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Generally, locks are advisory locks, where each thread cooperates by acquiring the lock before accessing the corresponding data.Some systems also implement mandatory locks, where attempting unauthorized access to a locked resource will force an exception in the entity attempting to make the access.
In software engineering, double-checked locking (also known as "double-checked locking optimization" [1]) is a software design pattern used to reduce the overhead of acquiring a lock by testing the locking criterion (the "lock hint") before acquiring the lock. Locking occurs only if the locking criterion check indicates that locking is required.
volatile int lock = 0; void critical {// Spin lock: loop forever until we get the lock; we know the lock was // successfully obtained after exiting this while loop because the // test_and_set() function locks the lock and returns the previous lock // value. If the previous lock value was 1 then the lock was **already** // locked by another ...
On any multi-processor system that uses the MESI contention protocol, such a test-and-test-and-set lock (TTAS) performs much better than the simple test-and-set lock (TAS) approach. [ 4 ] With large numbers of processors, adding a random exponential backoff delay before re-checking the lock performs even better than TTAS.
The downside is that write-preferring locks allows for less concurrency in the presence of writer threads, compared to read-preferring RW locks. Also the lock is less performant because each operation, taking or releasing the lock for either read or write, is more complex, internally requiring taking and releasing two mutexes instead of one.
[3] [4] [5] Hashing is an example of a space-time tradeoff. If memory is infinite, the entire key can be used directly as an index to locate its value with a single memory access. On the other hand, if infinite time is available, values can be stored without regard for their keys, and a binary search or linear search can be used to retrieve the ...
To release the lock upon exiting the critical section, process i sets level[i] to −1. That this algorithm achieves mutual exclusion can be proven as follows. Process i exits the inner loop when there is either no process with a higher level than level[i] , so the next waiting room is free; or, when i ≠ last_to_enter[ℓ] , so another ...
In computer science, a set is an abstract data type that can store unique values, without any particular order. It is a computer implementation of the mathematical concept of a finite set. Unlike most other collection types, rather than retrieving a specific element from a set, one typically tests a value for membership in a set.