RTOS Design

1
RTOS Design

• Some of the content of this set of slides is
  taken from the documentation existing on the
  FreeRTOS website
• www.freertos.org

2
RTOS Design

• Review states of a process
• Introduce queues that are used to keep track of
  processes
• Introduce the concepts of scheduler and
  dispatcher

3
RTOS Design
4
Process Suspension

• A process can be suspended in two ways
• involuntarily by the RTOS kernel
• Higher priority process preempts a low priority
  process
• a process can choose to suspend itself
• Wants to delay (sleep) for a fixed period of time
• Wait (block) for a resource to become available
  (e.g. serial port)
• A blocked or sleeping process is not able to
  execute, and will not be allocated any processing
  time.

5
Queues

• Queues are used to keep track of processes.
• Ready queue
• the set of processes that can be run if given
  access to the CPU.
• Blocked queue
• the set of processes that are waiting on some
  particular I/O device to become ready or on some
  event

6
Scheduler
7
Context Switch

• When the CPU has to execute a new process and the
  old one has not finished, the system must
• save the state of the old process
• then load the saved state for the new process.
• This is called a context switch.
• Context switch time is overhead and no useful
  work is done while the switch takes place.
• Context switching time depends upon hardware
  support.

8
Scheduler

• The scheduler is the part of the kernel
  responsible for deciding which process should be
  executing at any particular time. The kernel can
  suspend and later resume a process many times
  during the process lifetime.
• The scheduling policy is the algorithm used by
  the scheduler to decide which process to execute
  at any point in time.

9
Scheduling Decisions

• The scheduler selects from among the processes in
  memory that are ready to execute and allocates
  the CPU to one of them.
• Scheduling decisions may take place when a
  process
• switches from running to blocked state
• switches from running to ready state
• switches from blocked to ready state
• terminates

10
Dispatcher

• The dispatcher gives control of the CPU to the
  process selected by the scheduler, this involves
• switching context
• switching to user mode
• jumping to the proper location in the user
  program to restart that program
• It takes time to stop one process and start
  another. This is called dispatch latency.
• The dispatcher and scheduler are often referred
  to as the scheduler.

11
Summary

• Involuntary(by RTOS kernel) and voluntary
  suspension of process (by itself).
• Described ready and blocked queues.
• Reviewed context switch and introduced context
  switching time
• Described scheduler, scheduling policy, events
  for scheduling decisions, dispatcher and dispatch
  latency.