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CMPS 240 - Operating Systems

Lecture 8: Process Scheduling

Maurice Khabbaz, Ph.D. – Edited by Mohamad Zalghout, Ph.D.

Last Time

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

Critical Section Problem

Hardware-based Solutions

Software-based Solutions

Semaphores

Classical Problems of Synchronization

Monitors

Basic Concepts

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Single-CPU System: only one process can run at a time.
  • Other processes must wait until CPU is freed and can be rescheduled.

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Single-CPU System: only one process can run at a time.
  • Other processes must wait until CPU is freed and can be rescheduled.
• Multitasking OS can simultaneously interleave execution of multiple processes.
  • Several processes may be found in memory at any point in time.
  • On single-CPU machines → illusion of multiple processes running concurrently.
  • On multi-CPU machines → enable process concurrency and parallelism on different CPUs.
• Scheduling: main OS function that schedules all computer resources before use:
  • CPU is one of the primary computer resources.

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Single-CPU System: only one process can run at a time.
  • Other processes must wait until CPU is freed and can be rescheduled.
• Multitasking OS can simultaneously interleave execution of multiple processes.
  • Several processes may be found in memory at any point in time.
  • On single-CPU machines → illusion of multiple processes running concurrently.
  • On multi-CPU machines → enable process concurrency and parallelism on different CPUs.
• Scheduling: main OS function that schedules all computer resources before use:
  • CPU is one of the primary computer resources.
• Whenever CPU is idle:
  • CPU scheduler (STS(short-term scheduler)) selects a process from ready queue for execution.^ AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Single-CPU System: only one process can run at a time.
  • Other processes must wait until CPU is freed and can be rescheduled.
• Multitasking OS can simultaneously interleave execution of multiple processes.
  • Several processes may be found in memory at any point in time.
  • On single-CPU machines → illusion of multiple processes running concurrently.
  • On multi-CPU machines → enable process concurrency and parallelism on different CPUs.
• Scheduling: main OS function that schedules all computer resources before use:
  • CPU is one of the primary computer resources.
• Whenever CPU is idle:
  • CPU scheduler (STS(short-term scheduler)) selects a process from ready queue for execution.
  • CPU scheduling decisions take place when a process:
    1. Switches from running to waiting state.
    2. Switches from running to ready state.
    3. Switches from waiting to ready state.
    4. Terminates.

Ready^ AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

Waiting

start scheduler dispatch

interrupt I/O event or wait() I/O or event completion

Terminated exit() Running

New

• Single-CPU System: only one process can run at a time.
  • Other processes must wait until CPU is freed and can be rescheduled.
• Multitasking OS can simultaneously interleave execution of multiple processes.
  • Several processes may be found in memory at any point in time.
  • On single-CPU machines → illusion of multiple processes running concurrently.
  • On multi-CPU machines → enable process concurrency and parallelism on different CPUs.
• Scheduling: main OS function that schedules all computer resources before use:
  • CPU is one of the primary computer resources.
• Whenever CPU is idle:
  • CPU scheduler (STS(short-term scheduler)) selects a process from ready queue for execution.
  • CPU scheduling decisions take place when a process:
    1. Switches from running to waiting state.
    2. Switches from running to ready state.
    3. Switches from waiting to ready state.
    4. Terminates.

Ready^ AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

Waiting

start scheduler dispatch

interrupt I/O event or wait() I/O or event completion

Terminated exit() Running

New

5

• REMARK:
  • Scheduling under (1.) and (4.) is non-preemptive ( i.e. , cooperative ).
  • Otherwise, scheduling is preemptive.

Definitions

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Non-Preemptive Scheduling:
  • Process keeps CPU until either it terminates or it switches to waiting state.
  • Used on some HW platforms → does not require any special HW ( e.g. timer).
• Preemptive Scheduling:

Definitions

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

• Non-Preemptive Scheduling:
  • Process keeps CPU until either it terminates or it switches to waiting state.
  • Used on some HW platforms → does not require any special HW ( e.g. timer).
• Preemptive Scheduling:
  • Process releases CPU before terminating or willingly switching to waiting state.
  • When an interrupt arises, it preempts a process to handle the interrupt.

Dispatcher Module

• Component involved in CPU scheduling.
• Gives control of CPU to process selected by STS (short-term scheduler).
• Dispatching involves:
  • Switching context.
  • Switching to user mode.
  • Jumping to proper location in the user program to restart that program.

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D. 7

Dispatcher Module

• Component involved in CPU scheduling.
• Gives control of CPU to process selected by STS (short-term scheduler).
• Dispatching involves:
  • Switching context.
  • Switching to user mode.
  • Jumping to proper location in the user program to restart that program.
• Dispatcher should be as fast as possible:
  • It is invoked during every process switch.

AUB - CMPS 240 - M. J. KHABBAZ, Ph.D. 7

Burst Cycles

• CPU Burst Cycle (CPU-BC): cycle of CPU execution.
• I/O Burst Cycle (I/O-BC): wait for I/O. AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

add store

• When executing a process alternates between two burst cycles: read from file

Burst Cycles

• CPU Burst Cycle (CPU-BC): cycle of CPU execution.
• I/O Burst Cycle (I/O-BC): wait for I/O. AUB - CMPS 240 - M. J. KHABBAZ, Ph.D.

add store

• When executing a process alternates between two burst cycles: read from file

time