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