Embedded and Real-time Operating Systems The suitability of Linux

Operating Systems Before looking at embedded and real-time it would be good to review the role and position of the OS –Started after BIOS –Controls all application interaction with hardware –Generally is the kernel Role of file system(s), libraries, and applications –Is always memory resident in kernel space –Interface is through system calls for user processes

Operating System

Real-time requirements #1 Real-time systems can be classed as hard and soft –Hard real-time systems must meet processing deadlines, data must/cannot be missed/ignored. The system is safety critical, i.e. medical systems, military systems. –Soft real-time systems have real-time data but can afford to miss some data, force re-transmissions and don’t need to be too strict about deadlines. For example audio or video streaming –Many systems are a mix of above – the data is real- time, but there are tolerances in the system. White goods equipment, engine management.

Real-time requirements #2 There are 2 major requirements for real-time systems: latency and jitter –Latency is the amount of time required to service a real-time task. The task must be dealt with immediately, complete and cannot be interrupted by the operating system. For example a bar code read. –Jitter is periodic but crucial data. The data rate can be very short and unpredictable, so data could be lost. An example could be any sensor reading in a real- time environment. A system could involve both of these requirements

Standards The POSIX standard are for Portable Operating Systems. They are a set of graded standards for UNIX like OSs. They are supported by various governments (i.e. US), IEEE and ISO. POSIX d –Set of real-time extensions to UNIX like operating systems

POSIX Standards

Linux comparisons

Review of commercial real-time systems Operating SystemPOSIX ?CPUs supportedLatency average/Max microseconds VXWORKSYesARM, Pentium, PPC, M68K 1.7 – 6.8 WINCE.NETNoARM, Pentium, PPC, MIPS, SH 2.4/5.6 QNX Neutrino OSNoARM, Pentium, PPC, MIPS, SH 1.6/4.1 pSOS System3YesPPC, MPC, MIPS1.9/3.8

Linux Version Version RangePeriod October March March May June January September November January September May May January September November December March September 2005

Real-time Linux? From version Robert Love introduced a patch that allowed pre-emption in the kernel. –This means that higher priority task can interrupt (pre- empt) lower priority ones. This is now standard in the 2.6 kernels It allows Linux, as standard, to run soft real-time tasks (e.g. streaming video) with a latency of 1 millisecond (100s MHz Pentiums).

Pre-emptable Linux Kernel

Micro kernels The other versions of real-time Linux involve major patches to the existing systems RTLinux and RTAI linux are both micro-kernel patches. This means that the kernel is run as a low priority task from the micro-kernel Thus the micro-kernel can interrupt the Linux kernel to run a real-time task. These versions can run the POSIX version of real-time.

Micro-kernel real-time architecture

Nano-kernels This was developed by ADEOS as an open source real-time Linux –The micro kernel approach has been patented. This system runs various tasks and operating systems from a micro-kernel –It is rather similar to VMWare

Nano kernel real-time architecture

Resource kernel This is a modification to the standard kernel that allows high speed pre-emption. The resource kernel allows tasks to request and control resources (memory, CPU cycles) that a normal kernel would not allow. Can’t reach hard real-time latencies.

Resource kernel real-time architecture

Performance comparison