Operating Systems - Lecture 3

Process and Process State

Process

is a program in execution.

passive, active

A Program is ______; a process is _______.

• Address Space

• CPU State

• OS Resources

Process Structure

• New

• Running

• Waiting (Blocked)

• Ready

• Terminated

Process State

New

The process is about to be created but not yet created. It is the program that is present in secondary memory that will be picked up by OS to create the process.

Running

The process is chosen from the ready queue by the CPU for execution and the instructions within the process are executed by any one of the available CPU cores

Waiting (Blocked)

Whenever the process is _____  for an event, it enters the ______

state or ____state. The process continues to wait in the main memory and does not require CPU.

Ready

After the creation of a process, the process enters the ____ state. Processes that are ______ for execution by the CPU are maintained in a queue called ready queue.

Process Control Block (PCB)

• Each process is represented in the operating system by a _____ also called a task control block.

• It contains information associated with a specific process

• Process State

• Program Counter

• CPU registers

• CPU-Scheduling Information

• Memory-Management Information

• Accounting Information

• I/O status Information

Process Control Block includes:

Process State

The state may be new, ready, running, waited, halted, etc.

Program Counter

The counter indicates the address of the next instruction to be

executed for this process.

CPU Registers

• The registers vary in number and type, depending on the computer architecture.

• They include accumulators, index registers, stack pointers, and general-purpose registers.

CPU-Scheduling Information

This information includes a process priority, pointers to

scheduling queues, and any other scheduling parameters

Memory Management Information

This information may include such items as the value of the base and limit registers and the page tables, or the segment tables, depending on the memory system used by the operating system

Accounting Information

This information includes the amount of CPU and real time used, time limits, account numbers, job or process numbers, etc.

I/O Status Information

This information includes the list of I/O devices allocated to the process, a list of open files, etc.

Process Scheduler

selects an available process (possibly from a set of several available processes) for program execution on a core

• Ready Queue

• Wait Queue

Scheduling Queues

Ready Queue

contains those processes that are ready to run

Wait Queue

contains those processes that are waiting for a certain event to occur

Context Switch

is a task where switching the CPU core to another process requires performing a state save of the current process and a state restore of a different process

State Save

performs a state save of the current state of the CPU core

State Restore

to resume operations

• State save

• State restore

Context Switch

• Preemptive

• Non-preemptive

Categories in Scheduling falls into two categories:

Non-preemptive

• a process’s resource cannot be taken before the process has finished running.

• when a running process finishes and transitions to a waiting state, resources are switched.

Preemptive

• the OS assigns resources to a process for a predetermined period of time.

• the process switches from running state to ready state or from waiting state to ready state during resource allocation.

Long Term or Job Scheduler

• brings the new process to the Ready State.

• it controls the Degree of Multi-programming

• it increases efficiency by maintaining a balance between I/O and CPU-bound processes.

• they operate at a high level and are typically used in batch-processing systems.

Short-Term or CPU Scheduler

• is responsible for selecting one process from the ready state for scheduling it on the running state.

• all scheduling algorithms are used.

• is responsible for ensuring no starvation due to high burst time processes.

The Dispatcher

is responsible for loading the process selected by the Short-term scheduler on the CPU (Ready to Running State) Context switching is done by the dispatcher only

• Switching Context;

• Switching to user mode;

• Jumping to the proper location in the newly loaded program.

The dispatcher does the following:

Medium-Term Scheduler

• it is responsible for suspending and resuming process.

• it mainly does swapping (moving processes from main memory to disk and vice versa).

• swapping may be necessary to improve the process mix or because a change in memory requirements has overcommitted available memory, requiring memory to be freed up.

• is helpful in maintaining a perfect balance between the I/O bound and the CPU bound.

• it reduces the degree of multiprogramming.

Degree of Multi-programming

the number of processes present in a ready state at any point in time.