Hi Alex,
> This project looks great.
Thanks!
> >>
> We also have an extensive concept for cooperative, stackless multitasking,
> based on Dunkels Protothreads [19], which we expanded on with Resumable
> Functions [20] which are used in all device drivers to make communication
> non-blocking and allow multiple device drivers on the same bus to safely
> access it.
>
> This form of multithreading is very resource friendly since it does not
> require context switches and only the main call stack (literally 2 bytes of
> RAM per Protothread).
> However, this is quite an advanced topic and there are disadvantages to this
> approach compared to a full real-time operating system (power efficiency,
> hard real-time), such as mbed os or riot-os, however, they do not necessarily
> work well on AVRs.
> >>
>
> Could you please expand on the disadvantages to power-efficiency or hard
> real-time ? And how bad are those ?
Sure.
Some background first:
The protothreads are constantly being polled for state changes in their `run()`
method regardless of that being useful or not.
This makes it very simple to implement timer logics, since they do not require
to register callbacks, you simply wait until the timer expired:
xpcc::PeriodicTimer timer(200); // expire every 200ms
…
bool
MyAmazingThread::run() {
PT_BEGIN(); // protothread begins
while(1) {
PT_WAIT_UNTIL(timer.execute()); // yield until true
Led::toggle(); // toggle an led every 200ms
}
PT_END(); //
}
In your main loop you will have to call the `MyAmazingThread::run()`
continously for this to work.
Notice that this equals to an _implicit_ round robin scheduling, ie. you don’t
register with a dynamic scheduler class, but simply add your update function to
the main loop.
Other than that the Protothread behave very similarly to other thread
implementions, with the exception that you cannot have local variables, since
they are based on a stackless coroutine implementation).
This is a very simple multitasking concept and it works very well for most
applications.
I especially prefer the thread abstraction over using callbacks, as this can
lead to some very unreadable code.
Regarding power-efficiency:
Since you have to poll the threads even when no state change occurred, the CPU
is alway 100% utilized.
So this naive polling approach is obviously not very power efficient.
For our use this is not really a concern, since our robots consume around
10-15W just doing nothing, so the extra 10mA from the CPU doesn’t really matter
;-)
However, theoretically you could implement a sleep function here, by running
though the main loop only when a state change in hardware occurred.
For example, the timers only need to be polled every 1ms, since that is the
resolution of the clock.
So if all threads are waiting for a timer to expire, the CPU could to go sleep
and wake up on the next ms tick.
Similar approaches could be taken for peripherals. You only have to poll the
thread waiting on I2C/UART/SPI if a state change actually happened, etc.
I would like to investigate the possibilities of this in the distant future, as
that sounds promising.
Another reason for not being power-efficient is that no DMA support is
implemented yet.
Even though we have a DMA HAL for the STM32, the Controller and Stream
selection for every peripheral still needs to be implemented.
This will likely happen before considering a sleep concept.
However, please note, that for true power efficiency, you need a full blown
resource manager, that not only manages CPU sleep but also powers down unused
peripherals (clock gating) and even external peripherals (ie. sensors, external
memories).
Such advanced schemes can become very complicated very quickly (there is a ton
of research done on doing this in clever way) and will probably never be part
of xpcc.
Regarding hard real-time:
In my experience, you don’t need a hard real-time scheduler, when you can do
your things directly in hardware (using a timer for a motor controller for
example) or by enabling a high-priority interrupt handler and executing from
there.
If you do need actual hard real-time in your program, having a implicit
cooperatively scheduled main loop is not the way to go, since you cannot make
any guarantee on latency whatsoever.
So if you synchronize on a flag set during an interrupt, you will first have to
wait for the main loop to loop around to the thread receiving (“reading”) the
flag and acting on it.
However, the main loop gets called many thousands of times per second, so you
will most likely be totally fine.
So the protothreads are definitely soft real-time (very fast though), but you
can have the hard real-time if you want with interrupts.
Cheers,
Niklas
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