Realtime Operating Systems An Introduction

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Real-time Operating Systems An Introduction

• Swaminathan Sivasubramanian and Sai Sudhir A
• swami_at_cs.vu.nl saisud_at_iastate.edu

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Outline

• Capabilities of RTOS
• Some Services in detail
• Types of RTOS
• Case studies

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What is an RTOS

• Provides efficient mechanisms and services for
  real-time scheduling and resource management
• Must keep its own time and resource consumption
  predictable and accountable
• 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

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Capabilities

• Conformance to Standards (Real-Time POSIX)
• POSIX- Portable Operating Systems Interface, an
  API standard
• Defines Real-time and thread extensions for RTOS
• Prioritized scheduling, signals, high-resolution
  timer, IPC primitives
• Creation and management of threads

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Capabilities (contd..)

• Modularity and Scalability
• Footprint of the kernel how huge is the kernel?
• Can the kernel be scaled down to fit in the ROM
  of the system for small embedded applications
• I/O, File and Networking services provided by
  additional components

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RTOS Capabilities (contd..)

• Type of RTOS kernel
• Monolithic kernel tightly integrated services,
  less run-time overhead but not extensible
• Microkernel high run-time overhead but highly
  extensible

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RTOS Capabilities (contd..)

• Speed and Efficiency
• Run-time overhead most of the modern RTOSes are
  microkernels (more overhead), but unlike
  traditional microkernels theyve less overhead
• Run-time overhead is decreased by reducing the
  unnecessary context switch
• Important timings such as context switch time,
  interrupt latency, semaphore get/release latency
  must be minimum

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Capabilities (contd..)

• System Calls
• Non preemptable portions of kernel functions
  necessary for mutual exclusion are highly
  optimized and made short and deterministic
• Interrupt Handling
• Non preemptable portions of the interrupt handler
  routines are kept small and deterministic
• Interrupt handlers are scheduled and executed at
  appropriate priority

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RTOS Capabilities (contd..)

• 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
• Thread priorities be changed at run-time

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RTOS Capabilities (contd..)

• Priority Inversion Control
• Does it support Priority Inheritance or Ceiling
  protocols for scheduling?
• You can disable it to save the overhead of these
  mechanisms
• Clock and Timer Resolution
• Provides fine timer resolution

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Capabilities (contd..)

• Memory Management
• Can provide virtual-to-physical address mapping
• Traditionally does not do paging
• Can offer Memory Protection
• Networking
• Type of networking supported deterministic
  network stack or not, support for TCP/IP

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Some Services

• Timer Services
• System has at least one clock device consisting
  of a counter, a timer queue and an interrupt
  handler
• Content of counter gives current time, timer
  queue has pending timers associated with the
  clock device
• S/W clock associated with clock device, the
  latter raises interrupts periodically and kernel
  updates S/W clock according to current time

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Timer Services (contd..)

• Resolution of S/W clock equal to period of these
  interrupts (typically in milliseconds- not enough
  for most RT applns.)
• Finer the resolution, larger the amount of time
  kernel spends in responding to interrupts thus
  limiting the resolution
• Some implementations allow user threads to read
  the H/W clock directly for finer resolution by
  mapping H/W clock on to address space of
  application e.g. Pentium time stamp counter

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Some Services (contd..)

• Scheduling (Fixed priority)
• 256 priority levels
• Assigned priority- When the thread is created
  according to scheduling algorithm
• Current priority- Thread can inherit higher
  priority
• Ready queue for each priority level, thread
  placed in queue corresponding to its current
  priority

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Scheduling (contd..)

• Within each queue round-robin or FIFO scheduling
• Thread should change its own priority to support
  Dynamic priority scheduling, very expensive if
  this capability is provided through a system call
• So its better for the kernel to provide its own
  dynamic priority scheduling data structures

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Some Services (contd..)

• Memory Management
• Virtual Memory Mapping - Embedded RT systems may
  not support this, system directly creates
  physically contiguous blocks of memory, but
  fragmentation can occur
• Memory Locking Paging introduces
  unpredictability, RTOS can support paging so that
  memory demanding applns (editors, debuggers) can
  co-exist

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Memory management (contd..)

• Memory Protection Many RTOS dont provide
  protected address space for simplicity and light
  weight of system calls
• To control paging, application can pin its pages
  in memory so that they arent swapped out
• But change in one module might require retesting
  the entire system, so RTOS provides the
  application the choices in memory management

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TYPES of RTOS

• Commercial RTOS
• e.g., VxWorks (Wind River Systems), LynxOS (Lynux
  Works)
• Real-time flavor to commercial OS
• e.g., RT-Linux, KURT (Univ Kansas)
• Research kernels
• e.g., HARTS (UMich), Spring (UMass)

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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)

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Lynx Os (contd..)

• New KPIs can be added to the microkernel and can
  be configured to support I/O, file systems,
  TCP/IP, streams and sockets
• Here KPIs are multi-threaded, which means each
  KPI can create as many thread as it wants

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Lynx OS (contd..)

• 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

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Lynx OS (contd..)

• Lynx OS is a self hosted system wherein
  development can be done in the same system
• 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

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Lynx OS (contd..)

• 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

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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
  Priority Tracking

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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

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VxWorks (contd..)

• 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
• When memory boards are added dynamically, to
  increase the address space for interprocess
  communication
• The data is made non cacheable, to ensure cache
  consistency

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VxWorks (contd..)

• Reduced Context Switch time
• Saves only those register windows that are
  actually in use (on a Sparc)
• When a tasks 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

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ARTS - Distributed OS

• Distributed real-time OS provides a predictable
  distributed real-time computing environment

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ARTS (Contd..)

• 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
• How to synchronize the clocks in a distributed
  system?

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ARTS (contd..)

• 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