Post on 20-Jun-2015
transcript
Real Time Linux Operating Systems
Swaminathan SivasubramanianDependable Computing & Networking
LaboratoryIowa State University
swamis@iastate.eduswamis@iastate.edu
Linux as a Real-Time OS
Traditional RT Systems used custom built systems – which were not extensible I.e. tough to develop new applications
However, as technology improved, generic real-time OS became acceptable
In OS suited for extensible development Linux looks more appealing
Why Linux?• Linux (and its Real Time versions) are
free!!• Linux (and its Real Time versions) are
Open Source!!• Easy for developing RT applications
Why not Linux?
Linux didn’t have any corporate support until now
Linux, is a very good general purpose operating system, but not so for real-time OS (Why??)
Because, the design motive of a conventional OS and RTOS is different
Linux as your real-time solution?
• Could increase priority for “real-time” tasks and assume they get scheduled
• Problem – Linux optimizes average case whereas an RTOS should work under worst case assumptions
Linux – A Simplified View
Linux – conflicts with RT constraints
Coarse grained synchronization – long intervals when a task has exclusive use of data ( as fine grained – leads to lot of overhead reducing the average case performance)
Linux batch all operations for efficient use of H/W E.g freeing a list of pages when memory is
full reducing the worst case performance Linux doesn't preempt low-priority task during
system call Linux will make high priority tasks wait for low
priority to release resources
Real Time Linux approaches
Modify the current Linux kernel to guarantee RT constraints Used by KURT
Make the standard Linux kernel run as a task of the real-time kernel Used by RT-Linux, RTAI
Modifying Linux kernel
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.
Running Linux as a process of a second RT 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
RTAI Overview
Based on Real-Time Hardware Abstraction Layer (HAL) (also used in Windows NT)
HAL exports Linux data and functions related to hardware
HAL defines an interface between RTAI and Linux
RTAI – A Simplified View
RTAI Overview (continued)
Software architecture Interface to Linux hardware management
(HAL) 3 basic components: dispatcher, scheduler,
fifos 1 interface used in user tasks to initialize
and start the components RTAI is basically an interrupt dispatcher
(reroutes interrupts to Linux if necessary) e.g: Disk interrupt
KURT Overview Developed at University of Kansas Soft real-time system Refines the temporal granularity of Linux
Motivation: RT tasks may need a time resolution on the order of microseconds, while non-RT tasks may need a resolution of only milliseconds
Result: Timer interrupts are programmed to service earliest scheduled event (results in aperiodic timer interrupts)
KURT Overview (continued)
Not suitable for hard real-time systems 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
RT-Linux Overview
Open source Linux project Supports x86, PowerPC, Alpha Patch of the regular Linux kernel (simply
install the patch and recompile the kernel)
Provides an RT API for developers Runs Linux kernel as lowest priority
process
RT-Linux Task Structure
RT-Linux Interrupt Dispatcher
RT-Linux Overview (continued)
RT tasks are coded as modules Modules are inserted and removed at
users discretion Extremely good at handling periodic
tasks Communicates with non-RT kernel and
other RT tasks via fifo queues Tools are provided for graphical analysis
of RT execution
Problems with RT-Linux
Currently no support for aperiodic tasks Not very useful for complex RT systems Currently limited to simple problems
Montavista – A Faster Response Linux Kernel
Montavista Inc. provides a linux solution for embedded systems
The solution’s aim is to make the Linux kernel fully preemptable
It identifies the points where priority inversion occurs in Linux and makes those points fully preemptable
A good embedded solution not a complete RT solution.
Important References
RT-Linux : http://www.rtlinux.org RTAI :
http://www.aero.polimi.it/projects/rtai/contrib.htm
Montavista: http://www.mvista.com