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Preface
Redis is a very popular in-memory database known for its efficient performance and simple data structures. For a beginner of Redis, the biggest doubt about Redis so far is: Why does Redis use a single thread when processing tasks? Why is its efficiency so high and its speed so fast?
This article will be an answer to myself and other friends who have questions.
Why does Redis use single thread?
1. Simplify design
The reason why Redis uses a single-threaded model is mainly for simplicity of design. The single-threaded model makes the code structure of Redis clearer and easier to maintain and expand. Compared with complex multi-threaded or multi-process models, single-threading makes it easier to develop and maintain Redis.
2. Thread safety to avoid race conditions
Redis's single-threaded model provides natural thread safety. In a multi-threaded environment, data sharing can lead to race conditions, requiring the use of locks to protect shared data. This introduces complexity and performance overhead. The single-threaded model of Redis avoids these problems, because only one thread is performing operations at any given moment, and there is no need to worry about concurrency issues.
For example, use multiple Redis clients to connect to a Redis server at the same time and operate INCRa key at the same time:
127.0.0.1:6379> INCR key
(integer) 1
127.0.0.1:6379> INCR key
(integer) 2
127.0.0.1:6379> INCR key
(integer) 3
With a total of three increments, the final result is not likely to be less than 3 as in a multi-threaded environment.
3. Utilize CPU cache
Redis's single thread frequently accesses data in memory. Since data can usually be retained in the CPU cache, the latency of memory access is reduced and performance is improved. The multi-threaded model may cause data to be scattered in the memory of different threads, increasing the possibility of cache invalidation.
4. Avoid thread switching overhead
In a multi-threaded model, switching between threads will bring context switching overhead, including saving and restoring register states. These overheads can become significant in high concurrency situations. Redis's single-threaded model avoids these overheads and improves overall performance.
Why is it faster to use a single thread?
1. Memory storage
Redis stores all data in memory, and the read and write speed of memory is much higher than that of disk storage. This means that Redis can read and write data quickly without being limited by disk I/O speed.
2. Non-blocking I/O
Redis uses non-blocking I/O operations. This means that Redis will not block the entire process or thread when performing an I/O operation, but can continue to process other requests. This asynchronous operation can improve the performance of Redis in high concurrency environments.
3. I/O multiplexing notification mechanism
The single-threaded model of Redis does not mean that the entire Redis server is single-threaded. In addition to the main thread, Redis also uses I/O multiplexing notification mechanisms, such as epoll or select. These mechanisms allow Redis to continue processing other tasks while waiting for I/O to complete. When a task is ready, the notification mechanism wakes up the Redis task processing thread, thereby reducing the waiting time.
4. Simple data model
The data model of Redis is very simple and does not support many data structures. This reduces the complexity of internal operations, reduces the overhead of processing data, and improves execution efficiency.
In summary, Redis uses a single-threaded model to simplify design, improve thread safety, make full use of CPU cache, and avoid thread switching overhead. At the same time, the reason why Redis is able to maintain efficient performance under the single-threaded model is that it makes full use of the advantages of memory storage, non-blocking I/O, I/O multiplexing notification mechanism, and simple data model. This makes Redis perform well in most application scenarios, especially for highly concurrent read and write operations.