Performance Evaluation of Real-Time Operating Systems презентация

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.
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Many are now out of date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.
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
RTOS
Overview

Task Scheduling
Time Slicing
Task 1

Task Scheduling
Preemptive Priority
Task 1
Task 1

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)

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
Task A
Save context
of Task A
Start
Task B
Load context
of Task B

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
Task

Multiple Interrupt Latencies
Interrupt-to-Task Run
Interrupt Dispatch Time
Interrupt Service Routine
Other Interrupt
Pre-emption Disabled
Scheduling
Context Switch
Return from System Call
Interrupt
Task Run

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:
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
time required to preempt current thread
Interrupt latency
preempt current thread and start interrupt handler
Interrupt dispatch latency
switch from interrupt context to context of interrupted thread or next thread in queue
Thread creation and deletion

Task Switch Latency Test

Real-Time Consult Project
Performance Tests (objects, file, network):
Synchronization objects
time to create and delete a synchronization object
Exclusion objects
time to create and delete an exclusion object
File system operations
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 Real-Time 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
Target Hardware
Application Dependent Software
Standard Application Components
Device Drivers
R T E M S

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:
type of scheduler
number of priority levels.
current release of the system has over 200 options
Portable
Fully preemptable