Transcript Oracle Locks and Latches

Tuning Database Locks &
Latches
Hamid R. Minoui
Fritz Companies Inc.
NoCOUG May 16, 2001
The Challenge of Tuning
Oracle performance tuning requires a
good understanding of all the
components of a database system and the
way they operate and interact.
 This presentation addresses two types of
these components:

Database Locks and Latches
Need for locks and latches
To access shared resources concurrently by
other processes requiring access to the same
resources.
 To protect the contents of database objects
while they are being modified or inspected
by other processes
 To serialize access to SGA data structures

Locks & Latches

Oracle mechanisms for protecting and
managing SGA data structures and database
objects being accessed concurrently while
maintaining consistency and integrity
Differences between locks and
latches
Latches:


Provide only exclusive
access to protected
data structures
Request are not
queued, if a request
fails, process may try
later
Locks:


Allow serialized
access to some
resources
Requests for locks are
queued and serviced
in order
Locks & Latches
Latches:

Simple data structure

Protect resources that
are briefly needed
(LRU list)
Very efficient

Locks:



Complex data
structure that is further
protected by latch
Protect resources
needed for a longer
time (e.g. tables)
Less efficient
Categories of latches:
Solitary latches protecting one data
structure (majority of latches)
 Multiple latches protecting different parts of
a single data structure (grouped in a child-parent

relationship)

Latches protect locks (type varies depending on
type of locks)
Modes of latches

An Oracle process can request a latch in one
of two modes:
–
Willing-to-Wait Mode
If the requested latch is not immediately available,
the process will wait.
–
Immediate Mode (no-wait mode)
Then process will not wait if the requested latch is
not available and it continues processing
Latch free wait (spin & sleep)
1- Active wait or spin
–
When an attempt to get a latch in a willing-towait mode fails, the process will spin and try
again
2- Sleep
–
–
If the number of attempts reaches the value of
SPIN_COUNT parameter, the process sleeps
Sleeping is more expensive than spinning
Wakeup Mechanisms

Timeout
–

The operating system signals (wakes up) the
process when a set alarm is triggered
Latch wait posting
–
–
The next process to free the required latch will
wake up the process waiting for the latch
Initiated by the requesting process before going
to sleep by putting itself in a latch wait list
Benefit & cost of wait posting

Benefit:
–

The process is woken up as soon as the latch is freed
Cost:
– Requires protecting a latch wait list data
structure by yet another latch, namely latch
wait list latch
– When used extensively, it can result in a
secondary latch contention
Latch Contention

Latch contention has a significant impact on
performance when:
–
–


Enough latches are not available
A latch is held for a relatively long time
Latch contention can be resolved by increasing
specific init.ora parameters associated with latches
To detect latch contention latch statistics should be
examined
Dynamic Performance Views for
latches
Oracle collects statistics for the activity of
all latches and stores them in the dynamic
performance view V$LATCH.
 Latch statistics can be used to find
performance problems associated latch
contentions.

V$LATCH
 Each
row contains statistics for a specific type of
latch.
 Contains summary statistics for both non-parent
and parent latches grouped by latch number
(latch#).
 Should be the first point of reference when
investigating a suspecting latch contention.
Understanding the V$LATCH
Statistics
V$LATCH contains information such as:





GETS-Number of successful “willing-to-wait” requests for
a latch
MISSES- Number of times a “willing-to-wait” process had
to spin on the first try
SPIN_GETS - Number of times a latch is obtained after
spinning at least once
SLEEPS- Number of times a “willing-to-wait” process
slept
WAITERS_WOKEN- Number of times a wait was
awakened
V$LATCH Statistics (2)



WAITS_HOLDING - Number of waits while holding a
different latch
IMMEDIATE_GETS - Number of times obtained
without a wait
IMMEDIATE_MISSES - Number of times failed to get
without a wait
For the entire iterations for a latch request no more
than one gets, misses and spin_gets is recorded
 (gets-misses)
: Number of times a latch was obtained
without spinning at all
V$LATCHNAME


Holds information about decoded latch names
for the latches shown in V$LATCH
The rows of this view have one-to-one
correspondence to the rows of V$LATCH
Latches willing to wait
Query that shows the number of processes that had to
sleep, and the number of times they had to sleep.
This query is run by UTLESTAT.
SELECT name latch_name, gets, misses,
round((gets-misses)/decode(gets,0,1,gets),3) hit_ratio,
sleeps,
round(sleeps/decode(misses,0,1,misses),3) “sleeps/misses”
from
stats$latches
where gets != 0
order by name;
Evaluating the result
Hit_ratio:
Sleeps/Misses:
The ratio of gets to misses: (gets-misses)/gets
The ratio of sleeps to misses: sleeps/misses

Any latches that have a hit ratio below .99 should be
investigated.

Sleeps/misses is > 1 means there were processes that had to
sleep more than once before getting the latch

Increasing the parameter _LATCH_SPIN_COUNT can
increase the amount of CPU time a process will burn before
trying to acquire a latch (tunable in Oracle7)
Latches not willing to wait
For not willing-to-wait latches, the query the
immediate_gets and immediate_misses columns of the
v$latch view. It shows the statistics for not willing to
wait latches.
This query is run by UTLESTAT.
SELECT name latch_name, immed_gets nowait_gets,
immed_misses nowait_misses,
round((immed_gets/immed_gets+immed_misses),3) nowait_hit_ratio,
from
stats$latches
where immed_gets + immed_misses != 0
order by name;
Evaluating the result
nowait_gets - Number of times a request for a not-willingto-wait latch was successful
nowait_misses - Number of times a request for a notwilling-to-wait latch failed
nowait_hit_ratio - The ratio of nowait_misses to
nowait_gets: (nowait_gets - nowait_misses) / nowait_gets.
Nowait_hit_ratio should be as close to 1 as possible
V$LATCHHOLDER



Contains information about the current latch
holders.
Used to find the process (PID) & session (SID)
of the process and session holding the latch
identified by name (NAME) and address of the
latch (LADDR) being held.
In conjunction it with V$SESSION reveals the
identity of the user and process holding the
latch
V$LATCH_CHILDREN
These views contain statistics about child
latches and parent latches for multiple
latches
 Child latches with the same LATCH#
have the same parents
 The CHILD# column identifies the child
latch for the same parent

V$LATCH_PARENT
Has the same columns found in
V$LATCH
 The union of this view and
V$LACH_CHILDREN represents all
latches

V$LATCH_MISSES
Contains statistics about missed attempts to
acquire a latch
 NWFAIL_COUNT - Number of times that a
no-wait (immediate) acquisition of the latch
failed
 SLEEP_COUNT - Number of times that
acquisition attempts caused sleeps

Key Latches

Key latches impacting performance:
–
–
–
–
–
–
–
redo allocation
redo copy
cache buffers LRU
enqueues
row cache objects
library cache
shared pool
Latches using wait posting





By default latch-wait posting is enabled for the
library cache and shared pool latches
Wait posting can be entirely disabled by setting
_LATCH_WAIT_POSTING to 0 (default is 1)
Setting it to 2, enables it for all latches except for
cache buffers chains latch
Changing this parameter should be carefully
benchmarked
Disabling it can be beneficial where contention on
the library cache latch is severe
Sleeps Parameters

_MAX_EXPONENTIAL_SLEEP
–
–

The maximum duration of sleep (in seconds) under
an exponential back-off algorithm
default value is 2 second in Oracle8
_MAX_SLEEP_HOLDING_LATCH
–
–
The value to which maximum sleep time is
reduced, if the process is already holding other
latches
The default to 4 centiseconds
A sample query
To monitor the statistics for the redo allocation
latch and the redo copy latches:
SELECT name
“Latch”,
sum(gets)
“WTW gets”,
sum(misses)
“WTW misses”,
sum(immediate_gets)
“Immediate gets,
sum(immediate_misses)
“Immediate Misses”
FROM v$latch
WHERE name IN (’redo allocation’, ’redo copy’)
GROUP BY name
The redo allocation latch
• Controls the allocation of space for redo entries in
the redo log buffer.
• There is only one redo allocation latch to enforce the
sequential nature of the entries in the buffer.
•Only after allocation, the user process may copy the
entry into the buffer (copying on the redo allocation
latch).
• A process may only copy on the redo allocation latch
if the redo entry is smaller than a threshold size,
otherwise a redo copy latch is needed
The redo copy latch






Acquired before the allocation latch
Allocation latch is immediately released after
acquisition
User process performs the copy under the copy latch,
and releases the copy
User process does not try to obtain the copy latch
while holding the allocation latch.
Redo copy latch is released after the redo entry copy
System with multiple CPUs may have multiple redo
copy latches for the redo log buffer
Tuning redo allocation latch
Goal:
 Minimize the time that a process holds the latch
Achieved by:
 Reduce the frequency of “copying on the redo
allocation latch”.
How ?

Decrease LOG_SMALL_ENTRY_MAX_SIZE parameter
value which is the threshold for number and size of redo
entries to copied to redo allocation latch.
Tuning redo copy latch
Goal:
 Reduce contention on available copy latches
Achieved by:
 Adding more redo copy latches
How ?
 Set LOG_SIMULTANEOUS_COPIES up to twice
the number of CPUs
Cache buffer LRU latch





Controls buffers replacement in the buffer cache
Each LRU latch controls a set of buffers
Each latch should have at least 50 buffers in its set
Contention detected by querying v$latch,
v$session_event and v$system_event
Contention also exists if misses are higher than
3% in v$latch
Tuning LRU latch
Goal:
 Reduce cache buffer LRU latch contention
Achieved by:
 Having enough latches for the entire buffer cache.
How ?
 Set the maximum number of desired LRU latch
sets with DB_BLOCK_LRU_LATCHES up to
(number_of CPU’s)*2
 Adjust DB_BLOCK_BUFFERS.
Enqueue latch
This latch is used to protect the enqueue data
structure
To tune:
 Set ENQUEUE_RESOURCES to a value greater
than 10

Monitoring Wait Events
Wait events on any latch (latch free wait) are
recorded in WAIT and EVENT dynamic views:




V$SESSION_WAIT - Record events for which sessions are waiting
or just completed waiting (e.g. latch free wait)
V$SESSION_EVENTS - Record cumulative statistics events have
waited for each session (e.g. sessions latch free waits)
V$SYSTEM_EVENTS - Record cumulative wait statistics for all
sessions (e.g. latch free wait).
TIMED_STATISTICS must be enabled for the above statistics to
be recorded
v$session_wait for latch free wait

Wait parameters P1, P2 and P3 contain the
following values for latch free when the
process is waiting on a latch to be available
P1
Latch SGA address; corresponding with the ADDR
column of V$LATCH_PARENT & V$LATCH_CHILDREN
P2
Type of latch; corresponding with LATCH# column of
the V$LATCH family of views
P3
Number of times the process has slept trying to
acquire the latch
v$session_event &v$system_event





Symptoms of latch contention can be found in
these views
Updated when the process wakes up again
indicating the wait is over.
Sleep time is recorded
Consecutive sleeps during attempts to obtain a
single latch is recorded as separate waits
Latching statistics in the V$LATCH family are
only updated once the latch is acquired
Locks
Allow sessions to join a queue for a
resource that is not immediately available
 To achieve consistency and integrity
 Performed automatically by Oracle and
manually by users

Lock Usage

Transaction & Row-level locks
–
–

Buffer locks
–

Transactions imposing implicit locks on rows
In effect for the duration of the transaction
Short term block-level locks in force while
modifying blocks in cache
Data dictionary locks
–
Locks that protect data dictionary objects
Lock Modes

Applied to simple objects:
–
–
–

X - Exclusive
S - Shared
N- Null
Applied to compound objects:
–
–
–
SS - Sub-shared
SX- Sub-exclusive
SSX-Shared-sub exclusive
Enqueue Conversion
The operation of changing the mode of an
enqueue lock
Example:

1- Transaction T1 holds a lock on table TAB in SS
mode
2- T1 needs to update a row of TAB
3- Lock is converted to SX mode
ENQUEUE Locks





A sophisticated locking mechanism that uses fixed
arrays for the lock and the resource data structure
A request for a resource is queued
Permits several concurrent processes to share
known resources to varying levels
Can protect any object used concurrently
Many of Oracle locks
Enqueue Resources





The fixed array for enqueue resources is sized by
ENQUEUE_RESOURCES parameter.
Determines number of resources that can be
concurrently locked by the lock manager
Its default value is derived from SESSIONS
parameter
If set to a value greater than DML_LOCKS+20,
the provided value will be used
Increase if enqueues are exhausted
Enqueue Locks
A second fixed array used for enqueue
locking
 Size set by _ENQUEUE_LOCK
 Used by each session waiting for a lock or
holding a lock on a resource

Corresponding views
Each row in v$resource represents a locked
enqueue resource that is currently locked
 All locks owned by enqueue state objects
are shown in v$enqueue_lock
 All locks held by Oracle or locks and
outstanding requests for locks and latches
are recorded in v$lock

Enqueue wait
Occurs when an enqueue request or
conversion can not be granted at once
 An enqueue wait event is recorded by the
blocked process in the v$session_wait view

Enqueue statistics

Enqueue statistics recorded in V$SYSSTAT
–
–
–
–
–
enqueue waits
enqueue requests
enqueue conversions
enqueue timeouts
enqueue deadlocks
Deadlock Detection



Automatically performed by Oracle
Initiated when an enqueue wait times out and if:
– The resource type is deadlock sensitive
– The lock state for the resource in unchanged
When a session holding a lock on a resource
is waiting for a resource that is held by the
current session in an incompatible mode
DML Locks





Guarantees integrity of data being access and
modified concurrently for the entire transaction
Prevent destructive interference of conflicting
DML and/or DDL operations occurring at the
same time
Adds maintenance of locks conversion history
Locks are held during the entire transaction
Sessions with blocking transaction enqueue locks
always hold a DML lock as well
DML_LOCKS





DML_LOCKS - Max # of DML locks-one for each table
modified in a transaction. Equals the total number of locks
on tables currently references by all users.
If set to 0, DML locks are entirely disabled
V$LOCKED_OBJECTS reveals active slots
DISABLE TABLE LOCKS or ALTER TABLE can be
used to disable DML locks for particular tables
The free list data structure for DML locks is protected by
dml lock allocation latch
V$LOCK view
Records locks currently held as well as
outstanding requests for a lock or a latch
Key columns are:

ADDR: Memory address of object in locked state
SID: Id of session holding or requesting the lock
TYPE: type of user or system lock
ID1, ID2: Type dependent lock identifiers
LMODE, REQUEST: Mode the lock is held or requested
Example: Locked Users

If locking conflict are suspected, locked
users and the statement they are running can
be identified by the following query:
select b.username, b.serial#, d.id1, a.sql_text
from v$session b, v$lock d , v$sqltext a
where b.lockwait = d.kaddr
and a.address = b.sql_address
and a.hash_value = b.sql_hash_value
V$LOCKED_OBJECTS
Records information on all locks acquired
by all transactions including slot numbers
being used by locks
 Used to obtain session information for
sessions holding DML locks on crucial
database objects

Views created by catblock.sql



DBA_LOCKS: Gathers various lock statistics
translated into an easier to understand format
DBA_WRITERS: Provides information on
sessions waiting for locks on specific resources
and sessions that have those resources blocked
DBA_BLOCKERS: Provides information on
which sessions are holding up others
Other lock utilities
Utllockt.sql provided by Oracle
 The dbms_lock package
 Oracle Enterprise Manager
 Third-party tools

V$RESOURCE_LIMIT view
To monitor consumption of resources
 Reveals number of used slots in the fixed
array of lock structures
 Use it to adjust ENQUEUE_RESOURCE &
DML_LOCKS parameter settings

Other lock topics
Distributed transactions
 The Lock Manager
 LCKs processes
 Global locks
 Parallel cache management (PCM) locks

Resources




Oracle8i Internal Services for waits, latches, locks, and
memory by Steve Adams
Oracle Performance Tuning TIPS & TECHNIQUES by
Richard Niemiec
Oracle8i Tuning Manual
Oracle8i Reference Manual