Learn more about CAS in java

This article brings you related issues about java, which mainly introduces issues related to CAS, CAS (compare and swap), compare and exchange, which can solve multi-thread parallel situations Here is a mechanism of performance loss caused by using locks. I hope it will be helpful to everyone.

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CAS explanation:

CAS (compare and swap), Compare and swap. A mechanism that can solve the performance loss caused by using locks in multi-thread parallel situations. The CAS operation contains three operands—memory location (V), expected original value (A) and new value (B). If the value of a memory location matches the expected original value, the processor automatically updates the location to the new value. Otherwise, the processor does nothing. A thread gets the num value from the main memory and operates on num. When writing the value, the thread will compare the first num value obtained with the num value in the main memory. If they are equal, the changed value will be num is written into the main memory. If they are not equal, the comparison will be looped until successful.

CAS generation:

The volatile keyword is often used when modifying shared variables, but the volatile value has visibility and prohibits instruction reshooting (orderliness), and atomicity cannot be guaranteed. Although there is no problem in single thread, various problems will occur in multi-threading, causing on-site insecurity. Therefore, CAS was produced after jdk1.5 and uses CPU primitives (indivisible, continuous and uninterrupted) to ensure the atomicity of on-site operations.

CAS application:

The new java.util.concurrent(JUC) in JDK1.5 is built on CAS. Compared with the synchronized lock mechanism, CAS is a common implementation of non-blocking algorithms. Therefore, JUC has greatly improved its performance.

For exampleAtomicInteger class, AtomicInteger is thread-safe, the following is the source code

EnterunsafeSeedo whileself-loop, the self-loop here is in Judge the expected original value If it does not match the original value, the original value will be recycled and the CAS process will be repeated until the The new value was assigned successfully.

CAS Advantages

cas is an optimistic locking idea, and it is a non-blocking lightweight optimistic lock. Non-blocking refers to the failure or failure of a thread. Suspending should not affect the failure of other threads or the suspended algorithm.

CAS Disadvantages

1. The cycle time is long, the overhead is large, and it takes up CPU resources. If the spin lock fails for a long time, it will bring a lot of overhead to the CPU. If the JVM can support the pause instruction provided by the processor, the efficiency will be improved to a certain extent. The pause instruction has two functions. First, it can delay the pipeline execution instruction (de-pipeline) so that the CPU does not consume too much execution resources. The amount of delay depends on the implementation, and on some processors the delay is zero. Secondly, it can avoid the CPU pipeline flush caused by memory order violation when exiting the loop, thereby improving the execution efficiency of the CPU.
2. Only atomic operation of a shared variable can be guaranteed. When performing an operation on a shared variable, we can use cyclic CAS to ensure atomic operations. However, when operating on multiple shared variables, cyclic CAS cannot guarantee the atomicity of the operation. In this case, you can use a lock, or have a The trick is to merge multiple shared variables into one shared variable for operation. For example, there are two shared variables i=2, j=a, merge ij=2a, and then use CAS to operate ij. Starting from Java 1.5, JDK provides the AtomicReference class to ensure the atomicity between reference objects. You can put multiple variables in one object to perform CAS operations.
3. ABA problem

Solve the ABA problem (if the value is to consider the ending, you can ignore the problem without considering the process)

1. Add version number
2. AtomicStampedReference
   

## Starting from Java1.5, the JDK’s atomic package provides a class AtomicStampedReference to solve the ABA problem . The compareAndSet method of this class is to first check whether the current reference is equal to the expected reference, and whether the current flag is equal to the expected flag. If all are equal, then atomically set the value of the reference and the flag to the given update value.

Timing of using CAS

If the number of threads is small and the waiting time is short, spin lock can be used for CAS to try to get the lock, which is more efficient than synchronized.
1. The number of threads is large and the waiting time is long. It is not recommended to use spin locks and consumes a high amount of CPU.

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