You can write real time programs using standard Linux as long as you know how to control scheduling. In fact it turns out to be relatively easy and it enables the Raspberry Pi to do things you might not think it capable of. There are also some surprising differences between the one and quad core Pis that make you think again about real time Linux programming. 

 

If you are writing a real time system there are two things that should concern you - how fast the system can act and how poor this response can be in the worst case. 

After learning how to generate accurate and fast pulses we now have the ability to work with I/O down in the microsecond region, but we still have the problem that our program can be interrupted at any time by the operating system.

This means that our outputs and inputs can go drastically wrong. 

For example, if you generate a fast pulse train in the 1 microsecond range using a standard GPIO line and set a logic analyzer to trigger on a long pulse, you will eventually find one or more very long pulses -  typically in the millisecond range. This problem becomes worse the more the CPU is loaded as the operating system switches between tasks to make sure that everything has an opportunity to progress. 

The Problem

If you are familiar with microcontrollers such as the PIC, AMTEL or any dedicated mcu then this idea that there could be something getting between you and the hardware will be new. The majority of simple mcus do nothing but run the program you download. Any talk of an "operating system" generally refers to code that does the downloading or minimal system preparation. When you write a control loop then you can safely assume that the loop will run as you wrote it and without interruption - unless of course you have coded an interrupt handler. 

The point is that in many situations your program is the only program running and you are in complete charge of the processor. 

In the case of running a program on the Raspberry Pi's ARM the situation is very different. Your program is  just one of a number of programs running at any given time. The Raspberry Pi has up to four cores and this means that at most four programs can be running at any given time. The operating system is responsible for starting and stopping programs so that each and every program has a turn to run. 

This is called scheduling and it is a problem if you are trying to write a real time system. 

The problem is that you might write a program that toggles a GPIO line between high and low with a given timing, but whether this timing is honored depends on not just your program but on the operating system as well. You can't even be sure how the operating system will treat your program because it depends in a fairly complex way on what else is running on the system and exactly what the other programs are doing. 

Sometime this is expressed as your program execution being non-deterministic whereas in a simple mcu it is deterministic. This means that if you run the same program twice on on the Raspberry Pi you probably don't get exactly the same result but on an mcu this is a reasonable expectation. 

The whole subject of multi-tasking operating systems and scheduling in particular is a large one and it is usually taught as part of a computer science degree - but generally not as it applies to real time programming. What this means is that there is often a lot of guess work involved in getting programs with real time demands to work properly under general operating systems such as Linux. In fact it is often state that you can't do real time processing under Linux because you cannot even place a bound, an upper limit, on how long your program might be suspended by the OS. This isn't true and real time processing on standard Linux is possible - as long as you are able to live within the constraints. 

As an alternative you could opt to run a specially designed real time OS that does provide guarantees on how quickly a request will be serviced. There are specifically real time versions of Linux that you can install, but since version 2.6 the Linux Kernel has had sufficient real time facilities for many applications so you don't need to move to anything different to the standard Raspbian.

Before we continue it is important to realize that there is no way that a real time operating system can increase the speed of operation of the processor - the maximum speed of operation cannot be improved upon. In the case of the Raspberry Pi this means that you can achieve around the 1 microsecond pulse times if you are careful and no amount of real time programming is going to improve on this. 

What real time provides is higher consistency of that response time. It isn't perfect, however, and after we have used all of the features of real time Linux there will still be small periods of time when your program isn't operating and there is little to be done about this.

RT Scheduling

Every Linux thread is assigned a scheduling policy and a static priority. 

The normal scheduling algorithm, SCHED_OTHER, that Linux uses applies to all threads with static priority zero. If you are not using real time scheduling then all the threads run at priority zero. In place of a static priority each thread is assigned a dynamic priority, which increases each time it is passed over for execution by the scheduler. The scheduler gives the thread with the highest dynamic priority an opportunity to run for one quantum of time or for one time slice. A thread can be suspended before its time slice is up because it has to wait for I/O or because it is blocked in some other way. Any time a thread makes system call it is also a candidate to be suspended in favor of another thread. 

You have only a little control over the computation of the dynamic priority. All you can do is set its initial value using the nice command or setpriority. 

The normal scheduling algorithm doesn't provide much control over what runs. It is "fair" in the sense that all threads get a turn at running, but it isn't possible to set a thread to have a high priority so that it runs in preference to all others. 

To do this we need to look at the real time scheduling options. 

The most important for us is SCHED_FIFO and sometimes the closely related SCHED_RR.

These apply to threads, real time threads, with static priorities 1 to 99(high). 

The first thing to note is that a thread with priority greater than zero will always run in preference to a thread with priority zero and so all real time threads will run before a thread using the normal scheduling algorithm. 

What happens in FIFO is that the system maintains queues of threads that are ready to run at each priority. It then looks for the list with the highest priority with threads ready to run and it starts the thread at the head of the list. 

When a thread is started it is added to the back of its priority queue.

Once a FIFO thread gets to run it can be preempted by a thread with a higher static priority that is ready to run. 

If a FIFO thread is suspended because of a higher priority thread it goes back at the head of the queue. This makes it the next thread to resume. This is the sense in which the schedule is First In First Out FIFO - if a thread is suspended by another thread of higher priority that becomes runnable then it is restarted as soon as that thread that replaced it is suspended or stops running.

Finally if a thread explicitly yields (by calling yield) it goes to the end of its priority queue.