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Wait Events in RAC Session 362

Wait Events in RAC Session 362. Arup Nanda Longtime Oracle DBA arup.blogspot.com. What is This?. RAC Performance Tuning I teach a course – Performance Tuning in RAC Wait events are useful for understanding the bottlenecks All single instance wait events are applicable to RAC

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Wait Events in RAC Session 362

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  1. Wait Events in RACSession 362 Arup Nanda Longtime Oracle DBA arup.blogspot.com

  2. What is This? • RAC Performance Tuning • I teach a course – Performance Tuning in RAC • Wait events are useful for understanding the bottlenecks • All single instance wait events are applicable to RAC • RAC has some special cases • This session talks about those RAC-specific wait events • This is just a subset of the events; not a comprehensive list Wait Events in RAC

  3. What’s a Wait? • A process in Oracle can only be in three states • Doing something Useful (consuming CPU) ….. U • Idle, waiting for some work to be assigned ….. I • Waiting for something, e.g. ….. W • a block from disk • a lock • a latch (could be waiting on CPU) • Response time = U + I + W • We must accurately measure each component time before we decide what and how to tune Wait Events in RAC

  4. Cluster Coordination Node 1 Node 2 Buffer Cache Buffer Cache SCN1 SCN2 DBWR DBWR Checkpoint! LMS LMS Checkpoint! Database DBWR must get a lock on the database block before writing to the disk. This is called a Block Lock. RAC for Beginners

  5. Cache Fusion Will get it via interconnect session1 session2 Instance 1 Instance 2 Wants to modify it Has modified it 5 5 Wait Events in RAC

  6. Checking for Buffers How exactly is this “check” performed? • By checking for a lock on the block • The request comes to the Grant Queue of the block • Someone checks that no other instance has any lock • Instance 1 can read from the disk • i.e. Instance 1 is granted the lock Block Grant Queue Convert Queue SID1 SID5 SID2 SID6 SID3 SID7 Wait Events in RAC

  7. Master Instance • Only one instance holds the grant and convert queues of a specific block • This instance is called Master Instance of that block • Master instance varies for each block • The memory structure that shows the master instance of a buffer is called Global Resource Directory (GRD) • That is replicated across all instances • The requesting instance must check the GRD to find the master instance • Then make a request to the master instance for the lock Block Grant Queue Convert Queue SID1 SID5 SID2 SID6 SID3 SID7 Wait Events in RAC

  8. Scenario 1 Session Instance 1 Instance 2 • Session connected to Instance 1 wants to select a block from the table • Activities by Instance 1 • Check its own buffer cache to see if the block exists • If it is found, can it just use it? • If it not found, can it select from the disk? • If not, then check the other instances • How will it know which copy of the block is the best source? DB Wait Events in RAC

  9. Cache Fusion Node 1 Node 2 Buffer Cache Buffer Cache SMON SMON LMS LMS message buffer When node 2 wants a buffer, it sends a message to the other instance. The message is sent to the LMS (Lock Management Server) of the other instance. LMS then sends the buffer to the other instance. LMS is also called Global Cache Server (GCS). Wait Events in RAC

  10. Grant Scenario 2 • Check its buffer cache to see if the block exists • And the buffer is found. Can Instance1 use it? Not really. The buffer may be old; it may have been changed • LMS of node1 sends message to master of the buffer • Master checks the GES and doesn’t sees any lock • Instance 1 is granted the global block lock • No buffer actually gets transferred Wait Events in RAC

  11. Grant Scenario 3 • Instance 1 is the master • Then it doesn’t have to make a request for the grant • In summary, here are the possible scenarios when Instance1 requests a buffer • Instance1 is the master; so no more processing is required • No one has the lock on the buffer, the grant is made by the master immediately • Another instance has the buffer in an incompatible mode. It has to be changed. Wait Events in RAC

  12. Buffer States and Locks • Buffers can be gotten in two states • Current – when the intention is to modify • Shared Current – most recent copy. One copy per instance. Same as disk • Exclusive Current – only one copy in the entire cluster. No shared current present • CR – when the intention is to only select • Locks facilitate the state enforcement • XCUR for Exclusive Current • SCUR for Shared Current • No locking for CR Wait Events in RAC

  13. Placeholder Event • When the buffer is first requested, the session does not know which of the three paths it will go on to • Therefore it is assigned a placeholder event • This event is known as gccr block request (for Consistent Read requests) • If the request is made for the buffer in Current mode, the event is gc current block request • When one of the three options is chosen, the appropriate event replaces the placeholder event Wait Events in RAC

  14. Grant Event • If the session merely requests a grant from the master: • It waits with the gccr|current grant 2-way event • gccr grant 2-way, for requests for buffers in Consistent Read mode • gc current grant 2-way, for requests for buffers in Current mode Wait Events in RAC

  15. gc current|cr grant 2-way Instance 1 Instance 2 Block gc current grant 2-way gc current block request db file scattered read Grant Queue Convert Queue Session request granted SID1 SID5 SID2 SID6 LMS LMS GRD GES SID3 SID7 Database Wait Events in RAC

  16. Block Event • After the request is made • and assuming that the buffer is not in the local cache • the buffer may be found in another instance • Requestor requests the buffer from the holding instance • Possibilities: • Holder is also the master of that buffer • Holder is not the master • If the holder is the master, the requesting session waits with the event gccr|current block 2-way Wait Events in RAC

  17. gc current|cr block 2-way Instance 1 Instance 2 Block gc current block 2-way gc current block request Grant Queue Convert Queue Session SID1 SID5 SID2 SID6 LMS LMS GRD GES SID3 SID7 Database Wait Events in RAC

  18. gc current|cr block 3-way Instance 1 Instance 2 gc current block 3-way Block Session Grant Queue Convert Queue Master SID1 SID5 Instance 3 SID2 SID6 SID3 SID7 Holder Requestor Wait Events in RAC

  19. Grant –vs- Get Events There is no 3-way grant event, since the request is made to the master There is no 4-way block event; since there will a maximum of 2 hops: requestor  master  holder Wait Events in RAC

  20. Interpretation Wait Events in RAC

  21. gc current/cr block lost • Lost blocks while being transferred to the remote instance [in the interconnect] • Cause: may not be the network itself • Cause #1 Network • Network traffic drops the packets • Confirm from the ifconfig -a output • If packets are dropping, this could be a cause • Why? • Bad network configuration • CPU used for network processing • Cause #2 CPU • LMS process is CPU starved Block Grant Queue Convert Queue Instance 1 Instance 2 gc current block 2-way gc current block request SID1 SID5 Session LMS LMS SID2 SID6 GRD GES Database SID3 SID7 Wait Events in RAC

  22. gccurrent/crblock busy • What it Means • Session wants a block from the remote instance • Remote instance delays preparing the block to fulfill the request [CR or current] read • Cause: • Local delay on the remote instance • Most likely: an I/O bottleneck on the remote instance • block is being accessed by some session which is CPU starved • LGWR has not written a buffer to redo yet • Less likely: CPU starvation Wait Events in RAC

  23. gc current/cr block congested • Meaning • The instance has requested block from remote instance. • Remote instance has prepared the block and shipped it • but it has not reached the requesting instance within 1 ms. • What could be cause – network bottleneck? • Not necessarily. Causes: • Long run queues, causing the LMS process to be delayed in processing the incoming block • Heavy paging due to memory deficiency. This causes the blocks to be paged in before being processed. Wait Events in RAC

  24. Putting it All Together • Every Oracle process is either • Doing some productive work • Waiting for some work to be given (idle) • Waiting for some resource • Understand the reason for the wait • Devise a plan accordingly • RAC related wait events are manifestations of these issues mostly • Caused by • Network issues • LMS being overloaded • Blocks busy Wait Events in RAC

  25. Thank You! Session 362 • More Information: • Blog: arup.blogspot.com • 100 Things You Probably Didn't Know About Oracle Database http://bit.ly/evr05e • Twitter: arupnanda Wait Events in RAC

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