Guenadi N Jilevski's Oracle BLOG


Common Problems and symptoms – Wait events worth investigation

Common Problems and symptoms –  Wait events worth investigation

Let’s look at some of the wait events which are worth further investigation as they represent a potential performance problem if the wait time is excessive or the event wait time is among the top 5 list in AWR report.

global cache blocks lost: This statistic shows block losses during transfers. High values indicate network problems. The use of an unreliable IPC protocol such as UDP may result in the value for global cache blocks lost being non-zero. When this occurs, take the ratio of global cache blocks lost divided by global cache current blocks served plus global cache cr blocks served. This ratio should be as small as possible. Many times, a non-zero value for global cache blocks lost does not indicate a problem because Oracle will retry the block transfer operation until it is successful.

global cache blocks corrupt: This statistic shows if any blocks were corrupted during transfers. If high values are returned for this statistic, there is probably an IPC, network or hardware problem.

global cache open s and global cache open x: The initial access of a particular data block by an instance generates these events. The duration of the wait should be short, and the completion of the wait is most likely followed by a read from disk. This wait is a result of the blocks that are being requested and not being cached in any instance in the cluster database. This necessitates a disk read. When these events are associated with high totals or high per-transaction wait times, it is likely that data blocks are not cached in the local instance and that the blocks cannot be obtained from another instance, which results in a disk read. At the same time, suboptimal buffer cache hit ratios may also be observed. Unfortunately, other than preloading heavily used tables into the buffer caches, there is little that can be done about this type of wait event.

global cache null to s and global cache null to x: These events are generated by inter-instance block ping across the network. Interinstance block ping is when two instances exchange the same block back and forth. Processes waiting for global cache null to s events are waiting for a block to be transferred from the instance that last changed it. When one instance repeatedly requests cached data blocks from the other RAC instances, these events consume a greater proportion of the total wait time. The only method for reducing these events is to reduce the number of rows per block to eliminate the need for block swapping between two instances in the RAC cluster.

global cache cr request: This event is generated when an instance has requested a consistent read data block and the block to be transferred had not arrived at the requesting instance. Other than examining the cluster interconnects for possible problems, there is nothing that can be done about this event other than to modify objects to reduce the possibility of contention.

gc cr block lost – This event almost always represents a severe performance problem and can reveal network congestion involving discarded packets and fragments, packet reassembly or timeouts, buffers overflows, flow control. Checksum errors or corrupted headers are also often the reason for the wait event. It is worth investigating the IPC configuration and possible downstream network problems (NIC, switch etc). Operating system data needs to be gathered with ifconfig, netstat and sar to name a few. ‘cr request retry’ event is likely to be seen when ‘gc cr blocks lost’ show up.

gc buffer busy: This event can be associated with a disk I/O contention for example slow disk I/O due to rogue query. Slow concurrent scans can cause buffer cache contention. However, note than there can be a multiple symptoms for the same cause. It can be seen together with ‘db file scattered reads’ event.  Global cache access and serialization attributes to this event. Serialization is likely to be due to log flush time on another node or immediate block transfers.

congested: The events that contain  ‘congested’ suggest CPU saturation (runaway or spinning processes), long running queues, network configuration issues. It indicates performance problems. While investigating need to maintain a global view and remember that symptom and cause can be on different instances. This event can also happen if LSM cannot dequeue messages fast enough. gcs_server_processes init parameter controls number of LMS processes although in most of the cases the default value is sufficient.  Excessive memory consumption leading to memory swapping can be another reason.

busy:  The events that contain ‘busy’ indicate contention. It needs investigation by drilling down into either SQL with highest cluster wait time or segment statistics with highest block transfers. Also look at objects with highest number of block transfers and global serialization.

Gc [current/cr] [2/3]-way – If  we have 2 node cluster  we cannot get 3-way as only two RAC instances are available and therefore only 2-way   is possible as we can have at most two hops. If we have three or more RAC instances then 2-way or 3-way is possible. Event are received after 2 or 3 network hops immediately. The event is not a subject to any tuning except increasing private interconnects bandwidth and decreasing the private interconnects latency.

Gc [current/cr] grant 2-way – Event when grant is received immediately. Grant is always local or 2-way. Grant occurs when a request is made for a block image current or cr and no instance have the image in its local buffer cache. The requesting instance is required to do an I/O from data file to get the blocks. The grant simply is a permission from the LMS this to happen that is, the process to read the block from the data file. Grant can be either cr or current. Gc current grant is go read the block from the database files, while gc cr grant is read the block from disk and build a read consistent block once is read. The event is not a subject to any tuning except increasing private interconnects bandwidth and decreasing the private interconnects latency.

Gc [current/cr][block/grant] congested – means that it has been received eventually but with a delay because of  intensive CPU consumption, memory lack, LMS overload due to much work in the queues, paging, swapping. This is worth investigating as it provides a room for improvement. We will look at it later.

Gc [current/cr] block busy – Received but not sent immediately due to high concurrency or contention. This means that the block is busy for example somebody issue block recover command from RMAN. Variety of reasons for being busy just means cannot be sent immediately but not because of memory, LMS or system oriented reasons but Oracle oriented reasons. It is also worth investigating and we will look at it later.

Gc current grant busy – Grant is received but there is a delay due to many shared block images or load. For example we are extending the high water mark and we are formatting the block images or blocks with block headers.

Gc [current/cr][failure/retry] –  Not received because of failure, checksum error usually in the protocol of the  private interconnect  due to network errors or hardware problems. This is something worth investigating. Failure means that cannot receive the block image while retry means that the problem recovers and ultimately the block image can be received but it needs to retry.

Gc buffer busy – time between block accesses less than buffer pin time. Pin buffers can be in exclusive or shared mode depending if buffers can be modified or read only. Obviously if there is a lot of contention for the same block by different processes this event can manifest itself in grater magnitude. Buffer busy are global cache events as a request is made from one instance and the block is available in another instance and the block is busy due to contention.

Perform a top down approach for performance analysis can be helpful. We can start with ADDM analysis then continue with AWR detail statistics and historical data and last but not least ASH will provide you with finer-grained session specific data.

December 13, 2009 - Posted by | oracle

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