Real Time Operating Systems RTOS

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Real Time Operating Systems(RTOS)

• Stephen Ferzetti

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Types of Operating Systems

• An example of a hard real time system is a
  digital fly-by-wire control system of an
  aircraftNo lateness is accepted under any
  circumstances, otherwise the aircraft is not
  controllable.Useless results if late, if the
  control system does not respond timely, the
  result is a hole in the ground.Catastrophic
  failure, which needs no explanation in the case
  of an aircraft crash.Cost of missing deadline is
  infinitely high, the lives of people depend on
  the correct working of the control system of the
  aircraft.A soft real-time system can be a
  vending machineRising cost for lateness of
  results As it will take longer to treat a
  customer when the performance of the vending
  machine is degrading, less customers pay at this
  machine which results in less profits for the
  shop owner. Accept lower performance for
  lateness, it is not catastrophic when deadlines
  are not met. It will take longer to handle one
  client with the vending machine.Non real time
• http//www.omimo.be/encyc/techno/terms/defini/def.
  htmHARD

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Topics

• Why are Real Time Operating Systems Important
• Processes and Threads
• Scheduling
• Deadlocks

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What makes Operating Systems that are real time
so important?
Nuclear Plant Example

• Lets say there is a power plant
• And lets say that the temperature is rising
• And that there is 10 seconds to cool it down
  before it is too late
• But..

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Nuclear Plant

• Since the plant was running its anti-virus
  program at the time it missed its window
• Meltdown

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The Main Reasons for Having a RTOS

• Meeting deadlines
• Being able to break out of lower priority
  processes so that more important tasks can be
  accomplished
• When you break out of these processes that the
  data collected remains intact

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What is a Process?

• A process is an instance of a program running in
  a computer.

Whatis.com
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The Process Model

• A process can be broken into 2 parts
• Resource grouping Process says what it needs to
  run
• Execution The commands that need to occur for
  the command to run (handled by threads)

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Threads

• A thread has a program counter that keeps track
  of which instruction to execute next.
• It also has memory to store the variables it is
  using

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Threads and Processes
int main() int input coutltltEnter a
number cin gtgt input coutltlt you
enteredltltinput return 0

• Process
• Code
• Data
• Child Processes
• Resource List

• Resources
• Monitor
• Keyboard buffer

Process
Thread
Critical region
cout enter a number
cin input
coutyou entered
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Process Creation

• Processes are created when
• The system is initialized
• A process creates another process
• The user requests a new process
• Initiation of a batch job

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System Initialization

• There are always processes running in the
  background so when the system starts these
  initial processes begin

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Process Creates a Process

• A process that is running meets a condition that
  requires another process to run
• Real World Example
• A policeman is patrolling at night, then he
  sees something suspicious. He goes to
  investigate. If there is nothing he continues on
  his rounds. If there is something wrong he calls
  for backup and takes action.

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The User Requests a Process

• The user can start up a new program which will
  spawn new processes
• These programs may be something less important
  like opening an internet browser, or they may be
  critical and require immediate response like the
  triggering of an alarm

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Process States

• Running
• Ready
• Blocked

running
1
2
3
ready
blocked

• Process blocks for input
• Scheduler picks another process
• Scheduler picks this process
• Input becomes available

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Running

• When a process is running that means that it is
  accessing the processor and that its code is
  executing
• Requirements
• The process must have all of its devices
  allocated, and it must not be waiting for any
  other processes to run

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Ready

• The ready condition is when the process has all
  of its resources allocated and it is not waiting
  on any other processes, and all that it is
  waiting for is to have processor time

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Blocked

• A process is blocked when either it is waiting on
  other processes to complete, or it has not been
  able to allocate all of the devices that it needs

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Process Termination

• Ways a process is terminated
• Normal exit (voluntary)
• Error exit (voluntary)
• Fatal error (involuntary)
• Killed by another process (involuntary)

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Termination Example

• Normal Exit
• The document prints
• Error Exit
• The printer is out of paper
• Fatal Error
• The computer crashes
• Killed by another process
• The User cancels the job

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Scheduling

• When multiple processes are in the ready state a
  scheduler is required
• The scheduler uses a scheduling algorithm to
  decide which process can enter the critical region

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Types of Scheduling Algorithms

• Non-preemptive Picks a process to run and will
  allow it to run forever until it chooses to end
• Preemptive Allows the process to run, but at
  anytime it can force the process to move into the
  ready state and allow another process to run

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Deadlocks
Deadlock Condition when no processes will be
completed because they cannot get the resources
they require

• All of these conditions must be true for a
  deadlock to occur
• Mutual Exclusion resources may only be
  allocated to one process at a time
• Hold and Wait A process may allocate a resource
  and wait for others
• No Preemption A resource may not be forcibly
  taken away
• Circular Wait Processes

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Mutual Exclusion

• It is important for many resources to be mutually
  exclusive.
• If a printer was not mutually exclusive to one
  process at a time then if 2 processes were
  printing at the same time then the output would
  blend together and be unreadable.

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Hold and Wait

• When reserving the resources not all of them need
  to be available at that time
• The process can hold on to the ones that are
  available and wait until the others become
  available
• This is important because if a process were to
  wait to have everything it may never go into the
  ready state. This is because processes not
  requiring as much might continually jump in its
  position.

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No Preemption

• This means that a resource cannot be forcibly
  taken away from a process
• If preemption for resources was allowed then
  processes could constantly take away from each
  other and never get to the processor, or a
  process that was running and whose time expired
  might still need that resource

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Circular Wait

• Circular wait means that processes are holding
  resources that other processes need

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Deadlocks
Total Resources 1 printer 1 CD drive
Process B Holding CD Drive Requesting Printer
Process A Holding Printer Requesting CD Drive
Neither process is ever completed
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Strategies for dealing with deadlocks

• Ignore the problem
• Detection and recovery
• Dynamic avoidance by careful resource allocation
• Prevention, canceling one of the 4 conditions for
  deadlocks to occur

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Ignoring

• Ostrich Algorithm
• Do nothing
• If you know for a fact that a deadlock will occur
  very rarely, and the system is not a critical
  system you may choose to use this strategies
• This is totally unacceptable for RTOS because
  reliability is a major issue

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Detection and Recovery

• Check and see if you are in a deadlock and
  attempt to get out of it
• This means that the program will have to
  continually check and see if there is a deadlock
• This wastes too much time

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Avoidance

• This means that the program will continually
  check to see whether or not it is in a safe or
  unsafe condition
• Unsafe is if it is possible for a deadlock to
  occur
• Avoidance is also timely, but not as bad as
  detection and recovery

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Prevention

• To prevent a deadlock you must attack one of
  the conditions for a deadlock to occur
• Mutual exclusion
• Hold and wait
• No preemption
• Circular wait

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Attacking Circular Wait

• To attack the circular wait condition we place
  what resources need to be used in the order that
  they are used
• This means that the process cannot obtain any
  resource that is farther down before getting all
  the higher ones

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Attacking Circular Wait

• Process A
• CD drive
• Printer
• Monitor

In this example Process A may not hold onto the
monitor until having both the CD drive and the
Printer
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Attacking Hold and Wait

• This is the best strategy when using the real
  time operating system
• The strategy here is requiring the process to
  hold all that it requires before entering
  accessing the processor

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Attacking Hold and Wait

• The reason this works best is because if the
  process states up front what it needs then there
  is a list of what each has
• Therefore if a process needs to be executed
  immediately then it can kill the processes
  holding the resources it needs and be run
  immediately

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RTOS For Smart Sensors

• Smart sensors need to make decisions on their own
• Their decisions also need to be quick
• Also it is important that data be preserved

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Decision Making

• There must be a sense of priority, and there
  needs to be the ability to break out of processes
  to make deadlines
• Since real time systems have a clock specifically
  so that the system knows what time it is in the
  world around it is able to keep deadlines

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Data Preservation

• The systems use threading which can save all the
  current information even if the program is
  immediately ended
• This allows not only the quick response but also
  all the data at the occurrence to be saved

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Some Smart Sensor Topicswhere RTOS is Essential

• Scheduling Communication in Real-Time Sensor
  Applications Huan Li, University of
  Massachusetts, AmherstPrashant Shenoy,
  University of Massachusetts, AmherstKrithi
  Ramamritham, Indian Institute of Technology,
  India
• Static Real-Time Data Distribution Angela
  Uvarov, University of Rhode Island, Kingston,
  RILisa DiPippo, University of Rhode Island,
  Kingston, RIVictor Fay-Wolfe, University of
  Rhode Island, Kingston, RIKevin Bryan,
  University of Rhode Island, Kingston, RIPatrick
  Gadrow, University of Rhode Island, Kingston,
  RITimothy Henry, University of Rhode Island,
  Kingston, RIMatthew Murphy, University of Rhode
  Island, Kingston, RIPaul R. Work, Raytheon
  Company, Portsmouth, RILouis P. DiPalma,
  Raytheon Company, Portsmouth, RI
• Radio-Triggered Wake-Up Capability for Sensor
  Networks Lin Gu, University of VirginiaJohn A.
  Stankovic, University of Virginia

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Example

• An airplane has many sensors relaying data back
  to the pilot
• If something were to go wrong the sensors would
  need to send an error to the pilot, and take any
  other action (triggering an extinguisher)
• However it is almost as important that the
  sensors are able to record the data at the point
  of the problem so that the incident can be
  understood

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Homework

• Discuss the differences between preemptive and
  non-preemptive in relation to scheduling
• Pick a sensor you think would require a real time
  system and state why you picked it

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Questions
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References

• Modern Operating Systems 2nd edition, Tanenbaum
• Whatis.com
• Special thanks to Hussein for his help