What are the best ways to do close to real-time tasks on a non real-time OS/kernel?

Question

On a GNU/Linux machine, if one wants to do \"real-time\"(sub millisecond time critical) tasks, you almost invariably have to go through the lengthy, complex, and problem prone p

Answer 1

The sched_setscheduler(2) and friends allow you to use two different soft real-time schedulers, SCHED_FIFO SCHED_RR. Processes running under these schedulers are prioritised higher than regular processes. So as long as you only have a few of theses processes, and control the priorities between them, you can actually get pretty descent real-time responses.

As requested in a comment, here is the difference between SCHED_FIFO and SCHED_RR:

With the "real-time" schedulers, there are up to 100 different priorities (POSIX only requires 32 distinct levels, so one should use sched_get_priority_min(2) and sched_get_priority_max(2) to get the actual number. The schedulers both work by preempting processes and threads with lower priority, the difference is in how they handle tasks with the same priority.

SCHED_FIFO, is a first in first out scheduler (hence the name). This means that the task that hits the run queue first, is allowed to run until it is done, voluntarily gives up its space on the run queue, or is preempted by a higher priority task.

SCHED_RR, is a round robin scheduler. This means that tasks with the same priority are only allowed to run for a certain time quantum. If the task is still running when this time quantum runs out the task is preempted, and the next task in the run queue (with same priority) is allowed to run for up to its time quantum. As with SCHED_FIFO, higher priority tasks preempt lower priority ones, how ever, when a task which was preempted by a higher priority task is allowed to run again, then it's only allowed to run for the time left in its quantum. See the Noes-section in sched_rr_get_interval(2) for how to set the time quantum for a task.