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Commercial Real-time Operating Systems – An Introduction. Swaminathan Sivasubramanian Dependable Computing & Networking Laboratory [email protected] outline. Outline. Introduction RTOS Issues and functionalities LynxOS QNX/Neutrino VRTX VxWorks Spring Kernel Distributed RTOS ARTS

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commercial real time operating systems an introduction

Commercial Real-time Operating Systems – An Introduction

Swaminathan Sivasubramanian

Dependable Computing & Networking Laboratory

[email protected]

outline
outlineOutline
  • Introduction
  • RTOS Issues and functionalities
  • LynxOS
  • QNX/Neutrino
  • VRTX
  • VxWorks
  • Spring Kernel
  • Distributed RTOS
    • ARTS
    • MARS
commercial and research rtos
Commercial and Research RTOS
  • Commercial RTOSes different from traditional OS – gives more predictability
  • Used in the following areas such as:
    • Embedded Systems or Industrial Control Systems
    • Parallel and Distributed Systems
  • E.g. LynxOS, VxWorks, pSoS, Spring,ARTS, Maruti, MARS
  • Traditionally these systems can be classified into a Uniprocessor, Multiprocessor or Distributed Real-Time OS
rtos issues
RTOS – Issues
  • Real-Time POSIX API standard compliance
    • Whether pre-emptive fixed-priority scheduling is supported
    • Support for standard synchronization primitives
    • Support for light weight real-time threads
    • APIs used for task-handling
  • Scalability
    • Footprint of the kernel – how huge is the kernel?
    • Can the kernel be scaled down to fit in the ROM of the system?
rtos issues contd
RTOS – Issues (contd..)
  • Modularity
    • How does the functionalities like I/O, file system, networking services behave?
    • Can they be added at run-time or can they be changed at run-time?
    • Can a new service be added at run-time?
  • Type of RTOS kernel
    • Monolithic kernel – less run-time overhead but not extensible
    • Microkernel – high run-time overhead but highly extensible
rtos issues contd6
RTOS – Issues (contd..)
  • Speed and Efficiency
    • Run-time overhead – most of the modern RTOSes are microkernels, but unlike traditional RTOSes they’ve less overhead
    • Run-time overhead is decreased by reducing the unnecessary context switch
    • Important timings such as context switch time, interrupt latency, semaphore latency must be minimum
  • System Calls
    • Non preemptable portions of kernel functions necessary for mutual exclusion are highly optimized and made short and deterministic
rtos issues contd7
RTOS – Issues (contd..)
  • Interrupt Handling
    • Non preemptable portions of the interrupt handler routines are kept small and deterministic
    • Interrupt handlers are scheduled and executed at appropriate priority
  • Scheduling
    • Type of scheduling supported – RMS or EDF
    • Number of priority levels supported – 32 to be RT-POSIX compliant; many offer between 128-256
    • Type of scheduling for equal priority threads – FIFO or Round-Robin
    • Can thread priorities be changed at run-time?
rtos issues contd8
RTOS – Issues (contd..)
  • Priority Inversion Control
    • Does it support Priority Inheritance or Ceiling protocols for scheduling?
  • Memory Management
    • Can provide virtual-to-physical address mapping
    • Traditionally does not do paging
  • Networking
    • Type of networking supported – deterministic network stack or not
lynx os
Lynx OS
  • Microkernel design
    • Means the kernel footprint is small
    • Only 28 kilobytes in size
  • The small kernel provides essential services in scheduling, interrupt dispatching and synchronization
  • The other services are provided by kernel lightweight service modules, called Kernel Plug-Ins (KPIs)
  • New KPIs can be added to the microkernel and can be configured to support I/O, file systems, TCP/IP, streams and sockets
  • Can function as a multipurpose UNIX OS
lynx os contd
Lynx OS (contd..)
  • Here KPIs are multi-threaded, which means each KPI can create as many thread as it want
  • There is no context switch when sending a message to a KPI
    • For example, when a RFS (Request for Service) message is sent to a File System KPI, this does not request a context switch
    • Hence run-time overhead is minimum
    • Further, inter KPI communication incurs minimal overhead with it consuming only very few instructions
  • Lynx OS is a self hosted system – wherein development can be done in the same sytem
lynx os contd11
Lynx OS (contd..)
  • In such a system, there is a need for protecting the OS from such huge memory consuming applications (compilers, debuggers)
  • LynxOS offers memory protection through hardware MMUs
  • Applications make I/O requests to I/O system through system calls
  • Kernel directs I/O request to the device driver
  • Each device driver has an interrupt handler and kernel thread
lynx os contd12
Lynx OS (contd..)
  • The interrupt handler carries the first step of interrupt handling
  • If it does not complete the processing, it sets an asynchronous trap to the kernel
  • Later, when kernel can respond to the software interrupt, it schedules an instance of the kernel thread to complete the interrupt processing
qnx neutrino
QNX/ Neutrino
  • SMP RTOS – requires high end, networked SMP machines with GBs of physical memory
  • Microkernel design – kernel provides essential threads and real-time services
  • Other services are considered as resource managers and can be added or removed at run-time
  • The footprint of microkernel is 12kb.
qnx neutrino contd
QNX/ Neutrino (contd..)
  • QNX is a message passing operating system
    • Messages are basic means of interprocess communication among all threads
    • Follows a message based priority tracking feature
    • Messages are delivered at the priority order and the service provider executes at the priority of the highest priority clients waiting for service
    • So, if the highest priority task wants to do read some data from file, the file system resource manager will execute at this task’s priority
qnx neutrino contd15
QNX/ Neutrino (contd..)
  • When a service provider thread wants to provide service, then it creates a channel (for exchanging messages) with its service identifier for identification
  • To get a service from a provider, the client thread attaches it to the provider’s channel
  • Within the client, this connection is directly mapped to the file descriptor (so RFS can be sent directly to the file descriptor)
  • QNX messages are blocking unlike POSIX message standards
slide16
VRTX
  • VRTX has two multitasking kernels
    • VRTXsa
      • designed for performance
      • Provides priority inheritance, POSIX compliant libraries
      • Supports multiprocessing
      • System calls fully preemptable and deterministic
    • VRTXmc
      • designed for low power consumption
      • Used for cellular phones and hand-held devices
  • Rather than providing optional components provides hooks for extensibility – application can add its own system calls
vxworks
VxWorks
  • Monolithic Kernel
    • Leads to an improved performance with less run-time overhead
    • However the scalability is poor I.e. the footprint of the kernel is affected a little.
  • Provides interfaces specified by RT-POSIX standards in addition to its own APIs
  • Though not a multiprocessor OS, provides shared-memory objects: shared binary and counting semaphores
  • It has the standard MMU as a modern OS
    • Provides basic virtual-to-physical memory mapping
    • Allows to add new mappings and make portions of memory non cacheable
vxworks contd
VxWorks (contd..)
    • When memory boards are added dynamically, to increase the address space for interprocess communication
    • The data is made non cacheable, to ensure cache consistency
  • Reduced Context Switch time
    • Saves only those register windows that are actually in use (on a Sparc)
    • When a task’s context is restored, only the relevant register window is restored
    • To increase response time, it saves the register windows in a register cache – useful for recurring tasks
spring kernel
Spring Kernel
  • Goal – development of dynamic, distributed real-time system
  • System is a network of multiprocessors, each multiprocessor containing one or more processors, I/O subsystems
  • I/O subsystem is a separate entity from Spring kernel, handling non-critical I/O, slow I/O devices and fast sensors
  • Design Principle – Segmentation & Reflection
spring kernel contd
Spring Kernel (contd..)
  • Segmentation
    • dividing resources of the systems into units
    • Size of unit depends on application requirements
    • Helps in determining the resource constraints of online scheduling algorithms
  • Reflection
    • Concept of reasoning its own state and its environments
    • Required for handling situations in highly dynamic environments (where handcrafting is infeasible)
spring kernel contd21
Spring Kernel (contd..)
  • Scheduling – consists of 4 modules
    • Process-resident dispatcher – simply removes the task from Global System Task Table (GSTT)
    • Local Scheduler (per processor) – responsible for locally guaranteeing that a new task can make its deadline and for ordering processor specific tasks in STT
    • Global Scheduler – finds a site for execution for any task that cannot be locally guaranteed
    • Meta Level Controller – can adapt various parameters by noticing significant changes
spring kernel contd22
Spring Kernel (contd..)
  • Memory Management
    • OS is core-resident
    • No dynamic memory allocation to eliminate large and unpredictable delays (due to page faults and page replacements)
    • Kernel pre-allocates a fixed number of instances of the some of kernel data structures
    • Tasks are accepted dynamically if the necessary data structures are available
  • Inter-Process Communication
    • Mailboxes and communication primitives are used for communication
    • No need for semaphores since mutual exclusion is taken care in scheduling
arts distributed os
ARTS - Distributed OS
  • Distributed real-time OS – provides a predictable distributed real-time computing environment
  • Distributed computing environment
    • Heterogeneous computing environment
    • Need for global view of the system and resources
    • No over-utilization and under-utilization of a particular system in a distributed system
    • Guaranteeing predictability in such a system is difficult than in multiprocessor system case
arts contd
ARTS (Contd..)
    • How to synchronize the clocks in a distributed system?
  • Scheduling
    • Integrated time-driven scheduler
    • ITDS scheduler provides an interface between the scheduling policies and the rest of the operating system
    • Allows different scheduling policies to exist (though only one can be used at a time)
  • Communication scheduling
    • Extended RMS for communication scheduling – integrating message and processor scheduling
mars distributed rtos
MARS – Distributed RTOS
  • Maintainable Real-Time System (MARS) –focuses on fault tolerance in distributed RTOS
  • Objective
    • To provide guaranteed timely response under peak load conditions
    • To support real-time testability by breaking up the system into subsystem
  • Time Driven System – system initiates activities at pre-determined times
    • Better performance than event driven systems
    • Control signals are based on the physical time, hence in presence of a global physical time – no need for control signals across subsystem interfaces
mars contd
MARS (Contd..)
  • System Architecture
    • MARS application consists of a set of clusters (autonomous subsystems), several components of a cluster connected by a real-time bus
    • Each component runs an identical copy of the operating system
    • Different clusters are connected through an inter-cluster interface, forming a network
    • Cluster consists of Fault-Tolerant Units (FTUs) consisting of replicated components providing redundancy
    • Shadow components update their own internal state and monitor the operation of active components
    • Shadow becomes active, when active fails
    • Each message is also sent twice on real-time bus
mars contd27
MARS (Contd..)
  • Fault Tolerance
    • Addresses both transient and permanent faults
    • Messages have checksums and h/w comp. are self-checking
    • Uses robust storage structures
    • Application software detects errors by executing each task twice (catching transient faults)
    • MARS is fail silent – component is turned on detecting first error to avoid fault propagation
    • Upon detection – shadow component takes over the final one
mars contd28
MARS (Contd..)
  • Tasks and messages
    • Tasks (periodic and aperiodic) are scheduled by static scheduling schemes
    • Hard real-time tasks are run at specific intervals that are known during system initialization
    • Soft real-time tasks are run at intervals not used by hard real-time tasks
    • Communication through message passing – also uses state messages (produced periodically at predetermined times), conveying state of the system
    • To avoid unpredictable delays in CSMA/CD protocols, MARS uses a TDMA protocol to provide collision-free access to Ethernet (atmost one hard RT message for each slot – remaining for soft RT messages)
mars contd29
MARS (Contd..)
  • MARS uses only one kind of interrupts – periodic clock interrupt.
    • Interaction with peripherals is through polling
  • Scheduling
    • Scheduling done offline
    • Assumes that the running task will yield the CPU at the end of its quantum
    • Task switching is done by major handler every 8 milliseconds
    • Change can be triggered by invoking a system call or receiving an appropriate message.
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