Download What is a Real-Time Operating Systems

Performance Evaluation
of Real-Time Operating Systems
RTEMS, RTLinux, eCos
Real-Time Systems
Systems that interact predictably with
events in the outside world
Examples:
Flight control systems
 Collision avoidance systems
 Satellite guidance systems
 Patient Monitoring System
 Control Systems for synchrotrons

Real-Time Operating Systems
Free the applications programmer from writing
code for task scheduling and dispatching.
Manage time-sharing of a processor for a number
of different tasks and interrupt sources, while
adhering to strict time constraints.
Widely used in all kinds of applications both on
PCs and in embedded systems.
Generally operate unattended -- consequences of
failure can be catastrophic.
What is available?
Many RTOSs have been developed:
Commercial
 Open source

RTOS vendors and developers publish
performance metrics to showcase their
products. However, their evaluations are:


not performed on a common platform
not comprehensive
Evaluation of an RTOS
How would you choose an appropriate RTOS for
your application?

Need for an impartial evaluation.
Several such projects have been conducted.

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Only evaluated some performance characteristics
Only looked at a few commercial operating systems
Many are now out of date
‘Real-Time Consult’ introduced a more
comprehensive project.

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Developed a thorough test methodology
Evaluated several commercial RTOSs
Have not evaluated open source RTOSs
Research Objective
Performance evaluation, on a common
platform, of three open source real-time
operating systems:
RTEMS
 RTLinux
 eCos

Real-Time Operating Systems
Overview
RTOS
Task
Scheduling
Task
Synchronization
Task
Dispatching
Task Scheduling
Time Slicing
Last
Arrival
Order
Task 3
Task 3
Task 2
First Task 1
Task 2
Task 1
Time
Task Scheduling
Preemptive Priority
High
Task 3
Priority
Task 2
Low Task 1
Task 2
Task 1
Time
Task Dispatching
Task-control blocks is the
most popular method of
identifying and managing
tasks.
Task-Control Block (TCB)
contains:




a context (e.g. program counter
and register contents)
an identification string
a status, such as ready,
executing or blocked
a priority (if applicable)
A Typical
Task Control Block
Program Counter
Task Status
Task ID Number
Register Contents
Pointer to next TCB
:
Priority
Other Context
Task Synchronization
Events
Messages
Semaphores
Mutexes
Desired Performance Characteristics
Tasks should run with:
minimal event latency (the time between the
triggering of a task and its start)
 minimal jitter (the variation in running times of
a task that is supposed to run at a fixed period).

Performance Evaluation
Throughput – speed at which the system
executes instructions
Responsiveness – how fast it starts to
handle in interrupt request
Determinism – how it reacts under load

how long does it take to finish what it is doing
and start handling the interrupt request
Performance Metrics
Low Level Tests
Context switching
 Interrupt latency
 Exclusion objects

Semaphores
 Mutexes


Synchronization events
Context Switch Time
Switch to Task B
Task A
Save context
of Task A
Load context
of Task B
PC
PC
Register 0
Register 0
:
:
:
:
Other Context
Other Context
Context Switch Time
Start
Task B
Time
Other Considerations
Not an isolated event
Affected by number of tasks pending
Depends on priorities of pending tasks
Amount of context to be saved
Interrupt Latency
Interrupt and Interrupt Dispatch
Latencies
Interrupt
Occurs
First Instruction in
Interrupt Service Routine
(ISR) is executed
Task
resumes
ISR ends
ISR
Task
Interrupt
Latency
Task
Interrupt
Dispatch Latency
Time
Multiple Interrupt Latencies
Interrupt-to-Task Run
Interrupt
Task Run
Interrupt Dispatch Time
Interrupt Service Routine
Other Interrupt
Pre-emption Disabled
Scheduling
Context Switch
Return from System Call
Interrupt Task Response Time
Real response to an interrupt often occurs in
a task synchronized by, but outside of, the
actual ISR.
Task response also depends on whether the
kernel is preemptable. If not, then kernel
call must be completed before ISR.
Performance Metrics
High Level Tests
Network throughput
Stress tests
Real-Time Consult Project
Evaluation and comparison of RTOS performance
Commercial systems only, to date
Evaluations available for:

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Intime 1.20
RTX 4.2
Hyperkernel 4.3
VxWorks/x86 5.3.1
pSOSystem/x86 2.2.6
QNX 6.1
CE 3.0
Real-Time Consult Project
Test Method: timing is measured using an
external PCI bus analyzer.
Two types of tests:
Performance Tests
 Stress Tests

Qualitative Evaluation (API richness, etc.)
Real-Time Consult Project
Performance Tests (context switch &
latencies):

Thread switch latency

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Interrupt latency

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preempt current thread and start interrupt handler
Interrupt dispatch latency
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time required to preempt current thread
switch from interrupt context to context of
interrupted thread or next thread in queue
Thread creation and deletion
Task Switch Latency Test
Task start
TSK0
Each Task
Create tasks
TSK1, …, TSKN
End of Test?
No
Write trace on PCI
Bus
TRC_TSK0_TS0
Yield
Processor
Task start
TSKN
Yes
Delete tasks
TSK1, …, TSKN
Task end
TSK0
Write trace on PCI
Bus
TRC_TSKN_TSN
Yield
Processor
Real-Time Consult Project
Performance Tests (objects, file, network):

Synchronization objects

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Exclusion objects

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time to create and delete an exclusion object
File system operations

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time to create and delete a synchronization object
creating and deleting files, reading from files and
writing to files in synchronous mode
Network stack
performance of TCP/IP stack
 bandwidth for various packet sizes, and CPU usage

Real-Time Consult Project
Stress Tests:
Two simultaneous interrupts
 Interrupt nesting
 maximum sustainable interrupt frequency
 maximum number of objects
 memory leaks

Focus of my Research
Evaluation of three Open Source RealTime Operating Systems:

RTEMS
Real Time Executive for Multiprocessor Systems

RTLinux
Real Time Linux

eCos
Embedded Configurable Operating System
RTEMS
Developed by the U.S. military as alternative to
using commercial RTOS
Small, easy to port
High level of user configurability
Kernel is preemptible
Application Dependent Software
Standard Application Components
Device Drivers
RTEMS
Target Hardware
RTLinux
Abstraction layer between the hardware and
the standard Linux kernel
Appears as actual hardware to standard Linux
kernel
 Lowest priority is assigned to standard Linux
kernel, which then runs as a independent task.

RTLinux executive is nonpreemptible
Leaves Linux kernel essentially untouched
so it doesn’t hinder future Linux
development
ECOS
Targeted at high-volume applications:


consumer electronics, telecommunications, automotive
other deeply embedded applications.
Configurable:


lets developer configure a system that best matches the
needs of the application.
Typical configuration options:


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type of scheduler
number of priority levels.
current release of the system has over 200 options
Portable
Fully preemptable
Typical Application
Control system for the Canadian Light
Source (CLS) uses an open source real-time
operating system and control software:
RTOS  RTEMS
 Control Software  EPICS

(Experimental Physics and Industrial Control System)