested
gc cr block lost
gc current block lost

gc cr block 2-way
gc cr block 3-way
gc current block 2-way
gc current block 3-way
gc remaster

gc cr grant 2-way
gc current grant 2-way

Grant Busy

Resource Busy CPU Busy

gc cr grant busy
gc cr grant congested
gc current grant busy
gc current grant congested

gc cr cancel
gc cr failure
gc current cancel
gc current split

gc cr disk read
gc cr disk request
gc current retry

Figure 17-5. Wait events grouped 12

gc buffer busy
gc buffer busy waits are normally global-level issues of buffer busy waits noticed on single-instance Oracle
databases. Buffer busy waits are caused by frequent requests for buffer space, and Oracle is unable to acquire such
buffer space and goes into wait mode. Poorly written SQL statements that perform high LIO are the reasons for
high wait times.
In a RAC environment, multiple waiters maybe queuing for the same block that has a global operation pending
for both the requestor and holder of the block. The wait times could also be caused by high network latencies and
slow service times of the servers. A user trying to pin a buffer that is globally busy will have to wait until the block has
arrived and the waiter at the head of the queue has released the buffer or the remote user has given up the buffer.
Whereas in the earlier releases of Oracle, buffer busy waits in a RAC environment were grouped under this wait event,
starting with Oracle database 11g Release 2, the “ gc buffer busy ” waits are categorized into two new wait categories:

gc buffer busy acquire •

gc buffer busy release •

The classification here is based on if the request was made from the local instance of the requestor, in which case
the session will wait on “ gc buffer busy acquire .” If the request was made from the remote instance, the current
instance will wait on “ gc buffer busy release ” event.

12 Vallath, Murali. Oracle 10g RAC Grid, Services & Clustering . Digital Press, 2007.