Real Time Scheduling

1
Real Time Scheduling

• Telvis Calhoun
• CSc 8320

2
Outline

• Introduction
• Real-Time Scheduling Overview
• Tasks, Jobs and Schedules
• Rate/Deadline Monotonic
• Deferrable Scheduling Xiong, 2005
• Real-Time Synchronization
• Priority Inheritance/Priority Ceiling Protocols

3
Introduction

• Real-Time systems include
• Automotive control systems
• Telecommunications
• Industrial Process Control
• Real-Time Systems impose temporal consistency
  constraints
• Database must closely represent the real-time
  system in real time.
• Value of data decreases with time.

4
Hard Real Time vs. Soft Real Time McObject, 2006

• Critical real-time systems such as nuclear power
  plants or fly-by-wire airplanes are Hard Real
  Time
• Non-critical real time systems where missed
  transactions only degrade system quality are Soft
  Real Time
• Firm real-time systems are similar to soft
  real-time, except late tasks are discarded.
• Scheduling algorithm depends on real time system
  characteristics

5
Tasks, Jobs and Schedules

• Aperiodic tasks arrive at the system at arbitrary
  times.
• Periodic tasks have predictable arrival,
  execution times and deadlines.
• Each task executes a job or performs some
  function in the real-time system.
• A real-time schedule assigns tasks to the CPU in
  order to satisfy the system temporal constraints.

6
Rate Monotonic

• Simplest type of real-time scheduling
• Tasks are periodic
• Tasks do not communicate with each other
• Tasks are scheduled according to fixed priorities
• Calculates the critical instant for each task.
• Task completion time considering all other higher
  priority tasks.
• If a tasks deadline is scheduled at the critical
  instant, the task always meets its deadline.

7
Deadline Monotonic

• Tasks are assigned priority
• Tasks with shortest deadline get the highest
  priority.
• Cannot guarantee a feasible schedule
• NOT optimal for fixed priority non-pre-emptive
  scheduling

8
Deferrable Scheduling for Fixed Priority Systems
Xiong, 2005

• Goal Actively schedule the maximum time between
  periodic sensor updates to minimize energy
  consumption.
• Target System Process control systems that
  require continuous sensor updates

9
DS Definitions

• Validity interval Time that a data item is
  considered fresh after an update transaction.
• Response Time Time required to retrieve data
  from a sensor.
• Transaction Deadline Time when a transaction
  must be complete

10
DS Basic Algorithm

1. Set the update transaction deadline (d) to be the
   end of the validity interval.
2. The transaction start time is derived backward
   from the deadline using the known response time
   (r).

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DS Basic Algorithm cont.

• Adjust the schedule for high priority preemption.
• Construct a hyper-period that executes the
  schedule repeatedly to decrease scheduling
  overhead. Xiong, 2005

12
Real-Time Synchronization

• Tasks cooperate and share resources in a
  pre-emptive, priority-based environment.
• Tasks may be synchronized using traditional
  mutexes or semaphores.
• Potential Issues
• Priority Inversion A situation in which a
  higher priority job is blocked by lower priority
  jobs for an indefinite period of time Sha, 1990
• Chain Blocking A situation in which a high
  priority task is blocked for more than one lock
  due to more than one lower priority task. Sha,
  1990

13
Priority Inversions Netrino, 2007

• Low priority task is using a shared resource.
• A medium-priority task preempts the
  lower-priority task.
• If the higher-priority task is otherwise ready to
  run, but a medium-priority task is currently
  running instead, a priority inversion is said to
  occur.

14
Priority Inheritance and Priority Ceiling
protocols SHA, 1990

• Priority Inheritance Protocol (PIP)
• Eliminates priority inversion problems
• The algorithm will increase the priority of a
  task to the maximum priority of any task waiting
  for any resource the task has a resource lock on.
• Priority Ceiling Protocol (PCP)
• Each resource is assigned a priority ceiling,
  which is a priority equal to the highest priority
  of any task which may lock the resource.

15
Hardware Support for Priority Inheritance Akgul,
2003

• SoCLC System-on-a-Chip Lock Cache
• Reduces on-chip memory traffic, provides
• A fair and fast lock hand-off, simplifies
  software, increases
• The real-time predictability of the system and
  improves performance.
• PIP Implemented in an RTOS

16
References

• Chow, Randy, et. al.,Distributed Operating
  Systems Algorithms, Addison Wesley, March 18,
  1997
• McObject LLC, Real-time Databases for Embedded
  Systems, Precision Data Management, 2006
• M. Xiong, S. Han and K.Y. Lam, A Deferrable
  Scheduling Algorithm for Real-Time Transactions
  Maintaining Data Freshness, IEEE Real-Time
  Systems Symposium, 2005.
• http//www.netrino.com/Publications/Glossary/Prior
  ityInversion.php, 2007
• Sha, L. Rajkumar, R. Lehoczky, J.P. Priority
  inheritance protocols an approach to real-time
  synchronization, Computers, IEEE Transactions on
  Volume 39, Issue 9, Sept. 1990 Page(s)1175
  1185
• Akgul, B.E.S. Mooney III, V.J. Thane, H.
  Kuacharoen, P. Hardware support for priority
  inheritance, Real-Time Systems Symposium, 2003.
  RTSS 2003. 24th IEEE 2003 Page(s)246 - 255