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QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
System Architecture
The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
The QNX OS Microkernel
The microkernel implements the core POSIX features used in embedded realtime systems, along with the fundamental QNX OS message-passing services.
Thread scheduling
When and how are scheduling decisions made?
Scheduling policies
Round-robin scheduling

QNX Momentics IDE User's Guide
This User's Guide describes version 8.0 of the Integrated Development Environment (IDE) that's part of the QNX Tool Suite.
QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
- Quickstart Guide
- System Architecture
  The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
  - The Philosophy of the QNX OS
    The primary goal of the QNX OS is to deliver the open systems POSIX API in a robust, scalable form suitable for a wide range of systems—from tiny, resource-constrained embedded systems to high-end distributed computing environments. The OS supports several processor families, including x86 and ARM.
  - The QNX OS Microkernel
    The microkernel implements the core POSIX features used in embedded realtime systems, along with the fundamental QNX OS message-passing services.
    - Design goals of the QNX OS
      Historically, the application pressure on BlackBerry QNX's operating systems has been from both ends of the computing spectrum—from memory-limited embedded systems all the way up to high-end SMP (symmetrical multiprocessing) machines with gigabytes of physical memory.
    - System services
    - Threads and processes
      When building an application (realtime, embedded, graphical, or otherwise), the developer may want several algorithms within the application to execute concurrently. This concurrency is achieved by using the POSIX thread model, which defines a process as containing one or more threads of execution.
    - Thread scheduling
      When and how are scheduling decisions made?
      - Scheduling priority
        Every thread is assigned a priority. The thread scheduler selects the next thread to run by comparing the priorities of READY threads (i.e., those capable of using a processor).
      - Scheduling policies
        FIFO scheduling
        Round-robin scheduling
        Sporadic scheduling
        The sporadic scheduling policy is generally used to provide a capped limit on the execution time of a thread within a given period of time.
        Manipulating priority and scheduling policies
        A thread's priority can vary during its execution, either from direct manipulation by the thread itself or from the kernel adjusting the thread's priority as it receives a message from a higher-priority thread.
      - IPC issues
        Since all the threads in a process have unhindered access to the shared data space, wouldn't this execution model trivially solve all of our IPC problems? Can't we just communicate the data through shared memory and dispense with any other execution models and IPC mechanisms?
      - Thread complexity issues
        Although threads are very appropriate for some system designs, it's important to respect the Pandora's box of complexities their use unleashes.
      - External code invoked by the scheduler
        The scheduler may use code that's external to the kernel to perform certain functions. This code can affect thread scheduling and, thus, the deterministic behavior of the QNX OS.
    - Synchronization services
      The QNX OS provides the POSIX-standard thread-level synchronization primitives, some of which are useful even between threads in different processes.
    - Clock and timer services
      Clock services are used to maintain the time of day, which is in turn used by the kernel timer calls to implement interval timers.
    - Interrupt handling
      No matter how much we wish it were so, computers are not infinitely fast. In a realtime system, it's absolutely crucial that CPU cycles aren't unnecessarily spent. But, it is important to balance this with safety, predictable behaviour, and an avoidance of run-time allocation or queueing. QNX OS implements a low-latency path to schedule user threads to handle hardware interrupts.
    - CPU offlining
      The CPU offlining feature allows a privileged application to stop procnto from using a CPU for scheduling threads.
  - Interprocess Communication (IPC)
    Interprocess Communication (IPC) plays a fundamental role in the transformation of the microkernel from an embedded realtime kernel into a full-scale POSIX operating system. As various service-providing processes are added to the microkernel, IPC is the glue that connects those components into a cohesive whole.
  - The Microkernel Instrumentation
    The microkernel (procnto-smp-instr) is equipped with a sophisticated tracing and profiling mechanism that lets you monitor your system's execution in real time. This instrumentation works on both single-CPU and SMP systems.
  - Process Manager
    The Process Manager can create and manage multiple POSIX processes, each of which may contain multiple POSIX threads.
  - Dynamic Linking
    In a typical system, a number of programs will be running. Each program relies on a number of functions, some of which will be standard C library functions, like printf(), malloc(), write(), etc.
  - Resource Managers
    To give the QNX OS a great degree of flexibility, to minimize the runtime memory requirements of the final system, and to cope with the wide variety of devices that may be found in a custom embedded system, the OS allows user-written processes to act as resource managers that can be started and stopped dynamically.
  - Filesystems
    The QNX OS provides a rich variety of filesystems. Like most service-providing processes in the OS, these filesystems execute outside the kernel; applications use them by communicating via messages generated by the shared-library implementation of the POSIX API.
  - Character I/O
    A key requirement of any realtime operating system is high-performance character I/O.
  - Networking Architecture
    As with other service-providing processes in the QNX OS, the networking services execute outside the kernel. Developers are presented with a single unified interface, regardless of the configuration and number of networks involved.
  - TCP/IP Networking
    Our TCP/IP stack is included in the io-sock networking stack and uses the common FreeBSD API.
  - High Availability
    The term High Availability (HA) is commonly used in telecommunications and other industries to describe a system's ability to remain up and running without interruption for extended periods of time.
  - What is Real Time and Why Do I Need It?
    Real time is an often misunderstood—and misapplied—property of operating systems. This appendix provides a summary of some of the critical elements of realtime computing and discusses a few design considerations and benefits.
  - Glossary
- OS Components & Operations
- Graphics
- Programming
- System Security Guide
  The QNX System Security Guide is intended for both system integrators who are responsible for the security of a QNX OS system and developers who want to create a QNX OS resource manager free from vulnerabilities.
- Utilities & Libraries
- Migrating to QNX OS 8.0
QNX Software in the Cloud
QNX Toolkit for Visual Studio Code
This User's Guide describes the QNX^® Toolkit for Visual Studio Code. The guide introduces you to the QNX Toolkit by explaining the QNX development environment and how to build, run, and debug your QNX^® Operating System (OS) applications and systems.
Typographical Conventions, Support, and Licensing
This section describes the typographical conventions used throughout the documentation, explains how to obtain technical support, and provides some information about licensing.

Round-robin scheduling

QNX SDP8.0System ArchitectureDeveloperUser

In round-robin scheduling, a thread selected to run continues executing until it:

voluntarily relinquishes control
is preempted by a higher-priority thread
consumes its timeslice

A timeslice is the unit of time assigned to every thread. Once it consumes its timeslice, a thread is preempted and the next READY thread at the same priority level is given the processor. A timeslice is 4 × the clock period. (For more information, see the entry for ClockPeriod() in the C Library Reference.)

As the following diagram shows, Thread A ran until it consumed its timeslice; the next READY thread (Thread B) now runs:

Figure showing round-robin scheduling — Figure 1Round-robin scheduling.

Note:

Apart from time slicing, round-robin scheduling is identical to FIFO scheduling.

Page updated: December 04, 2024