Analysis and application of Redis network IO model

Redis is an open source, high-performance key-value storage system that is widely used in big data, architecture design and other fields. Its efficient network IO model is an important foundation for its rapid response to requests. This article will introduce the network IO model of Redis and its implementation principles, and discuss its optimization methods in practical applications.

1. Redis’s network IO model

Redis’s network IO model chooses a combination of single thread and multiplexing. The basic process is as follows:

1. Redis first creates a listening socket through the socket function and binds the service port.
2. Redis enters the main loop, calls the select function in the main loop, adds the listening socket and the client connection socket to the monitoring list, and blocks until an event occurs.
3. When the listening socket has a connection request, the select function will return the connection request event. At this time, Redis receives the connection request through the accept function and creates a new client connection socket.
4. Then Redis adds the newly created client connection socket to the monitoring list.
5. Redis starts processing the request sent by the client. When there is a read event (that is, the client sends data), the select function returns the read event. At this time, Redis reads the request sent by the client through the read function.
6. Redis performs request processing, and after processing, the result is sent to the client connection socket through the send function.
7. The select function blocks again waiting for the event to occur.

The above is the network IO model process of Redis. Since Redis uses a single-threaded approach, it avoids the overhead of context switching and lock competition caused by multi-threading. Multiplexing technology allows one thread to process multiple client requests at the same time, thereby improving the system's concurrent processing capabilities.
2. Implementation principle of Redis network IO model

The multiplexing technology used by Redis is mainly implemented by using the select, poll, epoll and other functions provided by the Linux kernel. Among them, the select and poll functions support a limited number of file descriptors, while the epoll function can support a large number of concurrent connections and its performance is more efficient. Therefore, in Redis versions above Linux 2.6, the epoll function is preferred.

Redis will create an epoll handle when it starts, and add the listening socket (the main service port) to epoll for monitoring. When there is a new connection request, the connection is processed through the accept function, and the newly connected socket is added to the file descriptor set managed by epoll. When there is readable data, epoll will notify Redis, and Redis will read the request from the client, parse and process it according to the protocol, and finally write the response data back to the client.

It is worth noting that Redis uses non-blocking IO (Non-Blocking IO) method. The principle is to set the file descriptor to non-blocking mode, thereby utilizing the characteristics of the kernel's asynchronous IO to implement non-blocking read and write operations, avoiding the situation where the process is blocked waiting for the return of the IO operation. In non-blocking IO mode, when the read operation returns, there may still be unread data in the current file descriptor, so you need to use a loop to read until all the data is read. The writing operation is similar, and data needs to be written in a loop until all data is written.

3. Optimization of Redis network IO model

1. Disable TCP Nagle algorithm

TCP Nagle algorithm is a method to reduce small data packets on the network The number of algorithms to improve network transmission efficiency. However, in some scenarios, data needs to be sent immediately, such as user login and other operations. In this case, you cannot wait until the data reaches the optimal size before sending. At this time, you can disable the TCP Nagle algorithm by setting the TCP_NODELAY option and send data immediately.

2. Reduce frequent IO operations

In Redis, frequent IO operations will greatly reduce system performance. Therefore, when writing a Redis application, you can reduce the amount and number of data sent by optimizing the protocol, such as merging multiple requests into one request, etc. At the same time, when the client performs read and write operations, it can also minimize the sending of data packets smaller than the MTU to avoid frequently triggering IO operations.

3. Use connection pool

In a Redis application, the number of connections will increase as the number of concurrencies increases. If the TCP connection is re-established each time, it will cause resulting in greater system overhead. At this time, connection pool (Connection Pool) technology can be used. Connection pooling is a common technology and is often used in high-concurrency system development. The connection pool can manage multiple connections and reuse existing connections, thereby avoiding the frequent establishment and destruction of TCP connections.

4. Optimize memory allocation and release

Redis’ memory allocation and release is an important part of its application. Using common memory pool technology can reduce the number of memory allocations and releases, thereby improving system performance. In Redis, the encoding method corresponding to the string type is embstr or raw. The raw type does not use memory pool technology, while the embstr type uses the memory pool, so the embstr type should be used to store data as much as possible.

5. Multiple processes solve the bottleneck of single-threaded IO reuse

Although Redis uses a single-threaded approach to provide high-performance IO operations, it also feels its bottleneck. In this case, the functions of the daemon process in one process can be split through multiple processes, allowing each process to handle IO operations independently to improve the concurrency performance of the system.

4. Summary

The network IO model of Redis adopts a combination of single thread and multiplexing. Its efficient implementation scheme and optimization method can ensure efficient execution of system performance. In actual application development, it is necessary to choose the appropriate optimization method according to the specific situation to allow Redis to maximize its potential.

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