Originally Posted by songcaidao

Is the kernel schedule the non real-time process by CFS,and real-time process by Real-Time Scheduling policies?
What is real-time process?How to identify the real-time process?

Originally Posted by sundialsvcs

Linux is not designed to be a "real-time operating system (RTOS)," although it does possess scheduling-controls that allows it to come fairly close. (However: "fairly close," in terms of true real-time programming, is not good enough.) Linux is a general-purpose operating system, and it really can't be both at once.

Originally Posted by crosstalk

sundialsvcs is correct in defining what a real-time task is. However, as far as the kernel itself is concerned, a real-time process or thread is simply one with a priority (not nice value) greater than 0.

If processes A and B both want to run, there is only one processor, and A has a higher priority, A is guaranteed to run before B.

If processes A and B both want to run, there is only one processor, and both have the same priority, then their scheduling policy determines which actually gets to run. If their priorities are 0 (non-realtime), then the CFS scheduler determines which runs; otherwise, a real-time scheduling policy determines which gets to run.

To identify real-time processes, you can run the "top" program. Note that top shows a combined priority (priority and nice value), so any process with a priority of 0 to 39 in top has an actual priority of 0 and is not realtime; all processes with a priority of 40 or higher are real-time.



Not entirely true. Vanilla Linux is not an RTOS, but it can be patched via the PREEMPT_RT patchset to become hard-realtime capable (if properly configured and setup). With this patchset, Linux has both hard real-time and general-purpose capabilities. However, for safety-critical applications, Linux will probably never be trustworthy enough to use.

There are also RTOSs that run alongside Linux via a realtime hypervisor. An excellent example of this type of RTOS is Xenomai. The advantage of using such an RTOS over Linux with PREEMPT_RT is that it can perform better -- PREEMPT_RT typically only satisfies requirements in the couple-hundred-microsecond range; Xenomai satisfies requirements as small as tens of microseconds. Again, Xenomai + Linux is a complex system, and as a result, will probably never be trustworthy enough to use in life-critical applications.

Originally Posted by crosstalk

sundialsvcs is correct in defining what a real-time task is. However, as far as the kernel itself is concerned, a real-time process or thread is simply one with a priority (not nice value) greater than 0.

If processes A and B both want to run, there is only one processor, and A has a higher priority, A is guaranteed to run before B.

If processes A and B both want to run, there is only one processor, and both have the same priority, then their scheduling policy determines which actually gets to run. If their priorities are 0 (non-realtime), then the CFS scheduler determines which runs; otherwise, a real-time scheduling policy determines which gets to run.

To identify real-time processes, you can run the "top" program. Note that top shows a combined priority (priority and nice value), so any process with a priority of 0 to 39 in top has an actual priority of 0 and is not realtime; all processes with a priority of 40 or higher are real-time.



Not entirely true. Vanilla Linux is not an RTOS, but it can be patched via the PREEMPT_RT patchset to become hard-realtime capable (if properly configured and setup). With this patchset, Linux has both hard real-time and general-purpose capabilities. However, for safety-critical applications, Linux will probably never be trustworthy enough to use.

There are also RTOSs that run alongside Linux via a realtime hypervisor. An excellent example of this type of RTOS is Xenomai. The advantage of using such an RTOS over Linux with PREEMPT_RT is that it can perform better -- PREEMPT_RT typically only satisfies requirements in the couple-hundred-microsecond range; Xenomai satisfies requirements as small as tens of microseconds. Again, Xenomai + Linux is a complex system, and as a result, will probably never be trustworthy enough to use in life-critical applications.

Originally Posted by trukna

There are two priority ranges in Linux.

1) The first is the nice value, a number from –20 to +19 with a default of 0. Larger nice values correspond to a lower priority
You can list all of the processes on your system and their respective nice values (under the column marked NI) with the command ps -el.

2) The second range is the real-time priority. The values are configurable, but by default range from 0 to 99, inclusive. Opposite from nice values, higher real-time priority values correspond to a greater priority. All real-time processes are at a higher priority than normal processes.
You can see a list of the processes on your system and their respective real-time priority (under the column marked RTPRIO) with the command ps -eo state,uid,pid,ppid,rtprio,time,comm.
A value of “-” means the process is not real-time.


The Linux scheduler is modular and have scheduler classes i.e enabling different algorithms to schedule different types of processes. Each scheduler class has a priority.
The base scheduler code iterates over each scheduler class in order of priority. The highest priority scheduler class that has a runnable process wins, selecting who runs next.

Linux provides two real-time scheduling policies, SCHED_FIFO and SCHED_RR. The normal, not real-time scheduling policy is SCHED_NORMAL

The Completely Fair Scheduler (CFS) is the registered scheduler class for normal processes (SCHED_NORMAL).

The real-time policies (SCHED_FIFO and SCHED_RR) are managed not by the Completely Fair Scheduler, but by a special real-time scheduler.

Originally Posted by songcaidao

Thank you!
Can I say that the kernel choose the scheduling policy by the type of process?Is that when a real-time(or not real-time) process use out it's timeslice,the kernel will use the real-time(or normal) scheduling policies to select next process?