[concurrent programming] atomic operation CAS

syn Blocking based lock mechanism ,1、 Blocked threads have a high priority ,2、 What if the thread that gets the lock does not release the lock ？3、 A lot of competition , Consume cpu, At the same time, it brings deadlock or other security .

Atomic manipulation ： Indivisible operation

CAS(compare and swap, Optimism lock , Lock free programming )

CAS Principle

CAS(Compare And Swap), The instruction level guarantees that this is an atomic operation

Three operators ：  A memory address V, A desired value A, A new value B

The basic idea ： If the address V Top value and expected value A equal , Just give the address V new value B, If not , Do nothing .

loop （ Dead cycle , The spin ） It's going on in the CAS operation

CAS The problem of

ABA problem ：A---》B----》A, resolvent ： Introduce version number

The cost problem ：CAS The operation has been unsuccessful for a long time ,cpu It's a constant cycle

compareAndSet
    public void increament() {
        while (true){
            int i = atomicI.get();
            boolean suc = atomicI.compareAndSet(i, ++i);
            if (suc) {
                break;
            }
        }
    }