Waits, Enqueues, and Latches - Expert Oracle RAC Performance Diagnostics and Tuning - page 587

In a RAC environment, the LMD background process demonstrates enqueue activity. When more and more users
submit requests that are to be completed by the LMS process while the LMS is busy catering to other user requests,
the LMD will enqueue/queue these requests until such time that the LMS has completed its requests and is ready to
accept new requests. Some of the commonly encountered enqueues are listed in Table 17-1 .

Table 17-1. Enqueues

Enqueue Name Resource Description

TS Temporary Segment Contention Serializes accesses to temp segment

WF AWR Flush Contention The enqueue is used to serialize the flushing of

snapshots

CF Control file Transaction Contention Synchronizes accesses to the control file

PS PX Process Reservation Contention Parallel Execution Server Process reservation and

synchronization

KSV slave startup PV Syncstart Synchronizes slave start_shutdown

TX Transaction Index Contention Lock held on an index during a split to prevent

other operations on it

SQ Sequence cache Contention Lock to ensure that only one process can

replenish the sequence cache

PS PX Process Reservation Contention Parallel Execution Server Process reservation and

synchronization

DF Datafile Online in RAC Contention Enqueue held by foreground or DBWR when a

datafile is brought online in RAC

RD RAC Load RAC Load Update RAC load info

HW Segment High Water Mark Contention Lock used to broker the high water mark during

parallel inserts

We now discuss a few of the common enqueues found in a RAC environment.

TX—Transaction
As soon as a transaction is started, a TX enqueue is needed. A session can be waiting on a TX enqueue for
several reasons:

• row lock contention —Another session is locking the requested row. These locks are more
prominently visible in a RAC environment when multiple instances try to access the same row
from the block.

• index contention —When a transaction is inserting a row in an index, it would have to wait for
an index block split operation being done by another transaction.

• allocate ITL entry —There is no free ITL (interested transaction list) in the block header
because one or more other sessions have rows locked in the same block.

Among the various reasons listed, lack of ITLs could be the initial reason for high TX enqueues. Normally, ITLs
are added dynamically; however, there could be insufficient free space in the block to add an ITL.