Linux – Why CPU spent time on IO(wa)

central-processing-unitiolinuxlinux-kernel

I know wa (in top) measures the CPU time on waiting for I/O. Many articles say that.

But I am confused that, based on 2 knowledge points:

  1. if a process uses a system call to read disk, the process is blocked.
  2. If a process is blocked, it is cannot be scheduled running on CPU.

Right?

It seems there no time for CPU waiting on I/O… What happens?

If recommend some books or articles for me to further reading, so much the better.

Best Answer

The CPU idle status is divided in two different "sub"-states: iowait and idle.

If the CPU is idle, the kernel then determines if there is at least one I/O currently in progress to either a local disk or a remotely mounted disk (NFS) which had been initiated from that CPU. If there is, then the CPU is in state iowait. If there is no I/O in progress that was initiated from that CPU, the CPU is in idle state.

So, iowait is the percentage of time the CPU is idle AND there is at least one I/O in progress initiated from that CPU.

The iowait counter states that the system can handle more computational work. Just because a CPU is in iowait state does not mean that it can't run other threads or processes on that CPU.

So, iowait is simply a form of idle time.

Related Topic