gc buffer busy acquire and gc buffer busy release

gc buffer busy acquire and gc buffer busy release

The gc buffer busy acquire and gc buffer busy release wait events specify the time the remote instance locally spends accessing the requested data block. In Oracle 11g you will see gc buffer busy acquire wait event when the global cache open request originated from the local instance and gc buffer busy release when the open request originated from a remote instance. In Oracle 10g these two wait events were represented in a single gc buffer busy wait, and in Oracle 9i and prior the "gc" was spelled out as "global cache" in the global cache buffer busy wait event. These wait events are all very similar to the buffer busy wait events in a single-instance database and are often the result of:

Hot Blocks - multiple sessions may be requesting a block that is either not in buffer cache or is in an incompatible mode. Deleting some of the hot rows and re-inserting them back into the table may alleviate the problem. Most of the time the rows will be placed into a different block and reduce contention on the block. The DBA may also need to adjust the pctfree and/or pctused parameters for the table to ensure the rows are placed into a different block.

Inefficient Queries - as with the gc cr request wait event, the more blocks requested from the buffer cache the more likelihood of a session having to wait for other sessions. Tuning queries to access fewer blocks will often result in less contention for the same block.

http://docs.oracle.com/cd/E11882_01/rac.112/e16795/monitor.htm#RACAD8308

Contention-Related Wait Events

The main wait events for contention-related waits are:

• gc current block busy

• gc cr block busy

• gc buffer busy acquire/release

The contention-related wait event statistics indicate that a block was received which was pinned by a session on another node, was deferred because a change had not yet been flushed to disk or because of high concurrency, and therefore could not be shipped immediately. A buffer may also be busy locally when a session has already initiated a cache fusion operation and is waiting for its completion when another session on the same node is trying to read or modify the same data. High service times for blocks exchanged in the global cache may exacerbate the contention, which can be caused by frequent concurrent read and write accesses to the same data.

The gc current block busy and gc cr block busy wait events indicate that the local instance that is making the request did not immediately receive a current or consistent read block. The term busy in these events' names indicates that the sending of the block was delayed on a remote instance. For example, a block cannot be shipped immediately if Oracle Database has not yet written the redo for the block's changes to a log file.

 

这里 CR 和 current 是不同的概念，如果是读的话，那就是cr request，如果是更改的话，那就是current request。

 

In comparison to block busy wait events, a gc buffer busy event indicates that Oracle Database cannot immediately grant access to data that is stored in the local buffer cache. This is because a global operation on the buffer is pending and the operation has not yet completed. In other words, the buffer is busy and all other processes that are attempting to access the local buffer must wait to complete.

The existence of gc buffer busy events also means that there is block contention that is resulting in multiple requests for access to the local block. Oracle Database must queue these requests. The length of time that Oracle Database needs to process the queue depends on the remaining service time for the block. The service time is affected by the processing time that any network latency adds, the processing time on the remote and local instances, and the length of the wait queue.

The average wait time and the total wait time should be considered when being alerted to performance issues where these particular waits have a high impact. Usually, either interconnect or load issues or SQL execution against a large shared working set can be found to be the root cause.

Load-Related Wait Events

The main wait events for load-related waits are:

• gc current block congested

• gc cr block congested

The load-related wait events indicate that a delay in processing has occurred in the GCS, which is usually caused by high load, CPU saturation and would have to be solved by additional CPUs, load-balancing, off loading processing to different times or a new cluster node.For the events mentioned, the wait time encompasses the entire round trip from the time a session starts to wait after initiating a block request until the block arrives.