Updated: 11 December 1998

OpenVMS Programming Concepts Manual

This chapter describes the use of the lock manager to synchronize access to shared resources. It contains the following sections:

Section 17.1 describes how the lock manager synchronizes processes to a specified resource.

Section 17.2 describes the concepts of resources and locks.

Section 17.3 describes how to use the SYS$ENQ and SYS$ENQW system services to queue lock requests.

Section 17.4 describes specialized features of locking techniques.

Section 17.5 describes how to use the SYS$DEQ system service to dequeue the lock.

Section 17.6 describes how applications can perform local buffer caching.

Section 17.7 presents a code example of how to use lock management services.

17.1 Synchronizing Operations with the Lock Manager

Cooperating processes can use the lock manager to synchronize access to a shared resource (for example, a file, program, or device). This synchronization is accomplished by allowing processes to establish locks on named resources. All processes that access the shared resources must use the lock management services; otherwise, the resources are not effective.

To synchronize access to resources, the lock management services provide a mechanism that allows processes to wait in a queue until a particular resource is available.

The lock manager does not ensure proper access to the resource; rather, the programs must respect the rules for using the lock manager. The rules required for proper synchronization to the resource are as follows:

• The resource must always be referred to by an agreed-upon name.
• Access to the resource is always accomplished by queuing a lock request with the SYS$ENQ or SYS$ENQW system service.
• All lock requests that are placed in a wait queue must wait for access to the resource.

A process can choose to lock a resource and then create a subprocess to operate on this resource. In this case, the program that created the subprocess (the parent program) should not exit until the subprocess has exited. To ensure that the parent program does not exit before the subprocess, specify an event flag to be set when the subprocess exits (use the completion-efn argument of LIB$SPAWN). Before exiting from the parent program, use SYS$WAITFR to ensure that the event flag is set. (You can suppress the logout message from the subprocess by using the SYS$DELPRC system service to delete the subprocess instead of allowing the subprocess to exit.)

Table 17-1 summarizes the lock manager services.

Table 17-1 Lock Manager Services

Routine	Description
SYS$ENQ(W)	Queues a new lock or lock conversion on a resource
SYS$DEQ	Releases locks and cancels lock requests
SYS$GETLKI(W)	Gets information about the lock database

17.2 Concepts of Resources and Locks

A resource can be any entity on the operating system (for example, files, data structures, databases, executable routines). When two or more processes access the same resource, you often need to control their access to the resource. You do not want to have one process reading the resource while another process writes new data, because a writer can quickly invalidate anything being read by a reader. The lock management system services allow processes to associate a name with a resource and request access to that resource. Lock modes enable processes to indicate how they want to share access with other processes.

To use the lock management system services, a process must request access to a resource (request a lock) using the Enqueue Lock Request (SYS$ENQ) system service. Three arguments are required to the SYS$ENQ system service for new locks:

• Resource name---The lock management services use the resource name to look for other lock requests that use the same name.
• Lock mode to be associated with the requested lock---The lock mode indicates how the process wants to share the resource with other processes.
• Address of a lock status block---The lock status block receives the completion status for a lock request and the lock identification. The lock identification is used to refer to a lock request after it has been queued.

The lock management services compare the lock mode of the newly requested lock to the mode of other locks with the same resource name. New locks are granted in the following instances:

• If no other process has a lock on the resource.
• If another process has a lock on the resource and the mode of the new request is compatible with the existing lock.
• If another process already has a lock on the resource and the mode of the new request is not compatible with the lock mode of the existing lock, the new request is placed in a queue, where it waits until the resource becomes available. When the resource becomes available, the process is notified that the lock has been granted.

Processes can also use the SYS$ENQ system service to change the lock mode of a lock. This is called a lock conversion.

17.2.1 Resource Granularity

Many resources can be divided into smaller parts. As long as a part of a resource can be identified by a resource name, the part can be locked. The term resource granularity describes the part of the resource being locked.

Figure 17-1 depicts a model of a database. The database is divided into areas, such as a file, which in turn are subdivided into records. The records are further divided into items.

Figure 17-1 Model Database

The processes that request locks on the database shown in Figure 17-1 may lock the whole database, an area in the database, a record, or a single item. Locking the entire database is considered locking at a coarse granularity; locking a single item is considered locking at a fine granularity.

In this example, overall access to the database can be represented by a root resource name. Access to areas in the database or records within areas can be represented by sublocks.

Root resources consist of the following:

• Resource domain
• Resource name
• Access mode

Subresources consist of the following:

• Parent resource
• Resource name
• Access mode

Because resource names are arbitrary names chosen by applications, one application may interfere (either intentionally or unintentionally) with another application. Unintentional interference can be easily avoided by careful design, such as using a registered facility name as a prefix for all root resource names used by an application.

Intentional interference can be prevented by using resource domains. A resource domain is a namespace for root resource names and is identified by a number. Resource domain 0 is used as a system resource domain. Usually, other resource domains are used by the UIC group corresponding to the domain number.

By using the SYS$SET_RESOURCE_DOMAIN system service, a process can gain access to any resource domain subject to normal operating system access control. By default, each resource domain allows read, write, and lock access by members of the corresponding UIC group. See the OpenVMS Guide to System Security for more information about access control.

17.2.3 Resource Names

The lock management system services refer to each resource by a name composed of the following four parts:

• A name specified by the caller
• The caller's access mode
• The caller's UIC group number (unless the resource is systemwide)
• The identification of the lock's parent (optional)

For two resources to be considered the same, these four parts must be identical for each resource.

The name specified by the process represents the resource being locked. Other processes that need to access the resource must refer to it using the same name. The correlation between the name and the resource is a convention agreed upon by the cooperating processes.

The access mode is determined by the caller's access mode unless a less privileged mode is specified in the call to the SYS$ENQ system service. Access modes, their numeric values, and their symbolic names are discussed in the OpenVMS Calling Standard.

The default resource domain is selected by the UIC group number for the process. The system domain can be accessed by setting the LCK$M_SYSTEM when you request a new root lock. Other domains can be accessed using the optional RSDM_ID parameter to SYS$ENQ. You need the SYSLCK user privilege to request systemwide locks from user or supervisor mode. No additional privilege is required to request systemwide locks from executive or kernel mode.

When a lock request is queued, it can specify the identification of a parent lock, at which point it becomes a sublock (see Section 17.4.8). However, the parent lock must be granted, or the lock request is not accepted. This enables a process to lock a resource at different degrees of granularity.

17.2.4 Choosing a Lock Mode

The mode of a lock determines whether the resource can be shared with other lock requests. Table 17-2 describes the six lock modes.

Table 17-2 Lock Modes

Mode Name	Meaning
LCK$K_NLMODE	Null mode. This mode grants no access to the resource. The null mode is typically used as an indicator of interest in the resource or as a placeholder for future lock conversions.
LCK$K_CRMODE	Concurrent read. This mode grants read access to the resource and allows sharing of the resource with other readers. The concurrent read mode is generally used when additional locking is being performed at a finer granularity with sublocks or to read data from a resource in an "unprotected" fashion (allowing simultaneous writes to the resource).
LCK$K_CWMODE	Concurrent write. This mode grants write access to the resource and allows sharing of the resource with other writers. The concurrent write mode is typically used to perform additional locking at a finer granularity, or to write in an "unprotected" fashion.
LCK$K_PRMODE	Protected read. This mode grants read access to the resource and allows sharing of the resource with other readers. No writers are allowed access to the resource. This is the traditional "share lock."
LCK$K_PWMODE	Protected write. This mode grants write access to the resource and allows sharing of the resource with users at concurrent read mode. No other writers are allowed access to the resource. This is the traditional "update lock."
LCK$K_EXMODE	Exclusive. The exclusive mode grants write access to the resource and prevents the sharing of the resource with any other readers or writers. This is the traditional "exclusive lock."

17.2.5 Levels of Locking and Compatibility

Locks that allow the process to share a resource are called low-level locks; locks that allow the process almost exclusive access to a resource are called high-level locks. Null and concurrent read mode locks are considered low-level locks; protected write and exclusive mode locks are considered high-level. The lock modes, from lowest- to highest-level access, are:

• Null
• Concurrent read
• Concurrent write
  Protected read
• Protected write
• Exclusive

Note that the concurrent write and protected read modes are considered to be of equal level.

Locks that can be shared with other locks are said to have compatible lock modes. High-level lock modes are less compatible with other lock modes than are low-level lock modes. Table 17-3 shows the compatibility of the lock modes.

Table 17-3 Compatibility of Lock Modes

Mode of	Mode of Currently Granted Locks
Requested Lock	NL	CR	CW	PR	PW	EX
NL	Yes	Yes	Yes	Yes	Yes	Yes
CR	Yes	Yes	Yes	Yes	Yes	No
CW	Yes	Yes	Yes	No	No	No
PR	Yes	Yes	No	Yes	No	No
PW	Yes	Yes	No	No	No	No
EX	Yes	No	No	No	No	No

• NL---Null
• CR---Concurrent read
• CW---Concurrent write
• PR---Protected read
• PW---Protected write
• EX---Exclusive

A lock on a resource can be in one of the following three states:

• Granted---The lock request has been granted.
• Waiting---The lock request is waiting to be granted.
• Conversion---The lock request has been granted at one mode and is waiting to be granted a high-level lock mode.

A queue is associated with each of the three states (see Figure 17-2).

Figure 17-2 Three Lock Queues

When you request a new lock, the lock management services first determine whether the resource is currently known (that is, if any other processes have locks on that resource). If the resource is new (that is, if no other locks exist on the resource), the lock management services create an entry for the new resource and the requested lock. If the resource is already known, the lock management services determine whether any other locks are waiting in either the conversion or the waiting queue. If other locks are waiting in either queue, the new lock request is queued at the end of the waiting queue. If both the conversion and waiting queues are empty, the lock management services determine whether the new lock is compatible with the other granted locks. If the lock request is compatible, the lock is granted; if it is not compatible, it is placed in the waiting queue. You can use a flag bit to direct the lock management services not to queue a lock request if one cannot be granted immediately.

17.2.7 Concepts of Lock Conversion

Lock conversions allow processes to change the level of locks. For example, a process can maintain a low-level lock on a resource until it limits access to the resource. The process can then request a lock conversion.

You specify lock conversions by using a flag bit (see Section 17.4.6) and a lock status block. The lock status block must contain the lock identification of the lock to be converted. If the new lock mode is compatible with the currently granted locks, the conversion request is granted immediately. If the new lock mode is incompatible with the existing locks in the granted queue, the request is placed in the conversion queue. The lock retains its old lock mode and does not receive its new lock mode until the request is granted.

When a lock is dequeued or is converted to a lower-level lock mode, the lock management services inspect the first conversion request on the conversion queue. The conversion request is granted if it is compatible with the locks currently granted. Any compatible conversion requests immediately following are also granted. If the conversion queue is empty, the waiting queue is checked. The first lock request on the waiting queue is granted if it is compatible with the locks currently granted. Any compatible lock requests immediately following are also granted.

17.2.8 Deadlock Detection

A deadlock occurs when any group of locks are waiting for each other in a circular fashion.

In Figure 17-3, three processes have queued requests for resources that cannot be accessed until the current locks held are dequeued (or converted to a lower lock mode).

Figure 17-3 Deadlock

If the lock management services determine that a deadlock exists, the services choose a process to break the deadlock. The chosen process is termed the victim. If the victim has requested a new lock, the lock is not granted; if the victim has requested a lock conversion, the lock is returned to its old lock mode. In either case, the status code SS$_DEADLOCK is placed in the lock status block. Note that granted locks are never revoked; only waiting lock requests can receive the status code SS$_DEADLOCK.

The OpenVMS lock manager was modified for OpenVMS Version 7.1. Some internal restrictions on the number of locks and resources available on the system have been eased and a method to allow enqueue limit quota (ENQLM) of greater than 32767 has been added. No changes were made to the interface and no programming changes to applications are required to take advantage of these changes.

While most processes do not require very many locks simultaneously (typically less than 100), large scale database or server applications can easily exceed this threshold.

Specifically, the OpenVMS lock manager includes the following enhancements:

• ENQLM greater than 32767 allowed
• Sub-resources and sub-locks greated than 65535 allowed
• Resource hash table greater than 65535 allowed
• LOCKIDTBL size restrictions removed (LOCKIDTBL_MAX obsolete)

If you set an ENQLM value of 32767 in the SYSUAF, the operating system treats it as no limit and allows an application to own up to 16,776,959 locks, the architectural maximum of the OpenVMS lock manager. The following sections describe in more detail these features.

17.2.9.1 Enqueue Limit Quota (ENQLM)

Before the release of OpenVMS Version 7.1, the previous limit for the total number of locks a single process could own was 32767. This limit was enforced unless the process ran in a privileged mode and used the NOQUOTA flag. Further attempts to acquire locks would result in an error (SS$_EXQUOTA). Because applications generally use the lock manager for internal synchronization, this error was usually fatal to the application. While most processes do not require very many locks simultaneously (typically less than 100), large scale database or server applications can easily exceed this threshold.

Now with the release of OpenVMS Version V7.1, an ENQLM value of 32767 in a user's SYSUAF record is treated as if there is no quota limit for that user. This means that the user is allowed to own up to 16,776,959 locks, the architectural maximum of the OpenVMS lock manager.

The current maximum SYSUAF ENQLM value of 32767 is not treated as a limit. Instead, when a process is created that reads ENQLM from the SYSUAF, if the value in the SYSUAF is 32767, it is automatically extended to the new maximum. The Create Process (SYS$CREPRC) system service has been modified to allow large quotas to be passed on to the target process. Therefore, a process can be created with an arbitrary ENQLM of any value up to the new maximum if it is initialized from a process with the SYSUAF quota of 32767.

The behavior of the process quota and creation limit (PQL) parameters for the default and minimum ENQLM quotas for detached processes has not been changed. The default SYSGEN values for the parameters have been raised accordingly.

17.2.9.2 Sub-Resources and Sub-Locks

The previous maximum value for the number of sub-resources or sub-locks in a resource or lock tree (parent/children relationships) was 65535. The internal structures were reorganized from word to longword counters, which can handle sub-resource and sub-lock counts up to the current architectural limits of the lock manager. No programming or interface changes were made. As a result, SS$_EXDEPTH errors no longer occur.

In a mixed-version OpenVMS Cluster, only nodes running OpenVMS Version 7.1 are able to handle these large lock trees. Large scale locking applications should be restricted to running on a subset of nodes running OpenVMS Version 7.1, or the entire cluster should be upgraded to OpenVMS Version 7.1 to avoid unpredictable results.

17.2.9.3 Resource Hash Table

The resource hash table is an internal OpenVMS lock manager structure used to do quick lookups on resource names without a lengthy interactive search. Like all such tables, it results in a tradeoff of consuming memory in order to speed operation. A typical tuning goal is to have the resource hash table size (RESHASHTBL system parameter) about four times larger than the total number of resources in use on the system. Systems with memory constraints or not critically dependent on locking speed could set the table to a smaller size.

Previously, the limit for the RESHASHTBL was 65535, based on the word field used for the parameter and the algorithm used to develop the hash index. This limit has been removed. The new maximum for the RESHASHTBL is 16,777,216 (2²⁴), which is the current architectural maximum for the total number of resources possible on the system.

No external changes are apparent from this modification. Large memory systems that use very large resource namespaces can take advantage of this change to gain a performance advantage in many locking operations. There is no mixed-version OpenVMS Cluster impact related to this change.

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