Oracle7 Parallel Server Concepts and Administrator's Guide Go to Product Documentation Library
Go to books for this product
Go to Contents for this book
Go to Index

Go to previous file in sequence Go to next file in sequence

Parallel Cache Management Instance Locks

The planning and allocation of PCM locks is one of the most complex tasks facing the Oracle Parallel Server database administrator. This chapter provides a conceptual overview of PCM locks.

This chapter covers the following topics:

See Also: "Allocating PCM Instance Locks" [*] for details on how to plan and assign these locks.

PCM Locks and How They Work

This section covers the following topics:

Figure 9 - 1 highlights PCM locks in relation to other locks used in Oracle.

Figure 9 - 1. Oracle Locking Mechanisms: PCM Locks

What PCM Locks Are

Parallel cache management locks, or PCM locks, are the instance locks which manage the locking of blocks in datafiles. They can cover one or more data blocks or index blocks. OPS uses these instance locks to coordinate access to shared resources. The Distributed Lock Manager maintains the status of the instance locks.

PCM locks ensure cache coherency by forcing instances to acquire a lock before modifying or reading any database block. PCM locks allow only one instance at a time to modify a block. If a block is modified by an instance, the block must first be written to disk before another instance can acquire the PCM lock, read the block, and modify it.

PCM locks use the minimum amount of communication to ensure cache coherency. The amount of cross-instance activity--and the corresponding performance of a parallel server--is evaluated in terms of pings. A ping occurs each time a block must be written to disk by one instance so that another instance can read it.

Note that busy systems can have a great deal of locking activity, but do not necessarily have pinging. If data is well partitioned, then the locking will be local to each node--therefore pinging will not occur.

How PCM Locks Work

For optimal performance, the OPS administrator must allocate PCM locks to datafiles. You do this by specifying values for initialization parameters which are read at startup of the database. The "Allocating Locks" chapter describes this procedure in detail.

You use the initialization parameter GC_FILES_TO_LOCKS to specify the number of PCM locks which cover the data blocks in a data file or set of data files. The smallest granularity is one PCM lock per datablock. Typically, a PCM lock covers a number of datablocks. PCM locks usually account for the greatest proportion of instance locks in a parallel server.

The first instance that starts up will initialize the lock value block for each DLM resource. The lock value block is an area in each DLM resource where lock history information can be stored. When an instance obtains a lock on the resource in exclusive mode, it owns the lock value block and can store information in it. The lock value block is read, for example, when converting a lock from null to exclusive; after an operation is performed and the lock is converted from exclusive to null mode, the lock value block is written.

Hashed PCM locks are initially acquired in null mode. All specified hashed locks are allocated at instance startup, and deallocated at instance shutdown. Because of this, hashed locks entail more overhead and longer startup time than fine grain locks. The advantage of hashed PCM locks, however, is that they do not need to be continually acquired and released.

Fine grain locking (also known as DBA locking) is more dynamic than hashed locking. For example, if you set GC_RELEASABLE_LOCKS to 10000 you can obtain up to ten thousand fine grain PCM locks. However, locks are allocated only as needed by the DLM. At startup Oracle allocates lock elements, which are obtained directly in the requested mode (normally shared or exclusive mode).

Figure 9 - 2 illustrates the way PCM locks work. When instance A reads in the black block for modifications, it obtains the PCM lock for the black block. The same scenario occurs with the shaded block and Instance B. If instance B requires the black block, the block must be written to disk because instance A has modified it. The ORACLE process communicates with the LCK process in order to obtain the lock from the DLM.

Figure 9 - 2. How PCM Locks Work

PCM Locks Are Owned by Instance LCK Processes

Each instance has at least one LCK background process. If multiple LCK processes exist within the same instance, the PCM locks are hashed over the LCK processes. This means that each LCK process is only responsible for a subset of the PCM locks.

Locks Are Converted from One Mode to Another

A PCM lock is "owned" or controlled by an instance when a block covered by that lock (in shared or exclusive mode) enters the buffer cache belonging to the instance.

LCK processes maintain PCM locks on behalf of the instance. The LCK processes obtain and convert hashed PCM locks; they obtain, convert, and release fine grained PCM locks.

Locks Can Be Owned by Multiple Instances

A PCM lock owned in shared mode is not disowned by an instance if another instance also requests the PCM lock in shared mode. Thus, two instances may have the same data block in their buffer caches because the copies are shared (no writes occur). With hashed locking only, different data blocks covered by the same PCM lock can be contained in the buffer caches of separate instances. This can occur if all the different instances request the PCM lock in shared mode.

Number of Blocks per Hashed PCM Lock

Typically, a hashed PCM lock overs a number of data blocks. (Fine grain PCM locks can have even a one-to-one correspondence to datablocks.)

The number of hashed PCM locks assigned to datafiles and the number of data blocks in those datafiles determines the number of data blocks covered by a single PCM lock.

If the size of each file, in blocks, is a multiple of the number of PCM locks assigned to it, then each hashed PCM lock covers exactly the number of data blocks given by the equation.

If the file size is not a multiple of the number of PCM locks, then the number of data blocks per hashed PCM lock can vary by one for that datafile. For example, if you assign 400 PCM locks to a datafile which contains 2,500 data blocks, then 100 PCM locks cover 7 data blocks each and 300 PCM locks cover 6 blocks. Any datafiles not specified in the GC_FILES_TO_LOCKS initialization parameter use the remaining PCM locks. The number of remaining PCM locks equals the value of GC_DB_LOCKS less the sum of all #locks values specified in GC_FILES_TO_LOCKS.

If n files share the same hashed PCM locks, then the number of blocks per lock can vary by as much as n. If you assign locks to individual files, either with separate clauses of GC_FILES_TO_LOCKS or by using the keyword EACH, then the number of blocks per lock does not vary by more than one.

If you assign hashed PCM locks to a set of datafiles collectively, then each lock usually covers one or more blocks in each file. Exceptions can occur when you specify contiguous blocks (using the "!blocks" option) or when a file contains fewer blocks than the number of locks assigned to the set of files.

Attention: One lock is always reserved for database additions. Thus, if all but one lock are already distributed explicitly to existing files, and new files are added to the database, it is possible that entire files may end up sharing a single lock. To avoid such situations, carefully examine GC_* parameter values when adding files.


The following example illustrates how hashed PCM locks can cover multiple blocks in different files. Figure 9 - 3 assumes 44 PCM locks assigned to 2 files which have a total of 44 blocks. GC_FILES_TO_LOCKS is set to A,B:44

Block 1 of a file does not necessarily begin with lock 1; a hashing function determines which lock a file begins with. In file A, which has 24 blocks, block 1 hashes to lock 32. In file B, which has 20 blocks, block 1 hashes to lock 28.

Figure 9 - 3. Hashed PCM Locks Covering Blocks in Multiple Files

In Figure 9 - 3, locks 32 through 44 and 1 through 3 are used to cover 2 blocks each. Locks 4 through 11 and 28 through 31 cover 1 block each; and locks 12 through 27 cover no blocks at all!

In a worst case scenario, if two files hash to the same lock as a starting point, then all the common locks will cover two blocks each. If your files are large and have multiple blocks per lock (on the order of 100 blocks per lock), then this is not an important issue.

Periodicity of Hashed PCM Locks

Note also the periodicity of PCM locks. Figure 9 - 4 shows a file of 30 blocks which is covered by 6 PCM locks. This file has hashed to begin with lock number 5. As suggested by the shaded blocks covered by PCM lock number 4, use of each lock forms a pattern over the blocks of the file.

Figure 9 - 4. Periodicity of Hashed PCM Locks

Pinging: Signaling the Need to Update

On a parallel server, a particular data block can only be modified by a single instance at a time. If one instance modifies a data block which another instance needs, then each instance's locks on the data block must be converted accordingly. The first instance must write the block to disk before the other instance can read it. This is known as pinging a block. The LCK process uses the Distributed Lock Manager to signal this need between the two instances.

Note that data blocks are pinged only when a block that is held in the exclusive current (XCUR) state in the buffer cache of one instance, is needed by a different instance. In some cases, therefore, the number of PCM locks covering data blocks may have little impact on whether a block gets pinged. You can have lost a PCM lock on a block and still have a row lock on it: pinging is independent of whether a commit has occurred. You can modify a block, but whether or not it is pinged is independent of whether you have made the commit.

If you have partitioned data across instances and are doing updates, you can have a million blocks each on the different instances, and still not have any pinging. As shown in Figure 9 - 5, if a single PCM lock covers one million data blocks in a table which are read/write into the SGA of instance X, and another single PCM lock covers another million data blocks in the table which are read/write into the SGA of instance Y, then regardless of the number of updates, no data blocks will ever ping between instance X and instance Y.

Figure 9 - 5. Partitioning Data to Avoid Pinging

With read-only data, both instance X and instance Y can hold the PCM lock in shared mode, and no pinging will take place. This scenario is illustrated in Figure 9 - 6.

Figure 9 - 6. No Pinging of Read-only Data

Lock Mode and Buffer State

The state of a block in the buffer cache relates directly to the mode of the lock held upon it. For example, if a buffer is in exclusive current (XCUR) state, you know that an instance owns the PCM lock in exclusive mode. There can be only one XCUR version of a block in the database, but there can be multiple SCUR versions. To perform a modification, a process must get the block in XCUR mode.

Finding the State of a Buffer

To learn the state of a buffer, check the STATUS column of the V$BH dynamic performance table. This table provides information about each buffer header.

PCM Lock Mode Buffer State Name Description
X XCUR Instance has an EXCLUSIVE lock for this buffer
S SCUR Instance has a SHARED lock for this buffer
N CR Instance has a NULL lock for this buffer
Table 9 - 1. PCM Lock Mode and Buffer State

How Buffer State and Lock Mode Change

The following figure shows how buffer state and lock mode change as instances perform various operations on a given buffer. Lock mode is shown in parentheses.

Figure 9 - 7. How State of Buffer and Lock Mode Change

In Figure 9 - 7 the three instances start out with blocks in shared current mode, and shared locks. When Instance 1 performs an update on the block, its lock mode on the block changes to exclusive mode (X). The shared locks owned by Instance 2 and Instance 3 convert to null mode (N). Meanwhile, the block state in Instance 1 becomes XCUR, and in Instance 2 and Instance 3 becomes CR. These lock modes are compatible. Similar conversions of lock mode and block state occur when Instance 2 performs a SELECT operation on the block, and when Instance 3 performs a SELECT operation on it.

Note: The DLM concurrent read mode (CR) should not be confused with the consistent read buffer state (also abbreviated as CR).

Lock Modes May Be Compatible or Incompatible

When one process owns a lock in a given mode, another process requesting a lock in any particular mode succeeds or fails as shown in the following table.

Lock Requested:
Lock Owned
Table 9 - 2. Lock Mode Compatibility

How Initialization Parameters Control Blocks and PCM Locks

This section explains how certain initialization parameters control blocks and PCM locks.

GC_* Initialization Parameters

PCM locks are controlled by the initialization parameters listed below. Be sure to set all of these parameters for your application.

Parameter Description Value
GC_DB_LOCKS Sets maximum number of PCM locks for all datafiles.
Some of these are hashed locks specified by the GC_FILES_TO_ LOCKS parameter. The remaining locks form a pool of hashed locks to protect database blocks which are not explicitly mentioned in the GC_FILES_TO_LOCKS parameter.
If GC_DB_LOCKS is set to zero, then GC_FILES_TO_LOCKS is ignored and all data blocks (class 1) use fine grain locks. GC_DB_LOCKS must be set to the same value on all instances. It defaults to the value of DB_BLOCK_BUFFERS.
Note: The value of DB_BLOCK_ BUFFERS, however, does not need to be identical on all instances. If you explicitly set GC_DB_LOCKS to the same value on all instances, then the value of DB_BLOCK_BUFFERS can vary from instance to instance.
GC_FILES_TO_LOCKS Gives the mapping of hashed locks (GC_DB_LOCKS) to blocks within each datafile.
The meaning of this parameter has changed. Previously, files not mentioned in this parameter (or files added later) were assigned the remaining hashed locks. You can now have multiple entries of GC_FILES_TO_LOCKS.
The configuration string for GC_FILES_TO_LOCKS now includes a value of zero for the number of locks. This indicates that the blocks are protected by fine grain locks.
Instances must have identical values.
GC_FREELIST_GROUPS Determines the number of locks to specify for free list group blocks. Default is 5 times the value of GC_SEGMENTS. Instances must have identical values.
GC_LCK_PROCS Sets the number of background lock processes (LCK0 through LCK9) for an instance in a parallel server. In shared mode, the value of this parameter must be greater than 1; the default value is 1. In exclusive mode, this parameter is ignored. Instances must have identical values.
GC_RELEASABLE_LOCKS Sets the maximum number of releasable locks in an instance. If persistent locking is supported by platform and DLM, these blocks will be allocated for releasable locks (as needed). If any other GC parameters contain a zero value, or if in GC_FILES_TO_ LOCKS you assign zero locks to a specific file number, then the GC_RELEASABLE_LOCKS parameter will be used to allocate releasable locks for those block classes or files. If, however, you do not explicitly set zero for any other GC parameters, or for a specific file, then this parameter is ignored. It does not affect the default locking mode. Support of fine grain locking is platform-specific. Defaults to the value of DB_BLOCK_BUFFERS, if releasable (fine grain) locks are being used. Larger values may be advantageous. No maximum value exists, except as imposed by space restrictions, or by the maximum number of locks which a single process can hold. This parameter will default to zero if releasable locks are not being used. Each instance can have a different number of releasable locks. If you want each block class to be releasable, you must set the corresponding parameter to zero. See Table 9 - 4.
GC_ROLLBACK_LOCKS For each rollback segment, specifies the number of instance locks available for simultaneously modified rollback segment blocks. The default value is 20. A value of zero causes fine grain locking to be used.
Instances must have identical values.
GC_ROLLBACK_SEGMENTS Specifies the maximum number of rollback segments system-wide, hence the number of hashed locks for undo segment headers (transaction tables). Hashed (non-releasable) locks are always used because no conflict between segment headers is possible. Default value is 20. A value of zero causes fine grain locking to be used.
Instances must have identical values.
GC_SAVE_ROLLBACK_LOCKS Reserves instance locks for deferred rollback segments, which contain rollback entries for transactions in tablespaces which have been taken offline Default value is 20. A value of zero causes fine grain locking to be used.
Instances must have identical values.
GC_SEGMENTS Specifies number of hashed locks to create for the segment header class of blocks. Default value is 10. A value of zero causes fine grain locking to be used.
Instances must have identical values.
GC_TABLESPACES Sets the maximum number of offline tablespaces which can contain outstanding transactions (see GC_SAVE_ROLLBACK_LOCKS) Default value is 5. A value of zero causes fine grain locking to be used.
Instances must have identical values.
Table 9 - 3. Parameters Which Control PCM Locks

Attention: All the GC_* parameters assume that hashed locking will be used. As of Oracle Parallel Server release 7.3, a value of 0 for any GC_* parameter causes fine grain locking. Exceptions are GC_RELEASABLE_LOCKS and GC_LCK_PROCS.

See Also: "Initialization Parameters" [*] for complete specifications for these initialization parameters.

"Allocating PCM Locks" chapter [*] provides information on how to set these parameters.

Parameters Which Protect Classes of Blocks

Oracle datafiles contain different classes of blocks. These blocks are protected by PCM locks which are governed by particular initialization parameters.

Each of the initialization parameters protects a different class of blocks. Database blocks belonging to an object will have at least two different classes: segment header class and data class. For each object there will be only one segment header block and the other blocks will be of class 1 data. As of Oracle Parallel Server release 7.3, if you have unlimited extents in a table you will have an additional class 6 block for extended segment headers.

Table 9 - 4 shows the GC_* parameters you can use to make different block classes releasable.

Class Description How to Make Class Releasable
1 Data or index blocks GC_FILES_TO_LOCKS: use to set number of locks, so that you can specify which files are releasable. GC_DB_LOCKS=0: if set, all files are releasable.
2 Sort blocks no PCM locks are used
3 Save undo blocks GC_SAVE_ROLLBACK_LOCKS=0
4 Segment headers GC_SEGMENTS=0
5 Save undo segment header blocks GC_TABLESPACES=0
6 Free list group blocks GC_FREELIST_GROUPS=0
7 System undo segment header blocks releasable if any other class is releasable; otherwise, they are covered by a single lock
8 System undo segment blocks GC_ROLLBACK_LOCKS=0
7 + (n * 2) User undo segment n header block releasable if any other class is releasable; otherwise, they are covered by a single lock
7 +( (n * 2) + 1) User undo segment n header block GC_ROLLBACK_LOCKS=0
Table 9 - 4. Initialization Parameters Which Make Block Classes Releasable

Note: In Table 9 - 4, n represents the rollback segment number: n = 0 indicates system rollback segment n > 0 indicates non-system rollback segment

If the GC_RELEASABLE_LOCKS initialization parameter is set, then you do not need to set corresponding GC_* parameters for the particular block classes you want to be releasable (fine grain) locks. You do need to set GC_* parameters if you want particular block classes to be hashed locks.

Classes of Blocks

The class of a block indicates the block's function and contents. Block classes include:

data blocks

These blocks contain data from indexes or tables.

sort blocks

These blocks contain data from on-disk sorts and internal temporary tables.

save undo blocks

This is also known as "deferred rollback segment". When you apply undo to an offline tablespace, the undo is stored in a deferred rollback segment. When the tablespace comes back online, the undo is automatically applied to the tablespace. Save undo blocks always reside in the system tablespace.

segment header blocks

The segment header block is the first block of a segment (table/index). It contains information about the segment and the extents in the segment.

save undo header blocks

These are header blocks for the "save undo" blocks.

free list blocks

Following the segment header is a block for each free list group specified in the storage clause of the CREATE TABLE or CREATE INDEX statement. Each block contains information on the freelists in that particular free list group.

(system) undo header blocks

These are also known as the rollback segment headers or transaction tables.

(system) undo blocks

These undo blocks are part of the rollback segment and provide storage for undo records.

Finding the Class of a Block

If a block is pinged heavily, you should determine the class of the block before making any changes to the GC_DB_LOCKS or GC_FILES_TO_LOCKS parameters. The problem may actually be with the setting of another GC_* parameter. To see the class of all blocks from the buffer cache, enter

        FROM V$BH

Protecting Blocks: Consider Data Block Content

When you allocate PCM locks to a datafile, you must take into consideration the content of the datablocks in the file. Figure 9 - 8 illustrates blocks in a sample Oracle datafile. The file's contents and parameters which protect each block, are listed in Table 9 - 5.

Figure 9 - 8. Blocks in a Sample Oracle Datafile

Block Contents Protection
0 Operating system header Not protected by PCM lock
1 Oracle file header Not protected by PCM lock
2 Segment header Protected by GC_SEGMENT
3 Freelist group 1 Protected by GC_FREELIST_GROUPS
4 Freelist group 2 Protected by GC_FREELIST_GROUPS
5 Data block Protected by GC_DB_LOCKS and GC_FILES_TO_LOCKS
6 Data block Protected by GC_DB_LOCKS and GC_FILES_TO_LOCKS
7 Data block Protected by GC_DB_LOCKS and GC_FILES_TO_LOCKS
8 End of table Protected by GC_DB_LOCKS and GC_FILES_TO_LOCKS
9 Rollback segment header Protected by GC_ROLLBACK_SEGMENTS
10 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
11 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
12 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
13 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
14 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
15 Data block for rollback segment Protected by GC_ROLLBACK_LOCKS
Table 9 - 5. Parameters Protecting Blocks in Sample Oracle Datafile

Classes of blocks, and the locks and parameters which affect them, imply some important guidelines for handling data blocks in your files

Segment header blocks, for example, are protected by GC_SEGMENTS. Freelist group blocks are controlled by GC_FREELIST_GROUP: the number of locks allocated for free list group blocks is equal to five times the value of GC_SEGMENTS, by default.

Do not allocate PCM locks for files which only contain the following, because class 1 blocks are not used for these files:

Two Methods of PCM Locking: Hashed and Fine Grain

This section compares the two methods for PCM locking: hashed locking and fine grain locking. You can use either or both kinds of PCM locks to protect the blocks in datafiles.

Allocation and Release of PCM Locks

Hashed PCM locks and fine grain PCM locks differ in the method by which they are allocated, and in whether or not they are released.

Allocation of Hashed Locks

Hashed locks are preallocated and statically hashed to blocks at startup time. The first instance which starts up creates a DLM resource and a DLM lock (in null mode) on the DLM resource for each hashed PCM lock. The first instance initializes each lock. The instance then proceeds to convert DLM locks to other modes as required. When a second instance requires a particular DLM lock, it waits until the lock is available and then converts the lock to the mode required.

Hashed PCM locks are never released; each will stay in the mode in which it was last requested. If the lock is required by another instance, it is converted to null mode. These locks are deallocated only at instance shutdown.

Allocation and Release of Fine Grain Locks

Fine grain PCM locks are acquired and released as needed. Since they are allocated only as required, the instance can start up much faster than with hashed locks. A DLM resource is created and a DLM lock is obtained only when a user actually requests a block. Once a fine grain lock has been created, it can be converted to various modes as required by various instances.

Fine grain locks are releasable: an instance can give up all references to the resource name during normal operation. The DLM resource is released when it is required for reuse for a different block. This means that sometimes no instance holds a lock on a given resource.

DLM Lock Elements and PCM Locks

Figure 9 - 9 illustrates the correspondence of lock elements to blocks in hashed and fine grain locking. A lock element (LE) is an Oracle-specific data structure that represents a DLM lock. There is a one-to-one correspondence between a lock element and a PCM lock in the DLM.

Figure 9 - 9. Hashed Locking and DBA Fine Grain Locking

Lock Elements for Hashed PCM Locks

For both hashed PCM locks and fine grain locks, you can specify more than 1 block per lock element. The difference is that hashed PCM locks are not releasable; the lock element name is fixed.

When the lock element is pinged, any other modified blocks owned by that lock element will be written along with the needed one. For example, in Figure 9 - 9, if LE is pinged when block DBA2 is needed, blocks DBA1, DBA3, DBA4, and DBA5 will all be written to disk as well--if they have been modified.

Lock Elements for Fine Grain PCM Locks

In fine grain locking, the name of the lock element is the name of the resource inside the DLM.

Although a fixed number of lock elements cover potentially millions of blocks, the lock element names change over and over again as they are associated with specific blocks that are requested. The lock element name (for example, LE7,1) contains the database block address (7) and class (1) of the block it covers. Before a lock element can be reused, the DLM lock must be released. You can then rename and reuse the lock element, creating a new resource in the DLM if necessary.

When using fine grain locking, you can set your system with many more potential lock names, since they do not need to be held concurrently. However, the number of blocks mapped to each lock is configurable in the same way as hashed locking.

Lock Elements for DBA Fine Grain PCM Locks

In fine grain locking you can set a one-to-one relationship between lock elements and blocks. Such an arrangement, illustrated in Figure 9 - 9, is called DBA locking. Thus if LE2,1 is pinged, only block DBA2 is written to disk.

Number of Blocks per PCM Lock

This section explains the ways in which hashed locks and fine grain locks can differ in lock granularity.

Hashed Locks for Multiple Blocks

Hashed PCM locks can protect more than one Oracle database block. The mapping of PCM locks to blocks in the database is determined on a file-by-file basis using initialization parameters specified when the first Oracle Parallel Server instance is started. The parameters can specify that the PCM lock protects a range of contiguous blocks within the file.

Hashed locks are useful in the following situations:

Situation Reason
when the data is mostly read-only A few hashed locks can cover many blocks without requiring frequent lock operations. These locks are released only when another instance needs to modify the data. Hashed locking can perform up to 100% faster than fine grain locking on read-only data with the Parallel Query Option. Note: If the data is strictly read-only, consider designating the tablespace itself as read-only. The tablespace will not then require any PCM locks.
when the data can be partitioned according to the instance which is likely to modify it Hashed locks which are defined to match this partitioning allow instances to hold disjoint DLM lock sets, reducing the need for DLM operations.
when a large set of data is modified by a relatively small set of instances Hashed locks permit access to a new database block to proceed without DLM activity, if the lock is already held by the requesting instance.
Table 9 - 6. When to Use Hashed PCM Locks

Hashed locks may cause extra cross-instance lock activity, since conflicts may occur between instances which are modifying different database blocks. The resolution of this false conflict ("false pinging") may require writing several blocks from the cache of the instance which currently owns the lock.

Fine Grain Locking: Locks for One or More Blocks

A fine grain lock can protect one or more Oracle database blocks. If you create a one-to-one correspondence between PCM locks and datablocks, then contention will occur only when instances need data from the same block. This level of fine grain locking is known as DBA locking. (A DBA is the data block address of a single block of data.) If you assign more than one block per lock, then contention will occur as in hashed locking.

On most systems an instance could not possibly hold a lock for each block of a database since SGA memory or DLM locking capabilities would be exceeded. Therefore, instances acquire and release fine grain locks as required. Since fine grain locks, lock elements, and resources are renamed in the DLM and reused, a system can employ fewer of them. The value of DB_BLOCK_BUFFERS is the minimum number of fine grain locks you can allocate.

DBA fine grain locks are useful when a database object is updated frequently by several instances. This advantage is gained as follows:

A disadvantage of fine grain locking is that overhead is incurred for each block read, and performance will be affected accordingly. (Acquiring a new lock and releasing it each time causes more overhead through the DLM than converting the mode of an existing lock from null mode to exclusive mode and back, as is done in hashed locking.)

See Also: "Fine Grain Examples" [*].

How Fine Grain Locking Works

Figure 9 - 10 shows how fine grain locking operates.

Figure 9 - 10. Lock Elements Coordinate Blocks (by Fine Grain Locking)

The foreground process checks in the SGA to see if the instance owns a lock on the block.

A lock element is created by both hashed locking and fine grain locking. It can be in either of two modes:

Note: Valid lock elements have a lock in the DLM; invalid lock elements do not. A free lock element indicates that a lock exists in the DLM which is not currently linked to this buffer; it is waiting on the LRU list. If a lock element is old, then there is a valid lock handle for the old name. It must be given a new name before Oracle can use it.

The V$LOCK_ELEMENT table shows the status of the lock elements.

Performance Effects of Fine Grain Locking

Fine grain locking may affect performance of OPS. Since releasable locks are more expensive (since they may cause a release lock and get lock on a buffer get), some operations may show a decreased level of performance when run in this mode. However, other types of access to the database will improve with releasable fine grain locks. Fine grain locking may have the following results:

Applying Fine Grain and Hashed Locking to Different Files

Each datafile can use one or the other method of locking. For best results, you may need to use hashed locks on some datafiles, and fine grain locking on other datafiles.

You can selectively apply hashed and fine grain locking on different files. For example, you could apply locks as follows on a set of files:

GC_FILES_TO_LOCKS = "1=100:2=0:3=1000:4-5=0EACH" GC_RELEASABLE_LOCKS=10000

File Number Locking Mode Value in GC_FILES_TO_LOCKS
1 Hashed 100
2 Fine grain 0
3 Hashed 1000
4 Fine grain 0
5 Fine grain 0
Table 9 - 7. Selective Application of Hashed and Fine Grain Locking

How Locks Are Assigned to Blocks

This section explains how hashed locks and fine grain locks are assigned to blocks. (DBA locks, of course, have a one-to-one correspondence to blocks.)

File to Lock Mapping

Two data structures in the SGA control file to lock mapping. The first structure maps each file (DB_FILES) to a bucket (index) in the second structure. This structure contains information on the number of locks allocated to this bucket, base lock number and grouping factor.

For example:

GC_DB_LOCKS = 1000


Example 1

The following dynamic performance table maps each file number to a bucket (index) in the X$KCLFH dynamic performance table. The default bucket number is 0. If the file is specified in GC_FILES_TO_LOCKS it will be assigned a bucket number.

File Number Bucket
1 1
2 0
3 0
4 0
5 2
Table 9 - 8. X$KCLFI Dynamic Performance Table: Example 1

In the following dynamic performance table Oracle finds the number of locks assigned to each bucket, as determined by GC_FILES_TO_LOCKS. Bucket 0 obtains all the remaining locks (GC_DB_LOCKS minus GC_FILES_TO_LOCKS: in this example, 1000 - 700 = 300). For each specified entry in GC_FILES_TO_LOCKS there is an entry in the X$KCLFH table. The grouping factor is determined by the ! option in the GC_FILES_TO_LOCKS strings.

Bucket Locks Grouping Start
0 300 1 0
1 500 1 300
2 200 1 800
Table 9 - 9. X$KCLFH Dynamic Performance Table: Example 1

Example 2

Here is a further example of how hashed locks are assigned to blocks.

If GC_FILES_TO_LOCKS were set to "1-3=500:4-5=200!5EACH", then the structures would look like this:

File Number Bucket
1 1
2 1
3 1
4 2
5 3
Table 9 - 10. X$KCLFI Dynamic Performance Table: Example 2

The value 1 - 3 = 500 means that the files (1, 2, and 3) will share 500 locks between them. That is why files 1, 2, and 3 point to bucket 1. The value 4 - 5 = 200!5EACH means that file 4 and file 5 will each have 200 blocks. That is why file 4 points to bucket 2 and file 5 points to bucket 5.

Bucket Locks Grouping Start
0 100 1 0
1 500 1 100
2 200 5 600
3 200 5 800
Table 9 - 11. X$KCLFH Dynamic Performance Table: Example 2

The !5 for files 4 and 5 is the grouping factor for each file. In this example 100 locks are not explicitly assigned, but are associated with bucket 0. These locks can be used for datafiles which may be added while the database is running.

Note: The X$ dynamic performance tables used in these examples are unsupported, and may change in future releases.

Number of Locks per Block Class

The following query shows the number of locks allocated per class:


The following query shows the number of hashed PCM locks:

 WHERE bitand(flag, 4)!=0;

The following query shows the number of fine grain PCM locks:

 WHERE bitand(flag, 4)=0;

Lock Element Number

For a data class block the file number is determined from the data block address (DBA), and the bucket is found through the X$KCLFI dynamic performance table. Data class blocks are hashed to lock element numbers as follows:

Other block classes are hashed to lock element numbers as follows:

The following procedure finds the lock element number based on the database address and class of a block:

     IF CLASS = 1
          INTO LE
          WHERE I.INDX =
           AND I.KCLFIBUK = H.INDX;
    END IF;
    IF CLASS = 2
       LE := -1;
    END IF;
        FROM X$LE
      LE := BASE + MOD(DBA, LOCKS);
    END IF;
    IF CLASS > 6
       IF MOD(CLASS, 2) = 1
          SELECT INDX
            INTO LE
            FROM X$LE
           WHERE LE_CLASS = CLASS;
    END IF;
    IF MOD(CLASS, 2) = 0
         FROM X$LE
       LE := BASE + MOD(DBA, LOCKS;
    END IF;
   END IF;

Examples: Mapping Blocks to PCM Locks


The following examples show different ways of mapping blocks to PCM locks, and how the same locks are used on multiple datafiles.

Note: These examples discuss very small sample files to illustrate important concepts. The actual files you manage will be significantly larger.

Figure 9 - 11. Mapping PCM Locks to Data Blocks

Example 1

Figure 9 - 11 shows an example of mapping blocks to PCM locks for the parameter value GC_FILES_TO_LOCKS = "1=60:2-3=40:4=140:5=30".

In datafile 1 of the figure, 60 PCM locks map to 120 blocks, which is a multiple of 60. Each PCM lock therefore covers two data blocks.

In datafiles 2 and 3, 40 PCM locks map to a total of 160 blocks. A PCM lock can cover either one or two data blocks in datafile 2, and two or three data blocks in datafile 3. Thus, one PCM lock may cover three, four, or five data blocks across both datafiles.

In datafile 4, each PCM lock maps exactly to a single data block, since there is the same number of PCM locks as data blocks.

In datafile 5, 30 PCM locks map to 170 blocks, which is not a multiple of 30. Each PCM lock therefore covers five or six data blocks.

Each of the PCM locks illustrated in Figure 9 - 11 can be held in either read-lock mode or read-exclusive mode.

Example 2

The following parameter value allocates 500 PCM locks to datafile 1; 400 PCM locks each to files 2, 3, 4, 10, 11, and 12; 150 PCM locks to file 5; 250 PCM locks to file 6; and 300 PCM locks collectively to files 7 through 9:

GC_FILES_TO_LOCKS = "1=500:2-4,10-12=400EACH:5=150:6=250:7-9=300" 

This example assigns a total of (500 + (6*400) + 150 + 250 + 300) = 3600 PCM locks; therefore, the value of GC_DB_LOCKS must be at least 3600. You may specify more than this number of PCM locks if you intend to add more datafiles later.

Example 3

In Example 2, 300 PCM locks are allocated to datafiles 7, 8, and 9 collectively with the clause "7-9=300". The keyword EACH is omitted. If each of these datafiles contains 900 data blocks, for a total of 2700 data blocks, then each PCM lock covers 9 data blocks. Because the datafiles are multiples of 300, the 9 data blocks covered by the PCM lock are spread across the 3 datafiles; that is, one PCM lock covers 3 data blocks in each datafile.

Example 4

The following parameter value allocates 200 PCM locks each to files 1 through 3; 50 PCM locks to datafile 4; 100 PCM locks collectively to datafiles 5, 6, 7, and 9; and 20 data locks in contiguous 50-block groups to datafiles 8 and 10 combined:

GC_FILES_TO_LOCKS = "1-3=200EACH:4=50:5-7,9=100:8,10=20!50" 

In this example, a PCM lock assigned to the combined datafiles 5, 6, 7, and 9 covers one or more data blocks in each datafile, unless a datafile contains fewer than 100 data blocks. If datafiles 5 to 7 contain 500 data blocks each and datafile 9 contains 100 data blocks, then each PCM lock covers 16 data blocks: one in datafile 9 and five each in the other datafiles. Alternatively, if datafile 9 contained 50 data blocks, half of the PCM locks would cover 16 data blocks (one in datafile 9); the other half of the PCM locks would only cover 15 data blocks (none in datafile 9).

The 20 PCM locks assigned collectively to datafiles 8 and 10 cover contiguous groups of 50 data blocks. If the datafiles contain multiples of 50 data blocks and the total number of data blocks is not greater than 20 times 50 (that is, 1000), then each PCM lock covers data blocks in either datafile 8 or datafile 10, but not in both. This is because each of these PCM locks covers 50 contiguous data blocks. If the size of datafile 8 is not a multiple of 50 data blocks, then one PCM lock must cover data blocks in both files. If the sizes of datafiles 8 and 10 exceed 1000 data blocks, then some PCM locks must cover more than one group of 50 data blocks, and the groups might be in different files.

More Sample Hashed Settings of GC_FILES_TO_LOCKS

Examples 5, 6, and 7 show the results of specifying various values of GC_FILES_TO_LOCKS. In the examples, files 1 and 2 each have 16 blocks of data.

Example 5


In this example four locks are specified for files 1 and 2. Therefore, the number of blocks covered by each lock is 8 ((16+16)/4). The blocks are not contiguous.

Figure 9 - 12. GC_FILES_TO_LOCKS Example 5

Example 6


In this example four locks are specified for files 1 and 2. However, the locks must cover 8 contiguous blocks.

Figure 9 - 13. GC_FILES_TO_LOCKS Example 6

Example 7


In this example four locks are specified for file 1 and four for file 2. The locks must cover 4 contiguous blocks.

Figure 9 - 14. GC_FILES_TO_LOCKS Example 7

Sample Fine Grain Setting of GC_FILES_TO_LOCKS

The following example shows fine grain locking mixed with hashed locking.

Example 8


File 1 has hashed PCM locking with 4 locks. On file 2, fine grain locks are allocated on demand--none are initially allocated.

Figure 9 - 15. GC_FILES_TO_LOCKS Example 8

Go to previous file in sequence Go to next file in sequence
Prev Next
Copyright © 1996 Oracle Corporation.
All Rights Reserved.
Go to Product Documentation Library
Go to books for this product
Go to Contents for this book
Go to Index