I/O completion port's advantages and disadvantages

Question

Why do many people say I/O completion port is fast and nice model?
What is the I/O completion port\'s advantages and disadvantages?

I want to know some points

Answer 1

by using IOCP, we can overcome the "one-thread-per-client" problem. It is commonly known that the performance decreases heavily if the software does not run on a true multiprocessor machine. Threads are system resources that are neither unlimited nor cheap.

IOCP provides a way to have a few (I/O worker) threads handle multiple clients' input/output "fairly". The threads are suspended, and don't use the CPU cycles until there is something to do.

Also you can read some information in this nice book http://www.amazon.com/Windows-System-Programming-Johnson-Hart/dp/0321256190