Understanding Locks and Enqueues

How to Approach Common Blocking Lock Scenarios

By Mark J. BobakNovember 20th, 2008

Introduction

• Senior Oracle DBA at ProQuest Company• Working with Oracle since version 6• Present regularly at the local, national and

international level• Member of the OakTable Network since 2001

Disclaimer

• At least one statement in this presentation is incorrect, but it may be this one.

Agenda• What is an Enqueue?– Difference between locks, enqueues, and latches

• Characteristics of Enqueues• Most Common Types of Enqueues– TM – Table Modification Enqueue– TX – Transaction Enqueue

• Wait Scenarios of TM and TX enqueues• Anatomy of a row-level Lock

What is an Enqueue?

• An Enqueue protects shared resources from concurrent, incompatible accesses– Prevent two sessions from writing the same row

of a table at the same time– Facilitates enforcement of parent/child locking for

referential integrity– Prevent two sessions from updating the definition

of a table (drop/add column, etc) at the same time

What is an Enqueue? (p.2)

• Enqueues differ from latches:– Latches are the Database’s version of a mutex– Whereas latches (mostly) provide for mutually

exclusive access, enqueues allow for shared access, if mode is compatible

– Enqueues allow for enqueueing, that is, sessions waiting for access to an enqueue will queue in a line

– In the case of latches, sessions waiting for access must spin or sleep, and there’s no guarantee who will get the latch next

What is an Enqueue? (p.3)

• An Enqueue has three lists:– Holder: List of sessions that currently hold the

enqueue, and in what mode– Waiter: List of sessions that are waiting to acquire

the enqueue in a mode incompatible with what’s being held

– Converter: List of sessions that are currently holding the enqueue in one mode, and waiting to convert their lock to a more restrictive mode, incompatible with a mode already held by some other session

Ok, but what’s an Enqueue, really??

• An enqueue is an element in an array of enqueues that are allocated at SGA creation time. (Visible in X$KSQEQ, number of elements controlled by _enqueue_locks)

• It’s a state object, like so many other components in the SGA. (process state objects, session state objects, enqueue state objects, etc.)

Most Common Enqueue Types• List of most common enqueues– CF – Controlfile– DM – Database Mount– MR – Media Recovery– RS – ?? (not documented in list of enqueue names for 10g

or 11g)– RT – Redo Thread– TM – DML– TO – Temporary Table Object– TS – Temporary Segment or Tablespace– TX – Transaction– UL – User Lock

Most Common Enqueues

• Though there are many common enqueues, usually only TM and TX create problems for applications and developers.

• TM – DML, or table enqueue• TX – Transaction enqueue

TM Enqueue

• The TM or DML enqueue is taken at the table level, by any session doing DDL or DML.– DDL takes TM enqueue in ‘X’ (exclusive) mode,

which prevents other sessions doing any DML or DDL while the table definition is changing

– DML takes TM enqueue in ‘S’ (shared) mode, which allows for concurrent DML on the same table, but locks out DDL.

TM Enqueue (p. 2)

• Is used when foreign key relationships are enforced

• Locking strategy is much more aggressive in the case when indexes on the child table’s foreign key columns are missing.

• Missing indexes on child table foreign key columns is a common cause of waits and deadlocks.

TM Enqueue (p.3)

• One exception is direct load insert. In the case of direct load insert, a session will take the TM enqueue in ‘X’ mode. This prevents any other DML from taking place while the direct load is happening, in addition to blocking all DDL.

• TM enqueues may be disabled– At the instance level, via dml_locks=0 (If RAC, all

instances must have same value of dml_locks)– On a per table basis, with ‘alter table … disable locks;’

TM Enqueue (p. 4)

• There are two different wait scenarios you’ll likely encounter with the TM enqueue.

• ‘X’ mode waiting on ‘X’ mode being held.– This is likely to be a session holding ‘X’ mode

during direct load insert, while another session attempts to initiate a direct load into the same table.

• ‘SSX’ mode waiting on ‘SX’ mode being held.– This is due to unindexed foreign keys

TX Enqueue

• The TX or Transaction Enqueue is at the heart of Oracle’s row locking mechanism.

• The TX enqueue points to an undo segment header, slot within that header, and wrap number.

• Before images of any changes are stored in the body of that undo segment, and point to that segment header/slot/wrap.

TX Enqueue (p. 2)

• When a transaction commits, the only action that’s guaranteed to happen is that the undo segment slot is marked committed, and the TX enqueue is released.

• Many different wait scenarios may be observed, each with a different root cause.

TX Enqueue (p. 3)

• In every (known) case, the TX enqueue will be held in ‘X’ mode.

• If a session is waiting to acquire a TX enqueue in ‘X’ mode, it’s simple row-level locking or PK/UK enforement on an existing row.

• If a session is waiting to acquire TX enqueue in ‘S’ mode, there are several possibilities.– ITL slot shortage

TX Enqueue (p.4)

• If a session is waiting to acquire TX enqueue in ‘S’ mode, there are several possibilities. (cont.)– Bitmap index locking– Is the segment an IOT? If so, it’s simple row-level

locking– On insert of duplicate, uncommitted PK or UK– Very rarely, freelist contention (no longer

relevant if you’re using ASSM)

Monitoring Enqueues• V$LOCK– List of current enqueues, as well as sessions holding and

waiting on enqueues– Shows TYPE of enqueue, mode held, mode waited, as well

as ID1 and ID2 that help interpret information about the enqueue

• V$SESSION_WAIT– Can be used to quickly identify waiting session

• V$SESSION– Starting with 10g, V$SESSION has extended information,

including:• Wait information from V$SESSION_WAIT• And especially, BLOCKING_INSTANCE and BLOCKING_SESSION

Monitoring Enqueues (p. 2)

• If you’re still not on at least 10g, there are couple of other options– catblock.sql, available in

?/rdbms/admin/catblock.sql will create DBA_BLOCKERS, DBA_WAITERS, etc• As of 10g, still distributed, but, appears to be out of

date?

– Steve Adams’ locking scripts, available on his website, http://www.ixora.com.au/

Monitoring Enqueues (p. 3)

• Looking at V$LOCK, the TYPE column identifies the type of enqueue, i.e., TM, TX, UL, etc

• ID1 and ID2 may carry additional information, but are context-sensitive with respect to the enqueue TYPE

• For TM enqueue, ID1 is the OBJECT_ID of the object being locked, which can be referenced in DBA_OBJECTS, and ID2 is always 0

• For TX Enqueue, ID1 and ID2 hold the undo segment number, slot number, and wrap.

Monitoring Enqueues (p. 4)

• For TX enqueue, ID1 and ID2 must be decoded using this formula:– trunc(id1/power(2,16)) USN– bitand(id1,to_number('ffff','xxxx'))+0 SLOT– id2 SEQ/WRAP

Row-level Locking• The first DML in a session where a transaction does not

already exist will implicitly create a transaction.– An undo segment number, slot, and wrap will be assigned– TX enqueue (state object) will be instantiated

• When a row to be modified is identified, session will take an entry in the ITL of the data block, assign it to the transaction– USN/SLOT/WRAP will be written to ITL slot, reserving that

slot for the current transaction– Lock will be taken on the row, by setting the lock byte in

the row directory to point to the current transaction’s ITL slot

Demo Time!

• Row locking demo• If time, do more demos

Questions?

• Comments?• Criticisms?

More Information• Oracle Documentation

– http://tahiti.oracle.com/• Oracle MetaLink

– http://metalink.oracle.com/• OakTable Website

– http://oaktable.net/• Jonathan Lewis

– http://jonathanlewis.wordpress.com/– http://www.jlcomp.demon.co.uk/

• Steve Adam’s Website– http://www.ixora.com.au/

• Tom Kyte’s AskTom– http://asktom.oracle.com/