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Real-Time Operating Systems
Heechul Yun
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What is an OS?
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Real-Time Operating Systems Heechul Yun 1 What is an OS? 2 - - PowerPoint PPT Presentation
Real-Time Operating Systems Heechul Yun 1 What is an OS? 2 - - PowerPoint PPT Presentation
Real-Time Operating Systems Heechul Yun 1 What is an OS? 2 Operating Systems A program that acts as an intermediary between users and the computer hardware Applications Operating System Computer Hardware 3 Common OS Services Task
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Operating Systems
- A program that acts as an intermediary
between users and the computer hardware
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Applications Operating System Computer Hardware
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Common OS Services
- Task (process) management
- Inter process communication (IPC)
- Virtual memory
- Demand paging
- Filesystem
- Hardware interface and I/O handling
- Protection, error handling, security
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Real-Time Operating System
- OS for real-time systems
- Real-time systems
– correctness depends on not only logical correctness but also temporal one.
- Common requirements
– Bounded interrupt latency – Bounded scheduling latency – Bounded WCETs of system-calls – Support for real-time scheduling policies
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RTOS
- Commercial
– VxWorks – QNX – Nucleus – ..
- Free
– FreeRTOS – NuttX – ...
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Common RTOS Services
- Task (process) management
- Inter process communication (IPC)
- Virtual memory (▲)
- Demand paging (X)
- Filesystem
- Hardware interface and I/O handling
- Protection, error handling, security
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RTOS with Partitioning
- RTOS with time and space partitioning
– E.g., VxWorks 653, Integrity-178
- Time partitioning
– A partition’s access time to a shared resource (e.g., CPU) must be guaranteed regardless of others
- Space partitioning
– A partition’s memory cannot be corrupted by other partitions
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VxWorks 653
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Integrity-178
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Linux as RTOS
- Mainline Linux
– Pretty good real-time support these days – RT schedulers: SCHED_FIFO, RR, DEADLINE – Memory locking and other techniques
- Problems
– Kernel used to be non-preemptible (not anymore) – Still suffer long interrupt/scheduling latencies – Due to many long non-preemptible code sections
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Linux-RT (PREEMPT-RT patchset)
- Reduce non-preemptible code sections in
various kernel code paths
– Preemptible spin-locks (rtmutexes) – Interrupts are handled by kernel threads (preemptible)
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Scheduling Latency Comparison
- w/o vs. w/ PREEMPT_RT patch
- I/O heavy background tasks
13 (*) Figure source: A Comparison of Scheduling Latency in Linux, PREEMPT RT, and LITMUS-RT, OSPERT 2013
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Dual-Kernel Approach
- Linux runs on top of an RTOS
– RTLinux – RTAI – Xenomai – L4/Linux
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This Week
- SCHEDULING AND LOCKING IN MULTIPROCESSOR
REAL-TIME OPERATING SYSTEMS. Chapter 3.3. The Design and Implementation of LITMUSRT. Bj¨orn B. Brandenburg, PhD Thesis, 2011
– LITMUS-RT
- GPUSync: A Framework for Real-Time GPU
Management, RTAS’13
- Real-Time Multi-Core Virtual Machine Scheduling in
Xen, EMSOFT'14
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