Order insensitive combination locksedit
Other schemes also provide shared access for reading data. There is a tradeoff between decreasing lock overhead and decreasing lock contention when choosing the number of locks in synchronization. For example, consider the following C code:.
In a concurrent program, this order insensitive combination locksedit is incorrect because when one thread is halfway through transferanother might observe a state where amount has been withdrawn from the first account, but not yet deposited into the other account: The two types of locking are pessimistic locking and optimistic locking:. Views Read Edit View history.
From Wikipedia, the free order insensitive combination locksedit. These instructions allow a single process to test if the lock is free, and if free, acquire the lock in a single atomic operation. Archived from the original on A lock is designed to enforce a mutual exclusion concurrency control policy.
Alternatives to locking include non-blocking synchronization methods, like order insensitive combination locksedit programming techniques and transactional memory. This subtlety can increase the chance that a programmer will unknowingly introduce a deadlock. It is inefficient if the lock is held for a long time, or if the progress of the thread that is holding the lock depends on preemption of the locked thread. Software design patterns Concurrency control. An example in C follows:.
It provides exclusive access to the locked data. Coarse granularity, such as using table locks, tends to give the best performance order insensitive combination locksedit a single user, whereas fine granularity, such as record locks, tends to give the best performance for multiple users. Database locks can be used as a means of ensuring transaction synchronicity. Careless use of locks can result in deadlock or livelock.
In most cases, proper locking depends on the CPU providing a method of atomic instruction stream synchronization for example, the addition or deletion of an item into a order insensitive combination locksedit requires that all contemporaneous operations needing to add or delete other items in the pipe be suspended during the manipulation of the memory content required to add or delete the specific item. There is a tradeoff between decreasing lock overhead and decreasing lock contention when choosing the number of locks in synchronization. The second part of the problem is much more complicated. Other widely implemented access modes are exclusive, intend-to-exclude and intend-to-upgrade. Therefore, an application order insensitive combination locksedit often be more robust when it recognizes the burdens it places upon an operating system and is capable of graciously recognizing the reporting of impossible demands.
In a database management systemfor example, a lock could protect, in order of increasing granularity, part of a field, a field, a record, a data page, or an entire table. This page was last edited on 5 Februaryat An important property of a lock is its granularity.