How to Properly Set the Backlog Length for the PHP socket_listen Function?

When using PHP for socket programming, the socket_listen() function is commonly employed to listen for incoming connections. The "backlog" parameter specifies the maximum length of the pending connection queue. But how should you determine an appropriate value for this parameter?

1. Understanding Backlog

When a client attempts to connect to the server and the server does not immediately call socket_accept() to process the connection, the new connection is temporarily stored in a queue. The backlog parameter limits the length of this queue.

In simple terms, the backlog parameter sets the maximum number of pending connections the server can handle. Any connection requests exceeding this length will be rejected by the operating system.

2. How to Choose an Appropriate Queue Length

There is no fixed standard for queue length. It is typically set based on the server's processing capacity and the specific use case:

• For low concurrency and small applications: A value between 5 and 20 is sufficient.
• For medium to high concurrency applications: It is recommended to adjust based on actual stress tests, with common values ranging from 50 to 200.
• For very high concurrency or special applications: Larger values can be considered, but should be within the operating system's limits.

3. System Limits

It's important to note that the operating system imposes an upper limit on the backlog parameter:

• On Linux systems, the maximum value is typically determined by net.core.somaxconn.
• On Windows systems, the limit is usually between 5 and 200.

If the backlog value exceeds the system's allowed maximum, the operating system will automatically adjust it to the maximum allowable value.

4. Practical Recommendations

• Start by setting a reasonable initial value based on the expected concurrency, such as between 50 and 100.
• Perform stress testing and adjust based on overflow and rejected connection scenarios.
• Use socket_set_option to adjust other performance parameters, such as SO_REUSEADDR.

In conclusion, the queue length should be set based on the application scenario, server processing capacity, and system limitations. There is no fixed "optimal value," and practical testing and monitoring are key.