Table of contents
1.2 Application scenarios of CAS
2.1 Compare and swap operations
2.2 Implementation Principle of CAS
2.3 ABA problems and solutions of CAS
3.1 java.util.concurrent.atomic 包
3.2 AtomicInteger 与 AtomicLong
3.3 Solution to ABA problem: AtomicStampedReference and AtomicMarkableReference
4. Advantages and disadvantages of CAS
4.1 Advantages: lock-free, high concurrency performance
4.2 Disadvantages: number of spins, ABA problem
1. What is CAS
1.1 Definition of CAS
CAS, or Compare and Swap, is a technology used in concurrent programming to implement atomic operations in a multi-threaded environment. It is a lock-free algorithm for solving data synchronization problems in a multi-threaded environment. The CAS operation has three operands: the memory location V, the old expected value A, and the new value B to be written. Only if the value at the memory location is equal to the old expected value A, the new value B is written to the memory location V, otherwise it does nothing. CAS operations are atomic, ensuring data consistency and thread safety in a multi-threaded environment.
1.2 Application scenarios of CAS
CAS is mainly used to solve the problem of data competition in a multi-threaded environment. For example, when multiple threads modify shared variables at the same time, data inconsistency may occur. CAS is commonly used in the following scenarios:
- Counter operation: Increase or decrease the counter in a multi-threaded environment to ensure the correctness of the count.
- Concurrent container: When implementing concurrent containers (such as concurrent queues and concurrent collections), the atomicity of data operations in a multi-threaded environment is guaranteed.
- Thread Safety Checks: When checking and setting the state of an object, it ensures the correctness of the state.
- Optimistic lock implementation: In distributed databases and caches, it is used to implement the optimistic lock mechanism and reduce lock competition.
2. The principle of CAS
2.1 Compare and swap operations
The core of the CAS operation is the Compare and Exchange (Compare and Exchange) operation. It does this by comparing the current value of the memory location with the expected value, and if they are equal, writes the new value to the memory location; otherwise, does nothing. This process is atomic, i.e. it is not interrupted by other threads during the comparison and exchange operation.
2.2 Implementation Principle of CAS
The implementation of CAS operations depends on the underlying hardware support, and is usually implemented using atomic instructions provided by the processor. In modern multi-core processors, specific instructions (such as the CMPXCHG instruction under the x86 architecture) are usually used to perform CAS operations. These instructions guarantee that compare and exchange operations are performed within a single instruction cycle, thereby guaranteeing the atomicity of the operation.
2.3 ABA problems and solutions of CAS
Although the CAS operation can guarantee data consistency and thread safety, it may encounter ABA problems. The ABA problem refers to: if a value does not change before and after the CAS operation, but undergoes multiple changes during the operation, it may cause the CAS operation to be inaccurate.
A common way to solve the ABA problem is to use version numbers or flag bits. The AtomicStampedReference and AtomicMarkableReference classes are provided in the Java concurrent package to solve this problem. AtomicStampedReference uses a version number to mark, while AtomicMarkableReference uses a boolean mark bit.
3. CAS in Java
3.1 java.util.concurrent.atomic 包
Java provides the java.util.concurrent.atomic package, which contains some CAS-based atomic classes for implementing atomic operations in a multi-threaded environment. These classes include AtomicInteger, AtomicLong, AtomicBoolean, etc.
3.2 AtomicInteger 与 AtomicLong
AtomicInteger and AtomicLong are used for atomic operations on integer and long integers respectively. They provide a series of methods, such as get, set, getAndIncrement, getAndSet, etc., which can ensure the atomicity of operations in a multi-threaded environment.
3.3 Solution to ABA problem: AtomicStampedReference and AtomicMarkableReference
In order to solve the ABA problem that may be encountered in CAS operations, the Java concurrent package provides the AtomicStampedReference and AtomicMarkableReference classes. These two classes prevent the occurrence of ABA problems by introducing version numbers or flag bits on the basis of CAS operations.
4. Advantages and disadvantages of CAS
4.1 Advantages: lock-free, high concurrency performance
The CAS operation is a lock-free algorithm. Compared with the traditional lock mechanism, it reduces thread blocking and switching overhead, thereby improving concurrency performance. In a high-concurrency environment, CAS can make better use of the advantages of multi-core processors and improve system throughput.
4.2 Disadvantages: number of spins, ABA problem
One disadvantage of the CAS operation is that the number of spins may be too many. If the CAS operation cannot be successfully executed within a certain period of time, the thread will continue to spin and waste CPU resources. In addition, CAS operations may encounter ABA problems, which need to be resolved using version numbers or flag bits.