Tuning Global Cache - Expert Oracle RAC Performance Diagnostics and Tuning - page 382

LMD n
The global enqueue service deamon ( LMD n ) is a background process that manages enqueue requests. It processes
incoming enqueue message requests and controls access to global enqueues. The LMD process also handles global
deadlock detection and remote resource requests (remote resource requests are requests originating from another
instance). Examples of enqueues encountered in a RAC environment are the following:

• TX—Transaction lock: will be acquired for the following reasons:

• When a row is locked. TX Transaction lock is acquired when a transaction initiates its first

change and is held until the transaction does a COMMIT or ROLLBACK . It is used mainly as a
queuing mechanism so that other sessions can wait for the transaction to complete.

• Every transaction that needs to modify a block requires a slot in the ITL (interested

transaction list) of the block. If there are no slots in the ITL of the block, then the
requesting session will wait on one of the active transaction locks.

• IS—Instance State: can be encountered during enqueue operations to synchronize instances.

LCK0
The lock process ( LCK0 ) manages global requests for the row cache and library cache and is responsible for
cross-instance broadcasts. Its major use is for the row cache and library cache and for some cross-instance calls
(which are basically remote function invocations on all instances in the cluster). With LMD and LCK , the responsibility
for global enqueues is shared between the two. None of them is managing the buffer cache. This handles all requests
for resources other than data blocks.

LMHB
Global Cache/Enqueue Service Heartbeat Monitor ( LMHB ) was introduced in Oracle Database 11g Release 2 and
monitors the heartbeat of LMON , LMD , and LMS n processes to ensure they are running normally without blocking or
spinning.

ACMS
Atomic Control File to Memory Service (ACMS) introduced in Oracle Database 11g Release 2 is an agent that
contributes to ensuring distributed SGA memory updates. It is globally committed across SGAs on success or globally
aborted/rolled back if a failure occurs. This coordinates consistent updates to a control file resource with its SGA
counterpart on all instances in an Oracle RAC environment.
The ACMS process works with a coordinating caller to ensure that an operation is executed on every instance in
Oracle RAC despite failures. ACMS is the process in which a distributed operation is called. As a result, this process
can exhibit a variety of behaviors. In general, ACMS is limited to small, non-blocking state changes for a limited set of
cross-instance operations.