Real Time and Linux
Unless we state otherwise, assume we are talking about the 2.4.9 version of the Linux kernel. Version 2.4.9 was released in August 2001, although our statements, at least for the most part, are true for the last several years of kernel releases.
There are many qualities of an operating system that may be necessary or desirable for it to be appropriate for real-time applications. One list of features is included in the Comp.realtime FAQ at http://www.faqs.org/faqs/realtime-computing/faq/. That list contains such features as the OS being multithreaded and preemptible, and able to support thread priorities and provide predictable thread-synchronization mechanisms. Linux is certainly multithreaded, supports thread priorities and provides predictable thread-synchronization mechanisms. The Linux kernel is not preemptible.
The FAQ also says that one should know the OS behavior for interrupt latency, time for system calls and maximum time that interrupts are masked. Further, one should know the system-interrupt levels and device-driver IRQ (interrupt request line) levels, as well as the maximum time they take. We provide some timings for interrupt latency and interrupt masked ("interrupts blocked'') times in the benchmarking section below.
Many developers also are interested in having a deadline scheduler, a kernel that is preemptible in the millisecond range or better, more than 100 priority levels, user-space support for interrupt handlers and DMA, priority inheritance on synchronization mechanisms, microsecond timer resolution, the complete set of POSIX 1003.1b functionality and constant time algorithms for scheduling, exit(), etc. None of these capabilities are available in the standard kernel and the GNU C library.
Additionally, in practice, the magnitude of delays becomes important. The Linux kernel, in a relatively easily constrained environment, may be capable of worst-case response times of about 50ms, with the average being just a few milliseconds.
Andrew Morton suggests that one should not scroll the framebuffer, run hdparm, use blkdev_close or switch consoles (see http://www.uow.edu.au/~andrewm/linux/schedlat.html#ddt). These are examples of constraining the operating environment.
Some applications, however, may require response times on the order of 25µs. Such requirements are not satisfiable in applications that are making use of Linux kernel functionality. In such cases, some mechanism outside of the Linux kernel functions must be employed in order to assure such relatively short response-time deadlines.
We see, in practice, that a hard real-time operating system can assure deterministic response and provide response times that are significantly faster than those provided by general-purpose operating systems like Linux.
We see that Linux is not a real-time operating system because it always cannot assure deterministic performance and because its average and worst-case timing behavior is far worse than that required by many real-time applications. Remember that the timing behavior required by these many real-time applications generally is not a hardware limitation. For example, the Linux kernel response time may be on the order of a few milliseconds on a typical x86-based PC, while the same hardware may be capable of better than 20µs response times when running a real-time operating system.
The two reasons that the Linux kernel has such relatively poor performance on uniprocessor systems are because the kernel disables interrupts and because the kernel is not suitably preemptible. If interrupts are disabled, the system is not capable of responding to an incoming interrupt. The longer that interrupts are delayed, the longer the expected delay for an application's response to an interrupt. The lack of kernel preemptibility means that the kernel does not preempt itself, such as in a system call for a lower-priority process, in order to switch to a higher-priority process that has just been awakened. This may cause significant delay. On SMP systems, the Linux kernel also employs locks and semaphores that will cause delays.
User-space real-time applications require services of the Linux kernel. Such services, among other things, provide scheduling, interprocess communication and performance improvement. Let's examine a variety of system calls (the kernel's way of providing services to applications that are of special benefit to real-time application developers). These calls are used to constrain the operating environment.
There are 208 system calls in the Linux kernel. System calls usually are called indirectly through library routines. The library routines usually have the same name as the system call, and sometimes library routines map into alternative system calls. For example, on Linux, the signal library routine from the GNU C library, version 2.2.3, maps to the sigaction system call.
A real-time application may call nearly all of the set of system calls. The calls that are most interesting to us are exit(2), fork(2), exec(2), kill(2), pipe(2), brk(2), getrususage(2), mmap(2), setitimer(2), ipc(2) (in the form of semget(), shmget() and msgget()), clone(), mlockall(2) and sched_setscheduler(2). Most of these are described well in either Advanced Programming in the UNIX Environment, by W. Richard Stevens, or in POSIX.4: Programming for the Real World, by Bill O. Gallmeister. The clone() function is Linux-specific. The others, for the most part, are compatible with typical UNIX systems. However, read the man pages because there are some subtle differences at times.
Real-time applications on Linux also frequently are interested in the POSIX Threads calls, such as pthread_create() and pthread_mutex_lock(). Several implementations of these functions exist for Linux. The most commonly available of these is provided by the GNU C library. These so-called LinuxThreads are based on the clone() system call and are scheduled by the Linux scheduler. Some POSIX functions are available for POSIX Threads (e.g., sem_wait()) but not for Linux processes.
An application running on Linux ordinarily can be slowed down considerably, from its best case, by a number of factors. Essentially these are caused by contention for resources. Such resources include synchronization primitives, main memory, the CPU, a bus, the CPU cache and interrupt handling.
An application can reduce its resource contention for these resources in a number of ways. For synchronization mechanisms, e.g., mutexes and semaphores, an application can reduce their use, employ priority inheritance versions, employ relatively fast implementations, reduce the time in critical sections, etc. Contention for the CPU is affected by priorities. In this view, for example, lack of kernel preemption can be seen as a priority inversion. Contention for a bus is probably not significantly long to be of direct concern. However, know your hardware. Do you have a clock that takes 70µs to respond and holds the bus? Contention for a cache is affected by frequent context switches and by large or random data or instruction references.
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