My embedded RTOS notes

Started 8Jan2019, updated 9Jul2019

This page is in group Technology and is a blog note where I will try to have a fresh view (as of 2019) of what might be going on with concurrency and Real-Time Operating Systems (RTOS) in the small embedded systems world. Here I would think mostly of what might run on platforms like ARM-based Arduino boards. I work mostly with XMOS boards and program in the XC language, so I would tend to view this through those glasses. So I would think of a big loop in main as only a single-threaded start in the boy’s (or girl’s) room, and would want real support for processes and then the “thread-safer” would be the better. Remember that a small embedded system landed on a comet in 2014 (see below), and many of them are indeed safety-critical.

Intro

This note started its life as cut and paste from an earlier note: My WiFi101 library port. There in particular, concurrency in the Arduino world was discussed. I guess that also the My SafeRTOS notesIEC 61508 and (safe?) concurrency and IEC 61508 and programming paradigms may relate to this note.

Arduino concurrency

Looping as fast as possible leaves no room for concurrency

SimpleWebServerWiFi_Teig.ino contains loop(). It just loops. I made a counter (unsigned long) and printed out its value every one mill counts. It took about 56 seconds to count to 5 mill, so it’s about 11.2 μs to loop around just counting up that value. It runs as fast as it can.

This is bad architecture. Lucky for the demos that there are no background or foreground tasks that have own state. There is no task / process / thread here. It’s as single threaded as possibly possible.

Disclaimer: Of course the ARM processors have a lot of hardware blocks that would live their own lives. And they would communicate with the main loop of running objects through interrupts. This is concurrency that all processors would need and have, and they have had it since 1954? They certainly make even the simplest of processors do more than one thing quite seamlessly. But it’s not concurrency at the occam, Ada, go, XC etc. level that I want to have here. The quite new (2017) Apache mynewt OS is also interesting in this context. I do not want my aquarium code and the communication to be in some one outer loop. One could say that each process, task or thread consists of local loops all over the place in the software. Then there is a scheduler that controls when each loop is run. Basically this is the basic theme of all of my Technology blogs and papers. But there’s more to it:

Arduino Scheduler concurrency is low-level yield-based

Arduino supplies an “experimental” Scheduler,  Scheduler (see https://www.arduino.cc/en/Reference/Scheduler) that’s “cooperative” (they don’t cooperate on data, just on passing of control). It’s based on starting an additional loop with Scheduler.startLoop(new_loop). Then new_loop must run a Scheduler.yield so that the original loop also will run. This is the lowest level of concurrency with no support for communication between them and no structured way to avoid busy poll.

Have a look at their blinking LED example (here). No no-skew timer. No communication between the loops. No communication into or out of the loops. Besides, all cycles are burned. And the cognitive understanding av what a task is probably isn’t easy to grasp in the light of how to next solve the missing points.

Later, working with the port of the RadioHead library My aquarium’s data radioed through the shelf, I discovered lots of YIELD; in the code. Like in file RHGenericDriver.cpp:

// Blocks until a valid message is received
void RHGenericDriver::waitAvailable()
{
    while (!available())
	YIELD;
}

Now it can do other things instead of blocking. (Aside: I have discussed blocking in Not so blocking after all.) In file RadioHead.h YIELD is defined as yield(); for Arduino is 1.55 or more. I then searched my installed libraries and found the Scheduler files. In Scheduler.h:

// Copyright (C) 2012 The Android Open Source Project
class SchedulerClass {
public:
	SchedulerClass();
	static void startLoop(SchedulerTask task, uint32_t stackSize = 1024);
	static void start(SchedulerTask task, uint32_t stackSize = 1024);
	static void start(SchedulerParametricTask task, void *data, uint32_t stackSize = 1024);

	static void yield() { ::yield(); };
};

We see that it’s based on what we saw above, new Loops may be started as tasks. Looking inside Scheduler.cpp then coopTaskStart, coopDoYield are assembly code, for ARM Arduinos (I think).

Adding YIELD; or yield(); to my .ino top level file for testing the RadioHead library did compile. I use ARM Arduinos. It compiles, yes, but yield(); probably is empty code. I edited in Scheduler.cpp and made sure it would not compile. It did. I then did Sketch | Include Library | Scheduler, and #include <Scheduler.h> arrived in the code. Then it didn’t compile before I had fixed the code again. So, in the RadioHead library blocking seems to block since I cannot find and start or startLoop in the code that would give the code any place to yield to.

I have queried about the sanity of the above inferring on the radiohead-arduino group, see YIELD; in RadioHead code.

Later I also found Ivan Seidel’s ArduinoThread (https://github.com/ivanseidel/ArduinoThread) and downloaded it. It’s a wrapper around callbacks for «blinking of LEDs» and it does not use C++11’s std::thread class. I have found no Arduino stuff that uses those native threads. Also, there is no communication between them.

I will not do my whole story here. Almost all of my technical blog notes deal with concurrency. Just search for yield, generator, CSP, concurrency or blocking in the search window above, or visit the main Technology blog note list here and see if you find a thread to pull.

That being said, to me this Scheduler.yield may be a starting point for building a CSP-type Scheduler on top. This may be interesting in the future.

Zephyr operating system

Moved to My Zephyr RTOS notes (Historical note).

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