[Multithreading] lock fair / unfair locks, lock optimistic / pessimistic locking

Lock fair / unfair locks (multi-threaded execution order dimension)

Conceptual understanding

• Fair locks: lock before waiting to see if there is a thread, the thread top surface of any priority, first come first served .
• Unfair as: thread locking direct attempt to acquire the lock , not automatically get the tail to wait.

example

• ReentrantLock support both lock

// create a fair and non-locking, non-default fair locks

Lock nonFairLock= new ReentrantLock();

Lock nonFairLock= new ReentrantLock(false);

// create a fair lock structure parameter passing true

Lock fairLock= new ReentrantLock(true);

Applicable scene

• More direct use of unfair lock: Lock unfair 5-10 times higher than the fair-locking performance, because the need to maintain a fair lock queue in the case of multi-core, if the current thread is not the first queue can not get a lock, an increase of thread switching times.

 

Lock optimistic / pessimistic locking (dimensions multithreading share data)

Conceptual understanding

• Pessimistic locking: suppose will happen concurrency violation , by blocking all other threads to ensure data integrity.
• Optimistic locking: assuming that does not happen concurrency conflicts directly unlocked to complete an update, if conflict returns failure.

example

• Pessimistic locking: Synchronized multi-thread synchronization, exclusive, will be prone to deadlock.
• Optimistic locking: CAS mechanism, in short, there will be three operands, the current value of V memory variables, the expected value of the variable A, is about to update the value of B, when the need to update the variable, the variable value V directly and the expected value A , if the same, then B is directly updated; if not identical, then the current value of the variable to a desired value V refresh, and then try to re-update the comparison.

Applicable scene

• Optimistic locking: apply to data contention is not serious / retry little cost / fast requires a corresponding scene.
• Pessimistic locking: apply to serious contention data / retry high price scenario.