Steve Matovski
CSE 666

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Real Time Linux comes in two different kernels styles
A Module layer can be installed between the Linux soft-time OS and the Linux real-time OS
The other way is to change the kernel so operates in real-time
The latest release of the Linux mainstream kernel will now include real-time support.

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A currently installed Linux distribution is required to be modified by RT Linux module
The module requires the Linux kernel to be recompiled so that module layer can be installed
Once the kernel is recompile it is added to boot loader as a separate OS to boot
A fully configured OSs are available from commercial vendors

 The Linux soft-time OS runs as a process in real-time OS task scheduler list
 The priority of Linux soft-time OS is the lowest of all processes in RT
 All interrupts are rerouted to the real-time layer
 I/O drivers and device drivers are routed to both the real-time and soft-time OS

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Soft-time Linux kernel runs along side a real-time
Linux so non-real-time software can run on a realtime system like Firefox
A low level task scheduler, control of hardware devices and static memory create a predicable environment
Dual kernel system keep soft-time processes separated from real-time processes allowing for faults in soft-time not to affect the real-time system
Real-time tasks have full access to hardware

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RT Linux system still need time to boot. 30 second to
1 minute on average
The system’s RTOS size is still very large for an embedded system (minimum system size in MB)
Systems require large amounts of RAM and ROM to run processes
Part of OS is run as a task of real-time kernel so it is not a true real-time system. Soft-time processes have no guarantees
Communication between the real and soft OS is done through a FIFO queue

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RT Linux use only one scheduling algorithm at a time while QNX uses four different ones. EDF – RT Linux,
FIFO and Round Robin – QNX
Processes in QNX run in a protected memory space
QNX handles Normal UNIX processes (weighted processes) and threads. RT Linux only handle threads
RT Linux has large set of user period levels
POSIX timers support in QNX

 Multiprocessor system support in RT Linux
 RT Linux uses one short and periodic timers
 LynxOS support a wide range of processors and microcontrollers
 User task, system tasks and kernel tasks all run in protected memory spaces
 Dynamic memory management at low level

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VxWorks thread communication is handle by messaging queues, binary/counting/multi-exclusion semaphores, and sockets
VxWorks supports a wide array of microcontroller
VxWorks has dynamic memory management
VxWorks supports FAT, NFS, raw, TrueFFS (Flash file system), ISO9660 filesystems
VxWorks has a graphical timer debugger called
WindView

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RT Linux handles basic Rate-Monotonic scheduling by default
Earliest First Deadline is possible by adding a module to the RT kernel
Scheduling of programs is done by the programmer using a set of API for setting process parameters
Scheduling and communication between real-time and soft-time is handle by FIFO schedule queue also soft-time task are not given any deadline guarantees
Making inter-processes task very difficult
Semaphore module can be added to help scheduling
Since RT Linux is an open source project there are many different scheduling module being built but RM and EDF are standard