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Real-Time and Multimedia SystemsDr. M. Munlin 1
NETE0516 Operating Systems
Instructor: ผ.ศ . ดร . หมั�ดอามั�น หมั�นหลิ�น
Faculty of Information Science and TechnologyMahanakorn University of Technology
Email: [email protected]
Real-Time and Multimedia SystemsReal-Time and Multimedia Systems
Lecture#15
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Real-Time and Multimedia SystemsDr. M. Munlin 2
ObjectivesObjectives
To explain the timing requirements of real-time systems
To discuss the defining characteristics of real-time systems
To describe scheduling algorithms for hard real-time systems
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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Real-Time and Multimedia SystemsDr. M. Munlin 3
TopicsTopics
Features of Real-Time Systems Implementing Real-Time Operating Systems Real-Time CPU Scheduling VxWorks 5.x Multimedia Streaming Compression Techniques QoS An Example: Tiger Video Server
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Real-Time and Multimedia SystemsDr. M. Munlin 4
Overview of Real-Time SystemsOverview 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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Real-Time and Multimedia SystemsDr. M. Munlin 5
Features of Real-Time KernelsFeatures 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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Real-Time and Multimedia SystemsDr. M. Munlin 6
Implementing Real-Time Implementing Real-Time Operating SystemsOperating Systems
In general, real-time operating systems must provide:
(1) Preemptive, priority-based scheduling
Higher priority process can preempt.
Soft real-time
(2) Preemptive kernels
Allow a process running in kernel to be preempted
Hard real-time
Insert preemption point in a long system call, context switch to the high priority process, interrupt and return to system call
Windows XP are non-preemptive kernel
(3) Latency must be minimized
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Real-Time and Multimedia SystemsDr. M. Munlin 7
Minimizing LatencyMinimizing Latency
Event latency is the amount of time from when an event occurs to when it is serviced.
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Real-Time and Multimedia SystemsDr. M. Munlin 8
Interrupt LatencyInterrupt 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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Real-Time and Multimedia SystemsDr. M. Munlin 9
Dispatch LatencyDispatch 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 and Multimedia SystemsDr. M. Munlin 10
Real-Time CPU SchedulingReal-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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Real-Time and Multimedia SystemsDr. M. Munlin 11
Scheduling of tasks when PScheduling of tasks when P22 has a has a
higher priority than Phigher priority than P11
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Real-Time and Multimedia SystemsDr. M. Munlin 12
Rate Monotonic SchedulingRate Monotonic 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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Real-Time and Multimedia SystemsDr. M. Munlin 13
Missed Deadlines with Missed Deadlines with Rate Monotonic SchedulingRate Monotonic Scheduling
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Real-Time and Multimedia SystemsDr. M. Munlin 14
Earliest Deadline First (EDF) SchedulingEarliest Deadline First (EDF) Scheduling
Priorities are assigned according to deadlines:
the earlier the deadline, the higher the priority;
the later the deadline, the lower the priority.
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Real-Time and Multimedia SystemsDr. M. Munlin 15
Proportional Share SchedulingProportional Share Scheduling
T shares are allocated among all processes in the system.
An application receives N shares where N < T.
This ensures each application will receive N / T of the total processor time.
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Real-Time and Multimedia SystemsDr. M. Munlin 16
VxWorks 5.0VxWorks 5.0
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Real-Time and Multimedia SystemsDr. M. Munlin 17
Wind MicrokernelWind Microkernel
The Wind microkernel provides support for the following:
(1) Processes and threads;
(2) preemptive and non-preemptive round-robin scheduling;
(3) manages interrupts (with bounded interrupt and dispatch latency times);
(4) shared memory and message passing interprocess communication facilities.
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Real-Time and Multimedia SystemsDr. M. Munlin 18
MultimediaMultimedia
Multimedia data includes
- audio and video clips (i.e. MP3 and MPEG files)
- live webcasts
Multimedia data may be delivered to
- desktop PC’s
- handheld devices (PDAs, smart phones, etc.)
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Real-Time and Multimedia SystemsDr. M. Munlin 19
Media DeliveryMedia Delivery
Multimedia data is stored in the file system like othe 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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Real-Time and Multimedia SystemsDr. M. Munlin 20
StreamingStreaming
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 client’s computer.
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Real-Time and Multimedia SystemsDr. M. Munlin 21
Real-time StreamingReal-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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Real-Time and Multimedia SystemsDr. M. Munlin 22
Multimedia Systems CharacteristicsMultimedia 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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Real-Time and Multimedia SystemsDr. M. Munlin 23
CompressionCompression
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 @ 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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Real-Time and Multimedia SystemsDr. M. Munlin 24
Operating Systems IssuesOperating 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.
Guaranteeing QoS has the following effects in a computer system:
(1) CPU processing
(2) Scheduling
(3) File systems
(4) Network protocols
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Real-Time and Multimedia SystemsDr. M. Munlin 25
Quality-of-Service (QoS)Quality-of-Service (QoS) There are three levels of QoS
(1) Best-effort service - the system makes a best effort with no QoS guarantees.
(2) Soft QoS - allows different traffic streams to be prioritized, however no QoS guarantees are made.
(3) Hard QoS - the QoS rquirements are guaranteed.
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Real-Time and Multimedia SystemsDr. M. Munlin 26
Parameters Defining QoSParameters Defining QoS
Throughput - the total amount of work completed during a specific time interval.
Delay - the elapsed time from when a request is first submitted to when the desired result is produced.
Jitter - the delays that occur during playback of a stream.
Reliability - how errors are handled during transmission and processing of continuous media.
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Real-Time and Multimedia SystemsDr. M. Munlin 27
Resources on a file serverResources on a file server
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Real-Time and Multimedia SystemsDr. M. Munlin 28
CPU SchedulingCPU Scheduling
Multimedia systems require hard realtime scheduling to ensure critical tasks will be serviced within timing deadlines.
Most hard realtime CPU scheduling algorithms assign realtime processes static priorities that do not change over time.
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Real-Time and Multimedia SystemsDr. M. Munlin 29
Disk SchedulingDisk Scheduling
Disk scheduling algorithms must be optimized to meet the timing deadlines and rate requirements of continuous media.
Earliest-Deadline-First (EDF) Scheduling
SCAN-EDF Scheduling
The EDF scheduler uses a queue to order requests according to the time it must be completed (its deadline.)
SCAN-EDF scheduling is similar to EDF except that requests with the same deadline are ordered according to a SCAN policy.
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Real-Time and Multimedia SystemsDr. M. Munlin 30
Deadline and cylinder requests for Deadline and cylinder requests for SCAN-EDF schedulingSCAN-EDF scheduling
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Real-Time and Multimedia SystemsDr. M. Munlin 31
Network ManagementNetwork Management
Three general methods for delivering content from a server to a client across a network:
(1) Unicasting - the server delivers the content to a single client.
(2) Broadcasting - the server delivers the content to all clients, regardless whether they want the content or not.
(3) Multicasting - the server delivers the content to a group of receivers who indicate they wish to receive the content.
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Real-Time and Multimedia SystemsDr. M. Munlin 32
Standard HTTP Streaming ProtocolStandard HTTP Streaming Protocol
Standard HTTP is stateless whereby the server does not maintain the status of its connection with the client.
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Real-Time and Multimedia SystemsDr. M. Munlin 33
Real-time Streaming ProtocolReal-time Streaming Protocol
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Real-Time and Multimedia SystemsDr. M. Munlin 34
RTSP StatesRTSP States
SETUP - the server allocates resources for a client session.
PLAY - the server delivers a stream to a client session.
PAUSE - the server suspends delivery of a stream.
TEARDOWN - the server breaks down the connection and releases the resources allocated for the session.
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Real-Time and Multimedia SystemsDr. M. Munlin 35
Tiger video serverTiger video server
Video on demand for a large number of users
Quality of service
Scalable and distributed
Low cost hardware
Fault tolerant
Tiger
Network
Clients
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Real-Time and Multimedia SystemsDr. M. Munlin 36
Tiger architectureTiger architecture
Storage organization
Striping
Mirroring
Distributed schedule
Tolerate failure of any single computer or disk
Network support
Other functions
pause, stop, start
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Real-Time and Multimedia SystemsDr. M. Munlin 37
Tiger video file server hardware configurationTiger video file server hardware configuration
n+10 n+21 n+32 n+43 2n+1n
Controller
Cub 0 Cub 1 Cub 2 Cub 3 Cub n
ATM switching network
video distribution to clientsStart/Stop
requests from clients
low-bandwidth network
high-bandwidth
Cubs and controllersare standard PCs
Each movie is stored in 0.5 MB blocks (~7000) across all disks in the order of the disk numbers, wrapping around after n+1 blocks.
Block i is mirrored in smaller blocks on disks i+1 to i+d where d is the decluster factor
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Real-Time and Multimedia SystemsDr. M. Munlin 38
Tiger scheduleTiger schedule
block play time T block service
time t
slot 0
viewer 4 state
viewer client viewer state: slot 1
free
slot 2
free
slot 3
viewer 0 state
slot 4
viewer 3 state
slot 5
viewer 2 state
slot 6
free
slot 7
viewer 1 state
012
Network address of client
FileID for current movie
Number of next block
Viewer's next play slot
Viewer state:
slot 0
viewer 4 state
viewer client viewer state: slot 1
free
slot 2
free
slot 3
viewer 0 state
slot 4
viewer 3 state
slot 5
viewer 2 state
slot 6
free
slot 7
viewer 1 state
Cub algorithm:
Read the next block into buffer storage at the Cub.
Packetize the block and deliver it to the Cub’s ATM network controller with the address of the client computer.
Update viewer state in the schedule to show the new next block and play sequence number and pass the updated slot to the next Cub.
Clients buffer blocks and schedule their display on screen.
Stream capacity of a disk = T/t (typically ~ 5)Stream capacity of a cub with n disks = n x T/t