Process States

As soon as a procedure is created, the system assigns it a state. The process can be in one of several states. The state of the processes could be viewed on a system using the ps(C) command with the -el options. The "S" field shows the current state as a single letter.

The very important states for performance tuning are:

O

On processor - This denotes in which the processor is executing on the CPU in either user or system mode.

R

Runnable - This denotes in which the process is on a run queue and is ready-to-run. A runnable process has every resource which it needs to execute except the CPU itself.

S

Sleeping - This denotes in which the process is waiting for some I/O event to complete like as keyboard input or a disk transfer. A Sleeping processes are not runnable until the I/O resource becomes available.

The figure represents these states and the probable transitions among them.

On one CPU systems only one procedure can run on the CPU at a time. Whole other runnable processes have to wait on the run queue. The portion of the Kernel known as the scheduler chooses that process to run on the CPU(s). Whenever the scheduler needs to run a different process on the CPU, from the highest priority the run queue is scanned to the lowest for the first runnable process. Whenever a process becomes runnable, its priority is calculated through the Kernel and is placed on the run queue at which priority. Although it remains runnable, the priority of the process is recalculated once each second, and its position in the run queue is adjusted. Whenever there are no higher-priority runnable processes on the run queue, the procedure is placed on the CPU to run for a fixed amount of time known as a time slice.

Figure