increase maximum open files.

1. see here. Increasing the file descriptor limit

2. see APUE, getrlimit, setrlimit()

when to recv() returns?

Basically, there are three conditions:
* The receive buffer is full. Here, the receive buffer means either
the buffer we submit or the intermediate winsock buffer.
* 500ms has passed since the last arrival of data.
* Data arrives with the PUSH-bit set.

the original post is here.