select, poll, epoll

1. select

• Several major disadvantages of select:
  • Every time select is called, the fd set needs to be copied from user space to kernel space. This overhead becomes large when there are many fds.
  • At the same time, each select call requires the kernel to traverse all passed-in fds. This overhead also becomes large when there are many fds.
  • The number of file descriptors supported by select is too small; the default is 1024.
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2. poll

• Solves the third problem of select.
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3. epoll

• Solves all three disadvantages of select.
• 

3.1. LT

• Level triggered: as long as the read buffer is not empty, read events continue to be triggered; as long as the write buffer is not full, write events continue to be triggered.
• This is generally used because if an ET event is not handled properly after one trigger, there is no second chance.

3.2. ET

• Edge triggered: triggers once when the read buffer changes from empty to nonempty; triggers once when the write buffer changes from full to non-full.
  • For example, a user sends a large file and fills the write buffer. Later, when the buffer becomes writable, a transition from full to non-full occurs once.

4. References

• Summary of the Differences Between select, poll, and epoll - CNBlogs