So what is the listen.backlog directive responsible for and how to understand it?

Question

linux

Actually, what does the listen.backlog directive in php-fpm do? The definitions of this parameter are ambiguous, -1 == unlim say some, -1 == take settings from sysctl others say, 4096 (for example) = the value of the queue set by the handles, because all the previous options are decay.
If I understand correctly, the value of listen.backlog is the size of the pool queue, or am I wrong?!

Answer 1

listen.backlog is the backlog parameter of the TCP listen function of the socket on which the fpm hangs,
the backlog parameter is responsible for the size of the queue of simultaneously _waiting_ connections to the socket, that is, initiated (SYN - SYN,ACK - ACK), but not yet accepted by the server (established)
- 1 uses the current hard limit net.core.somaxconn, you can open the source and see for yourself. The default value in Linux is 128 which is more than enough for any php-fpm.