What are real-time capabilities of the stock 2.6 linux kernel?
Traditionally, the Linux kernel will only allow one process to preempt another only under certain circumstances: • When the CPU is running user-mode code • When kernel code returns from a system call or an interrupt back to user space • When kernel code code blocks on a mutex, or explicitly yields control to another process If kernel code is executing when some event takes place that requires a high priority thread to start executing, the high priority thread can not preempt the running kernel code, until the kernel code explicitly yields control. In the worst case, the latency could potentially be hundreds milliseconds or more. The Linux 2.6 configuration option CONFIG_PREEMPT_VOLUNTARY introduces checks to the most common causes of long latencies, so that the kernel can voluntarily yield control to a higher priority task waiting to execute. This can be helpful, but while it reduces the occurences of long latencies (hundreds of milliseconds to potentially seconds or more), it does not
