"Cross Instance call Enqueue"是一种在一个或多个instance实例间调用后台进程行为时用到的队列锁,具体调用的后台进程行为包括检查点checkpoint、日志切换logfile switch、shutdown实例、载入数据文件头等等。需要注意的是这种Enqueue Lock并不仅仅在RAC中使用，即便是单节点也会用到。CI锁的数量取决于并行执行Cross Instance Call调用的进程的总数。

SQL> col ksqsttyp for a20
SQL> col ksqstrsn for a20
SQL> col ksqstexpl for a80
SQL> set linesize 200 pagesize 2000;
SQL> select ksqsttyp,ksqstrsn,ksqstexpl from x$ksqst where ksqsttyp='CI';

KSQSTTYP             KSQSTRSN             KSQSTEXPL
-------------------- -------------------- --------------------------------------------------------------------------------
CI                   contention           Coordinates cross-instance function invocations

SQL> show parameter cluster_database

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
cluster_database                     boolean     FALSE
cluster_database_instances           integer     1


SQL> select * from v$enqueue_stat where eq_type='CI';

   INST_ID EQ TOTAL_REQ# TOTAL_WAIT#  SUCC_REQ# FAILED_REQ# CUM_WAIT_TIME
---------- -- ---------- ----------- ---------- ----------- -------------
         1 CI        595           0        595           0             0

当系统中出现有大量这种跨实例后台进程调用时，将出现CI队列锁的争用。假设在一个RAC场景中，同时有大量的回话开始对不同的数据表执行TRUNCATE截断操作，TRUNCATE的一个前提是在所有实例上(因为对象表的dirty buffer可能分布在多个实例上)发生对象级别的检查点(object level checkpoint)，检查点发生时CKPT进程会通知DBWR写出指定对象表相关的脏块，DBWR需要扫描Buffer Cache以找出脏块，而如果Buffer Cache很大那么扫描将花费大量的时间，而在此过程中前台进程将一直排他地持有着本地的CI队列锁，这就将造成CI锁的严重争用。 为了减少CI队列锁地争用，我们第一步所要做的是找出实际的Cross Instance call跨实例调用的类型。这里要另外提一下的是在10g以前不管是v$session_wait或statspack中都不会将enqueue锁等待事件的具体enqueue lock类型写明，一般需要我们从p1/p2/p3列中找出enqueue的具体身份，例如"WAIT #1: nam='enqueue' ela= 910796 p1=1128857606 p2=1 p3=4"，这里的p1为1128857606也就是16进制的43490006，高位的'4349'转换为ascii码也就是'CI'，而这里的p2/p3对应为V$lock中的ID1/ID2，ID1=1代表了"Reuse (checkpoint and invalidate) block range"，ID2=4代表了"Mounted excl, use to allocate mechanism"。 具体ID1/ID2代表的含义在不同版本中有所变化，可以参考下表:

Id1, Id2 Combination:

   Oracle 10gR1

27 TO 29  *Same as 9i R2

                30    process waiters after row cache requeue
                31    Active Change Directory extent relocation
                32    block change tracking state change
                33    kgl mulitversion obsolete
                34    set previous resetlogs data
                35    set recovery destination pointer
                36    fast object reuse request
                37    test ksbcic()
                38    ASM diskgroup discovery wait
                39    ASM diskgroup release
                40    ASM push DB updates
                41    ASM add ACD chunk
                42    ASM map resize message
                43    ASM map lock message
                44    ASM map unlock message (phase 1)
                45    ASM map unlock message (phase 2)
                46    ASM generate add disk redo marker
                47    ASM check of PST validity
                48    ASM offline disk CIC
                49    Logical Standby Sync Point SCN
                50    update SQL Tuning Base existence bitvector
                51    PQ induced Checkpointing
                52    ASM F1X0 relocation
                53    Scheduler autostart
                54    KZS increment grant/revoke counter
                55    ASM disk operation message
                56    ASM I/O error emulation
                57    DB Supp log cursor invalidation
                58    Cache global range invalidation
                59    Cache global object invalidation
                60    ASM Pre-Existing Extent Lock wait
                61    Perform a ksk action through DBWR
                62    ASM diskgroup refresh wait

   Oracle 10gR2

       30 to 62 *Same as 10gR1

               63    KCBO object checkpoint
               64    KCBO object pq checkpoint
               65    global health check event
               66    Oracle Label Security refresh
               67    thread internal enable
               68    cross-instance registration
               69    KGL purge unused subheaps
               70    clear pin instance flag
               71    Rolling operations CIC

   Oracle 9iR2

                 Id1   Meaning
                 ~~~   ~~~~~~

             25 TO 26  *Same as 9i R1

                 27    set Database Force Logging mode
                 28    invalidate cached file address translations
                 29    Cursor Unauthorize Mode
                 30    snapshot too old diagnosis
                 31    process waiters after row cache requeue

   Oracle 9iR1

                Id1    Meaning
                ~~~    ~~~~~~~~

             18 TO 24  *Same as Oracle 8i

                25     Update Dscn Tracking (ktcndt)
                26     Purge dictionary Object number Cache

   Oracle 8i
                Id1     Meaning
                ~~~     ~~~~~~~

             0 TO 17    *Same as Oracle 8(please see "Oracle8*" for the Meaning)

                18      Object reuse request
                19      Rolling release checks
                20      Propagate begin backup scn for a file
                21      Refresh top plan (for db scheduler)
                22      Clear checkpoint progress record
                23      Drop temp file
                24      Quiesce database Restricted

               Id2      Meaning
               ~~~      ~~~~~~~

               0x01     Used to pass in parameters
               0x02     Used to invoke the function in backgroud process
               0x03     Used to indicate the foreground has not returned
               0x04     Mounted excl, use to allocate mechanism
               0x05     Used to queue up interested clients

    Oracle 8*

		Id1 	Meaning
		~~~	~~~~~~~
                0       Checkpoint block range
                1       Reuse (checkpoint and invalidate) block range
                2       LGWR Checkpointing and Hot Backup
                3       DBWR syncronization of SGA with control file
                4       Log file add/drop/rename notification
                5       Write buffer for CR read
                6       Test call
                7       Invalidate KCK cache in all instances
                8       Alter rollback segment optimal
                9       Signal Query Servers/coordinator
                10      Create Remote parallel query Server
                11      Set Global Partitions
                12      Stop Disk Writes
                13      Drop Sort Segments
                14      Release unused space from Sort Segments
                15      Instance Recovery for Parallel operation Group
                16      Validate parallel slave Lock Value
                17      Check transaction state objects
                18      Flush blocks in object
                19      Rolling release checks
                20      Propagate begin backup scn for a file
                21      Clear checkpoint progress record

     Oracle 7

		Id1 	Meaning
		~~~	~~~~~~~
		0 	Flush buffers for reuse as new class
		1 	LGWR checkpointing and Hot Backup
		2 	DBWR synchronization of SGA with control file
		3 	Log file add/drop/rename notification
		4 	Write buffer for CR read
		5 	Test Call
		6 	Invalidate KCK cache in all instances
		7 	Alter rollback segment optimal
		8 	Signal Query Servers/coordinator
		9 	Create Remote Parallel Query Server
		10 	Set Global Partitions
		11 	Stop Disk Writes
		12 	Drop Sort Segments
		13 	Release unused space from Sort Segments
		14 	Instance Recovery for Parallel operation Group
		15 	Validate parallel slave Lock Value
		16 	Check Transaction State Objects

		Id2 	Meaning
		~~~	~~~~~~~
		1 	Pass in Parameters
		2 	Invoke the call in background process
		3 	Foreground has not returned yet
		4 	Used to allocate the CI call
		5 	Used to queue up interested clients