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 (process) management • Inter process communication (IPC) • Virtual memory • Demand paging • Filesystem • Hardware interface and I/O handling • Protection, error handling, security 4 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 5 RTOS • Commercial – VxWorks – QNX – Nucleus – .. • Free – FreeRTOS – NuttX – ... 6 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 7 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 8 VxWorks 653 9 Integrity-178 10 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 11 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) 12 Scheduling Latency Comparison (*) Figure source: A Comparison of Scheduling Latency in Linux, PREEMPT RT, and LITMUS-RT, OSPERT 2013 • w/o vs. w/ PREEMPT_RT patch • I/O heavy background tasks 13 Dual-Kernel Approach • Linux runs on top of an RTOS – RTLinux – RTAI – Xenomai – L4/Linux 14 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 15.