Linux Kernel 2.6: the Future of Embedded Computing, Part I
Applications involving the use of shared resources, such as shared memory or shared devices, have to be developed carefully to avoid race conditions. The solution implemented in Linux, called Mutex, ensured that only one task is using the resource at a time. Mutex involved a system call to the kernel to decide whether to block the thread or allow it to continue executing. But when the decision is to continue, the time-consuming system call was unnecessary. The new implementation in Linux 2.6 supports Fast User-Space Mutexes (Futex). These functions can check from user space whether blocking is necessary and perform the system call to block the thread only when it is required. When blocking is not required, avoiding the unneeded system call saves time. It also supports setting priorities to allow applications or threads of higher priority to have first access to the contested resource.
LinuxThreads, the current Linux thread library in Linux, is bad. In fact, "the fellow is as brain-damaged as LinuxThreads" is a common expression among kernel hackers. The improved threading model in 2.6 is based on a 1:1 threading model, one kernel thread for one user thread. It also includes in-kernel support for the new Native Posix Threading Library (NPTL). The kernel's internal threading infrastructure has been rewritten to allow the Native POSIX Thread Library to run on top of it.
NPTL brings an eight-fold improvement over its predecessor. Tests conducted by its authors have shown that Linux, with this new threading, can start and stop 100,000 threads simultaneously in about two seconds. This task took 15 minutes on the old threading model.
Along with POSIX threads, 2.6 provides POSIX signals and POSIX high-resolution timers as part of the mainstream kernel. POSIX signals are an improvement over UNIX-style signals, which were the default in previous Linux releases. Unlike UNIX signals, POSIX signals cannot be lost and can carry information as an argument. Also, POSIX signals can be sent from one POSIX thread to another, rather than only from process to process, like UNIX signals.
Embedded systems often need to poll hardware or do other tasks on a fixed schedule. POSIX timers make it easy to arrange any task to be scheduled periodically. The clock that the timer uses can be set to tick at a rate as fine as one kilohertz, so software engineers can control the scheduling of tasks with precision.
Hardware designs in the embedded world often are customized for special applications. It is common for designers to need to solve a design issue in an original way. For example, a purpose-built board may use different IRQ management than what a similar reference design uses. In order to run on the new board, Linux has to be ported or altered to support the new hardware. This porting is easier if the operating system is made of components that are well separated, making it necessary to change only the code that has to change. The components of Linux 2.6 that are likely to be altered for a custom design have been refactored with a concept called Subarchitecture. Components are separated clearly and can be modified or replaced individually with minimal impact on other components of the board support package.
By formalizing Linux's support for the slightly different hardware types, the kernel can be ported more easily to other systems, such as dedicated storage hardware and other components that use industry-dominant processor types.
In the embedded marketplace, simpler microcontrollers often are the appropriate choice when low cost and simplicity are called for. Linux 2.6 comes with the acceptance and merging of much of the uClinux project into the mainstream kernel. The uClinux project is the Linux for Microcontrollers project. This variant of Linux has been a major driver of support for Linux in the embedded market. Unlike the normal Linux ports we are accustomed to, embedded ports do not have all the features that we associate with the kernel, due to hardware limitations. The primary difference is these ports feature processors that do not feature an MMU, or memory management unit--what makes a protected-mode OS protected. Although these generally are true multitasking Linux systems, they are missing memory protection and other related features. Without memory protection, it is possible for a wayward process to read the data of, or even crash, other processes on the system. This may make them unsuitable for a multi-user system but an excellent choice for a low-cost PDA or dedicated device.
The 2.6 version of Linux supports several current microcontrollers that don't have memory management units. Linux 2.6 supports Motorola m68k processors, such as Dragonball and ColdFire, as well as Hitachi H8/300 and NEC v850. Also supported is the ETRAX family of networking microcontrollers by Axis.