EE512 System Programming

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EE512 System Programming
Lecture 17 Real-Time and Multimedia Systems
Nov. 26, 2009 Prof. Kyu Ho Park http//core.kaist.
ac.kr
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Real-Time SystemsCh.19,Silverschatz

• System Characteristics
• Features of Real-Time Systems
• Implementing Real-Time Operating Systems
• Real-Time CPU Scheduling
• Multimedia SystemsCh.20, Silverschatz
• What is Multimedia
• Requirements of Multimedia Kernels
• CPU Scheduling
• Disk Scheduling

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Objectives

• To explain the timing requirements of real-time
  systems
• To distinguish between hard and soft real-time
  systems
• To discuss the defining characteristics of
  real-time systems
• To describe scheduling algorithms for hard
  real-time systems

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Overview of Real-Time Systems

• A real-time system requires that results be
  produced within a specified deadline period.
• An embedded system is a computing device that is
  part of a larger system (I.e. automobile,
  airliner.)
• A safety-critical system is a real-time system
  with catastrophic results in case of failure.
• A hard real-time system guarantees that real-time
  tasks be completed within their required
  deadlines.
• A soft real-time system provides priority of
  real-time tasks over non real-time tasks.

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Features of Real-Time Kernels

• Most real-time systems do not provide the
  features found in a standard desktop system.
• Reasons include
• Real-time systems are typically single-purpose.
• Real-time systems often do not require
  interfacing with a user.
• Features found in a desktop PC require more
  substantial hardware that what is typically
  available in a real-time system.

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Virtual Memory in Real-Time Systems

• Address translation may occur via
• (1) Real-addressing mode where programs generate
  actual addresses.
• (2) Relocation register mode.
• (3) Implementing full virtual memory.

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Address Translation
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Implementing Real-Time Operating Systems

• In general, real-time operating systems must
  provide
• (1) Preemptive, priority-based scheduling
• (2) Preemptive kernels
• (3) Latency must be minimized

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Minimizing Latency

• Event latency is the amount of time from when an
  event occurs to when it is serviced.

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Interrupt Latency

• Interrupt latency is the period of time from when
  an interrupt arrives at the CPU to when it is
  serviced.

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Dispatch Latency

• Dispatch latency is the amount of time required
  for the scheduler to stop one process and start
  another.

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Real-Time CPU Scheduling

• Periodic processes require the CPU at specified
  intervals (periods)
• p is the duration of the period
• d is the deadline by when the process must be
  serviced
• t is the processing time

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Scheduling of tasks

• Scheduling of tasks when P2 has a higher priority
  than P1

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Rate Montonic Scheduling

• A priority is assigned based on the inverse of
  its period
• Shorter periods higher priority
• Longer periods lower priority
• P1 is assigned a higher priority than P2.

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Scheduling of tasks

• Missed Deadlines with Rate Monotonic Scheduling

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Earliest Deadline First Scheduling

• Priorities are assigned according to
  deadlinesThe earlier the deadline, the higher
  the priority
• the later the deadline, the lower the priority.

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Delayed Locking TechniqueJPLeeKPARK,2008
P
NP
RT
P
preemptible section
OS Latency
NP
non-preemptible section (critical section)
urgent interrupt
RT
real-time process section
P
RT
NP
urgent interrupt

• Entering a critical section is allowed only if
  the operation does not disturb the future
  execution of the real-time application
• OS Latency is significantly reduced at the
  expense of delaying execution of NP section

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Locking Decision
(a) Locking is prohibited (delayed)
enter ith section
P
RT
NP
tlock,i gt tintr,j ?
Y
N
tintr,j
tlock,i
urgent timer interrupt
decision point
Locking is prohibited
Locking is allowed
(b) Locking is allowed
enter wait queue
P
NP
P
RT
tlock,i
tintr,j

• tintr,j time remaining until the next j-th
  urgent interrupt
• tlock,i time taken to execute an i-th
  non-preemptible section (lock hold time)

decision point
urgent timer interrupt
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OS Latency
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Multimedia SystemsObjectives

• To identify the characteristics of multimedia
  data
• To examine several algorithms used to compress
  multimedia data
• To explore the operating system requirements of
  multimedia data, including CPU and disk
  scheduling and network management

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What is Multimedia?

• Multimedia data includes
• - audio and video clips (i.e. MP3 and MPEG
  files)
• - live webcasts
• Multimedia data may be delivered to- desktop
  PCs
• - handheld devices (PDAs, smart phones)

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Media Delivery

• Multimedia data is stored in the file system like
  other ordinary data.
• However, multimedia data must be accessed with
  specific timing requirements.
• For example, video must be displayed at 24-30
  frames per second. Multimedia video data must be
  delivered at a rate which guarantees 24-30
  frames/second.
• Continuous-media data is data with specific rate
  requirements.

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Streaming

• Streaming is delivering a multimedia file from a
  server to a client - typically the deliver occurs
  over a network connection.
• There are two different types of streaming
• 1. Progressive download - the client begins
  playback of the multimedia file as it is
  delivered. The file is ultimately stored on the
  client computer.
• 2. Real-time streaming - the multimedia file is
  delivered to - but not stored on - the clients
  computer.

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Real-time Streaming

• There are two types of real-time streaming
• (1) Live streaming - used to deliver a live
  event while it is occurring.
• (2) On-demand streaming - used to deliver media
  streams such as movies, archived lectures, etc.
  The events are not delivered in real-time.

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Multimedia Systems Characteristics

• Multimedia files can be quite large.
• Continuous media data may require very high data
  rates.
• Multimedia applications may be sensitive to
  timing delays during playback of the media.

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Compression

• Because of the size and rate requirements of
  multimedia systems, multimedia files are often
  compressed into a smaller form.
• MPEG Compression
• (1) MPEG-1 - 352 X 240 _at_ 30 frames/second
• (2) MPEG-2 - Used for compressing DVD and
  high-definition television (HDTV)
• (3) MPEG-4 - Used to transmit audio, video, and
  graphics. Can be delivered over very slow
  connections (56 Kbps)

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Operating Systems Issues

• The operating system must guarantee the specific
  data rate and timing requirements of continuous
  media.
• Such requirements are known as Quality-of-Service
  (QoS) guarantees.