Group 9
Tittle:Real Time Linux
Members:
Lê Viết Hợp
Biện Xuân Vĩnh
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Real Time Linux
Overview Real-Time
 System that has to perform its functions by responding to
synchronous or asynchronous events within a specified
amount of time.
 May be
 Soft RT – occasionally allowed to miss deadlines (eg.
Updating a video display)
 Hard RT – Meeting the deadline is absolutely critical (eg.
Rocket controller)
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Real Time Linux
Kenel Linux
Real Time Linux
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Linux is not Real Time
 The Linux kernel (< 2.6) is neither preemptive nor reentrant by
user processes.
 Context switch takes several hundred microseconds.
 Insufficient resource handling.
 Scheduling algorithm designed not for real time.
 Unbound amount of CPU time used by interrupt handlers
under Linux, and these run at a higher priority than any user
process.
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Real Time Linux
Overview Real Time Linux
 Inspired by MERT (Bell Labs 1978) – a full-fledged
Virtual Machine (VM) concept.
 It slips a small, simple, Real Time OS underneath Linux.
 Linux becomes an idle task for this OS.
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Real Time Linux
Implementation of Real Time
Linux
Basically, there are 2 ways to make Linux real
time:
 Dual-Kernel: is to insert a new code layer between
standard Linux Kernel and hardware layer.
 IEEE 1003.1d Kernel: is to implement some real-time
extensions within Standard Linux Kernel such as timer,
scheduling , preemtion logic, etc.
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Real Time Linux
Implementation of RT Linux(Contd.)
In more detail, there are 4 strategies to modify standard
Linux:
 Micro Kernel.
 Nano Kernel.
 Resource Kernel Extension.
 POSIX Real Time Extentions.
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Real Time Linux
Micro Kernel
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Real Time Linux
Nano Kernel
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Real Time Linux
Resource Kernel Extension
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Real Time Linux
Dual-Kernel
 Advantages
 Can make hard real time guarantees
 Easy to implement a new scheduler
 Disadvantages
 Initial port difficult, must know a tremendous amount about
underlying hardware
 Running a small real-time executive is not a substitute for a
full fledged RTOS
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Real Time Linux
POSIX Real Time Extentions
 Directly modify the standard kernel and provide
libraries that together implement the POSIX realtime extensions.
 No second kernel in this approach.
 The patches are made directly to the standard kernel
to implement the timers, signals, semaphores,
process memory locking, shared memory, priority
scheduling, synchronized and assynchronized I/O
specified in IEEE 1003.1d.
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Real Time Linux
POSIX Real Time Extentions
 Advantages
 Most problems, such as interrupt handling, already solved
 Less initial labor
 Disadvantages
 No guaranteed performance
 RT tasks don’t always have precedence over non-RT tasks.
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Real Time Linux
Performance Comparison with
other Real Time Kernel
Performance Characteristics:
Event Latency.
Periodic Jitter.
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Real Time Linux
Real Time Linux
RT Linux
• Developed by Victor Yodaiken at the New
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Mexico Institute of Technology in 1996.
An Open Source, Hard Real-Time Operating
System.
Based on Micro Kernel architecture.
Was originally a research project.Became a
FSM lab's commercial product.
2007, became Wind River Linux up to now.
Real Time Linux
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RTLinux's
implementation
• Implementation of a RT-linux plugin
which:
o Co-exists along with the Standard
Kernel.
o Is considered as a sitting layer
between the Standard Kernel and
Hardware layer.
• The Standard Kernel sees
RTLinux as an actual Hardware.
• Within the RT-linux plugin,there are:
o The RT-schedulers for scheduling
RT tasks and Linux Kernel.
o The tool which generates RT
FIFO for IPC.
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Real Time Linux
RT Linux's Scheduling
• Real-time tasks can be configured by user within a specific
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scheduler and priority.
Real-time tasks executed by RT kernel.
Real-Time tasks are set with the highest priority values.
The RT Kernel allows users to create their own schedulers. So
far, there have been 2 schedulers :
o Priority based preemptive.
o EDF based.
Allows users to set priorities for each task.
Standard Kernel is assigned the lowest priority among the RT
tasks.
Supporting: FIFO, Shared Memory and Semaphore in IPC.
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Real Time Linux
RTAI
The Linux RT
• Real-Time Application Interface
• Implemented by staff at Dipartimento di Ingegneria
Aerospaziale - Politecnico di Milano, Italy.
• An open-source project.
• Originally based on RT Linux.
• Based on Real-Time Hardware Abstraction Layer (RTHAL).
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Real Time Linux
RTAI's implementation
• Implementation a RTHAL within
Linux Kernel. RTHAL is:
o Real Time Hardware
Abstraction Layer.
o Implemented to communicate
with the Real-time Kernel.
• Implementation Real-time Kernel
which:
o Co-exist along with the Linux
Kernel.
o Is placed between hardware
and Linux Kernel.
• FIFO is also implemented.
Real Time Linux
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RTAI's scheduling
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RTAI treats the Linux kernel as a low priority real-time task.
Whenever a new task is added, it will be assigned higher priority than the
Linux kernel.
• The scheduler provides services which are used in various real-time operating
systems:
o Suspend.
o Resume.
o Yield.
o Make periodic.
o Wait until.
• Schedulers:
o UP (Uni-processor).
o SMP (symetric multi-processor).
o MUP (Multi-Uni-Processor).
• Supporting: FIFO, Shared Memory, Semaphore,Message queue and Mail
boxes in IPC.
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Real Time Linux
KURT
• A Soft real-time Operating system, named Kansas University's
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Real-Time Linux.
Developed at University of Kansas, started in 1997
Project in the Information and Telecommunications
Technology Center.
An implementation of the POSIX real-time extensions to
Linux.
Specified by IEEE 1003.1d.
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Real Time Linux
KURT's implementation
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Improving the resolution of the system clock to 10ms.
Using the same mechanism for managing time as RT-Linux.
Generating interrupt on demand.
Modifying the scheduler to include new
policy, SCHED_KURT besides those are defined by POSIX
such as SCHED_FIFO, SCHED_RR,....
• Adding new system calls to implement real-time functionality.
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Real Time Linux
KURT's scheduling
• KURT can’t guarantee priority of RT tasks over
non-RT tasks
• An RT task can be blocked by a non-RT task (eg:
during disk I/O) leading to priority inversion
• Suitable for soft RT systems.
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Real Time Linux
ADEOS
The Adaptive Domain Environment for Operating Systems.
• An open source, released in 2002.
• A provision of the GNU GPL (The GNU General Public
License)
• A real-time framework which allows operating systems to be
run in parallel.
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Real Time Linux
ADEOS's implementation
• Providing a Hardware abstract
layer sitting over the hardware
layer.
• Yet, it does not implement RT
Kernel.
• Providing a mechanism which
allows interruption of RT OS
with possible priority.
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Real Time Linux
Linux/RK
A Portable Resource Kernel in Linux.
• A resource kernel based on Linux.
• Developed by the Real-time and Multimedia Systems
Laboratory ,in1998.
• Led by Dr. Raj Rajkumar at Carnegie Mellon
University.
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Real Time Linux
Linux/RK's implementation
• Adding a resource kernel which is a
real-time kernel (operating system)
that :
o Provides to
applications Timely Guaranteed,
and Enforced access to System
Resources
o Allows Applications to specify
only their Resource
Demands leaving the Kernel to
satisfy those Demands using
hidden management schemes
• Providing a tool that allows user
to request, reserve, and be
guaranteed a nite percentage of
machine resources
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Real Time Linux
References
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Real-time linux report v2.0.0, by Aeolean Inc in NIST (Intelligent Systems
Division National Institute of Standards & Technology).
http://www.aero.polimi.it/projects/rtai/contrib.htm
Adaptive Domain Envioronment for Oprerating Systems, by Karim Yaghmour.
http://www.linuxsymposium.org/archives/OLS/Reprints-2001/yaghmour.pdf
Linux/RK: A Portable Resource Kernel in Linux, by Shuichi Oikawa and
Ragunathan Rajkumar.
CS397 Ambuj and Utkarsh Feb 2000
Real Time Linux
Thank you!
Q&A
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Real Time Linux