What is the maximum number of tasks supported in AUTOSAR compliant systems?
In Linux, I can check the maximum process IDs supported to get the maximum number of tasks supported.
However, I couldn't find any source that states the maximum number of tasks supported by AUTOSAR.
Thank you very much for your help!
Well, we are still in an embedded automotive world and not on a PC.
There is usually a tradeoff between the number of tasks you have and what it takes to schedule them and what RAM/ROM and runtime resources your configuration uses.
As already said, if you just need a simple timed loop with some interrupts in between, one task may be ok.
It might be also enough, to have e.g. 3 tasks running at 5ms, 10ms and 20ms cycle. But you could also schedule this in simple cases like this with a single 5ms task:
TASK(TASK_5ms)
{
static uint8 cnt = 0;
cnt++;
// XXX and YYY Mainfunctions shall only be called every 10ms
// but do a load balancing, that does not run 3 functions every 10ms
// and 1 every 5ms, but only two every 5ms
if (cnt & 1)
{
XXX_Mainfunction_10ms();
}
else
{
YYY_Mainfunction_10ms();
}
ZZZ_Mainfunction_5ms();
}
So, if you need something to be run every 5, 10 or 20ms, you put these runnables into the corresponding tasks.
The old OSEK also had a notion of BASIC vs EXTENDED Tasks, where only extended tasks where able to react on OsEvents. This tasks might not run cyclically, but only on configured OsEvents. You would have an OS Waitpoint there, where the tasks is more or less stopped and only woken up by the OS on the arrival of an event. There are also OSALARM, which could either directly trigger the activation of a OsTask, or indirectly over an Event, so, you could e.g. wait on the same Waitpoint on both a cyclic event from an OsAlarm or an OsEvent set by something else e.g. by another task or from an ISR.
TASK(TASK_EXT)
{
EventMaskType evt;
for(;;)
{
WaitEvent(EVT_XXX_START | EVT_YYY_START | EVT_YYY_FINISHED);
GetEvent(TASK_EXT, &evt);
// Start XXX if triggered, but YYY has reported to be finished
if ((evt & (EVT_XXX_START | EVT_YYY_FINISHED) == (EVT_XXX_START | EVT_YYY_FINISHED))
{
ClearEvent(EVT_XXX_START);
XXX_Start();
}
// Start YYY if triggered, will report later to start XXX
if (evt & EVT_YYY_START)
{
ClearEvent(EVT_YYY_START);
YYY_Start();
}
}
}
This direct handling of scheduling is now mostly done/generated within the RTE based on the events you have configured for your SWCs and the Event to Task Mapping etc.
Tasks are scheduled mainly by their priority, that's why they can be interrupted anytime by a higher priority taks. Exception here is, if you configure your OS and tasks to be not preemptive but cooperative. Then it might be necessary to also use Schedule() points in your code, to give up the CPU.
On bigger systems and also on MultiCore systems with an MultiCore OS, there will be higher nunbers of Tasks, because Tasks are bound to a Core, though the Tasks on different Cores run independently, except maybe for the Inter-Core-Synchronization. This can also have a negative performance impact (Spinlocks can stop the whole system)
e.g. there could be some Cyclic Tasks for normal BaseSW components and one specific only for Communication components (CAN Stack and Comm-Services).
We usually separate the communication part, since they need a certain cycle time like 5..10ms, since this cycle is used by the Comm-Stack for message transmission scheduling and also reception timeout monitoring.
Then there might be a task to handle the memory stack (Ea/Fls, Eep/Fee, NvM).
There might be also some kind of Event based Tasks to trigger certain HW-control and processing chains of measured data, since they might be put on different cores, and can be scheduled by start or finished events of each other.
On the other side, for all your cyclic tasks, you should also make sure, that the functions run within such task do not run longer than your task cycle, otherwise you get an OS Shutdown due to multiple activation of the same task, since your task is started again, before it actually finished. And you might have some constraints, that require some tasks to finish in your applications expected measurement cycle.
In safety relevant systems (ASIL-A .. ASIL-D) you'll also have at least one task fpr each safety-level to get freedome-from-interference. In AUTOSAR, you already specify that on the OSApplication which the tasks are assigned to, which also allows you to configure the MemoryProtection (e.g. WrAccess to memory partitions by QM, ASIL-A, ASIL-B application and tasks). That is then another part, the OS has to do at runtime, to reconfigure the MPU according to the OsApplications MemoryAccess settings.
But again, the more tasks you create, the higher the usage of RAM, ROM and runtime.
RAM - runtime scheduling structures and different task stacks
ROM - the actual task and event configurations
Runtime - the context switches of the tasks and also the scheduling itself
It seems to vary. I found that ETAS RTA offers 1024 tasks*, whereas Vector's MICROSAR OS has 65535.
For task handling, OSEK/ASR provides the following functions:
StatusType ActivateTask (TaskType TaskID)
StatusType TerminateTask (void)
StatusType Schedule (void)
StatusType GetTaskID (TaskRefType TaskID)
StatusType GetTaskState (TaskType TaskID, TaskStateRefType State)
*Link might change in future, but it is easy to search ETAS page directly for manuals etc.: https://www.etas.com/en/products/download_center.php
Formally you can have an infinite number of OsTasks. According to the spec. the configuration of the Os can have 0..* OsTask.
Apart from that the (OS) software uses data type TaskType for Task-Index variables. Therefore, if TaskType is of uint16 you could not have more than 65535 tasks.
Besides that, if you have a lot of tasks, you might re-think your design.
Related
The accepted answer at golang methods that will yield goroutines explains that Go's scheduler will yield control from one goroutine to another when a syscall is encountered. I understand that this means if you have multiple goroutines running, and one begins to wait for something like an HTTP response, the scheduler can use this as a hint to yield control from that goroutine to another.
But what about situations where there are no syscalls involved? What if, for example, you had as many goroutines running as logical CPU cores/threads available, and each were in the middle of a CPU-intensive calculation that involved no syscalls. In theory, this would saturate the CPU's ability to do work. Would the Go scheduler still be able to detect an opportunity to yield control from one of these goroutines to another, that perhaps wouldn't take as long to run, and then return control back to one of these goroutines performing the long CPU-intensive calculation?
There are few if any promises here.
The Go 1.14 release notes says this in the Runtime section:
Goroutines are now asynchronously preemptible. As a result, loops without function calls no longer potentially deadlock the scheduler or significantly delay garbage collection. This is supported on all platforms except windows/arm, darwin/arm, js/wasm, and plan9/*.
A consequence of the implementation of preemption is that on Unix systems, including Linux and macOS systems, programs built with Go 1.14 will receive more signals than programs built with earlier releases. This means that programs that use packages like syscall or golang.org/x/sys/unix will see more slow system calls fail with EINTR errors. ...
I quoted part of the third paragraph here because this gives us a big clue as to how this asynchronous preemption works: the runtime system has the OS deliver some OS signal (SIGALRM, SIGVTALRM, etc.) on some sort of schedule (real or virtual time). This allows the Go runtime to implement the same kind of schedulers that real OSes implement with real (hardware) or virtual (virtualized hardware) timers. As with OS schedulers, it's up to the runtime to decide what to do with the clock ticks: perhaps just run the GC code, for instance.
We also see a list of platforms that don't do it. So we probably should not assume it will happen at all.
Fortunately, the runtime source is actually available: we can go look to see what does happen, should any given platform implement it. This shows that in runtime/signal_unix.go:
// We use SIGURG because it meets all of these criteria, is extremely
// unlikely to be used by an application for its "real" meaning (both
// because out-of-band data is basically unused and because SIGURG
// doesn't report which socket has the condition, making it pretty
// useless), and even if it is, the application has to be ready for
// spurious SIGURG. SIGIO wouldn't be a bad choice either, but is more
// likely to be used for real.
const sigPreempt = _SIGURG
and:
// doSigPreempt handles a preemption signal on gp.
func doSigPreempt(gp *g, ctxt *sigctxt) {
// Check if this G wants to be preempted and is safe to
// preempt.
if wantAsyncPreempt(gp) && isAsyncSafePoint(gp, ctxt.sigpc(), ctxt.sigsp(), ctxt.siglr()) {
// Inject a call to asyncPreempt.
ctxt.pushCall(funcPC(asyncPreempt))
}
// Acknowledge the preemption.
atomic.Xadd(&gp.m.preemptGen, 1)
atomic.Store(&gp.m.signalPending, 0)
}
The actual asyncPreempt function is in assembly, but it just does some assembly-only trickery to save user registers, and then calls asyncPreempt2 which is in runtime/preempt.go:
//go:nosplit
func asyncPreempt2() {
gp := getg()
gp.asyncSafePoint = true
if gp.preemptStop {
mcall(preemptPark)
} else {
mcall(gopreempt_m)
}
gp.asyncSafePoint = false
}
Compare this to runtime/proc.go's Gosched function (documented as the way to voluntarily yield):
//go:nosplit
// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
func Gosched() {
checkTimeouts()
mcall(gosched_m)
}
We see the main differences include some "async safe point" stuff and that we arrange for an M-stack-call to gopreempt_m instead of gosched_m. So, apart from the safety check stuff and a different trace call (not shown here) the involuntary preemption is almost exactly the same as voluntary preemption.
To find this, we had to dig rather deep into the (Go 1.14, in this case) implementation. One might not want to depend too much on this.
A little bit more on this to complete #torek's answer.
Goroutines are interruptible when there is a syscall, but also when a routine is waiting on a lock, a chan or sleeping.
As #torek's said, since 1.14 routines can also be preempted even when they do none of the above. The scheduler can mark any routine as preemptible after it ran for more than 10ms.
More reading there: https://medium.com/a-journey-with-go/go-goroutine-and-preemption-d6bc2aa2f4b7
We have a throttling implementation that essentially boils down to:
Semaphore s = new Semaphore(1);
...
void callMethod() {
s.acquire();
timer.recordCallable(() -> // call expensive method);
s.release();
}
I would like to gather metrics about the impact semaphore has on the overall response time of the method. For example, I would like to know the number of threads that were waiting for acquire, the time spend waiting etc., What, I guess, I am looking for is guage that also captures timing information?
How do I measure the Semphore stats?
There are multiple things you can do depending on your needs and situation.
LongTaskTimer is a timer that measures tasks that are currently in-progress. The in-progress part is key here, since after the task has finished, you will not see its effect on the timer. That's why it is for long running tasks, I'm not sure if it fits your use case.
The other thing that you can do is having a Timer and a Gauge where the timer measures the time it took to acquire the Semaphore while with the gauge, you can increment/decrement the number of threads that are currently waiting on it.
I understand the notion of update_rq_clock as it updates the run queue clock on system tick periodically. But this function calls update_rq_clock_task(). What is the purpose behind this function?
Within update_rq_clock the difference between the CPU timestamp and the run queue clock is calculated (The rq->clock variable represents the last clock read from the CPU). That difference is added to the rq->clock and to the rq->clock_task (Which is the same as rq->clock - time for interrupts and stolen time) through update_rq_clock_task.
There are a couple of options within the function, which you can activate with kernel build options. But basically it breaks down to:
...
rq->clock_task += delta;
...
update_rq_clock_pelt(rq, delta);
...
So, both functions together update the clock of the run queue and the clock of the run queue without accounting for interrupts and stolen time (unless you activated that accounting through the kernel options), so the actual time that the tasks used.
So I have a half duplex bus driver, where I send something and then always have to wait a lot of time to get a response. During this wait time I want the processor to do something valuable, so I'm thinking about using FreeRTOS and vTaskDelay() or something.
One way to do it would off be splitting the driver up in some send/receive part. After sending, it returns to the caller. The caller then suspends, and does the reception part after a certain period of time.
But, the level of abstraction would be finer if it continues to be one task from the user point of view, as today. Therefore I was thinking, is it possible for a function within a task to suspend the task itself? Like
void someTask()
{
while(true){
someFunction(&someTask(), arg 1, arg 2,...);
otherStuff();
}
}
void someFunction(*someSortOfReferenceToWhateverTaskWhoCalled, arg1, arg2 ...)
{
if(something)
{
/*Use the pointer or whatever to suspend the task that called this function*/
}
}
Have a look at the FreeRTOS API reference for vTaskSuspend, http://www.freertos.org/a00130.html
However I am not sure you are going about controlling the flow of the program in the correct way. Tasks can be suspended on queues, events, delays etc.
For example in serial comms, you might have a task that feeds data into a queue (but suspends if it is full) and an interrupt that takes data out of the queue and transmits the data, or an interrupt putting data in a queue, or sending an event to a task to say there is data ready for it to process, the task can then wake up and process the data or take it out of the queue.
One thing I think is important though (in my opinion) is to only have one suspend point in any task. This is not a strict rule, but will make your life a lot easier in most situations.
There a numerous other task control mechanisms that are common to most RTOS's.
Have a good look around the FreeRTOS website and play with a few demo's. There is also plenty of generic RTOS tutorials on the web. It it worth learning how use the basic features of most RTOS's. It is actually not that complicated.
I need to use task_scheduler_init to limit the number of threads to a number under of cores, however TBB ignores the number & always uses the number of cores (8 in this case).
This doesn't look like normal behavior to me. Please note that it is not a possibility for me to use a different version of TBB.
Snippet:
task_scheduler_init scheduler(nb_thread);
tbb::parallel_for(
tbb::blocked_range<size_t>(0, size),
[&](const tbb::blocked_range<size_t>& subrange) {
int tid = syscall(SYS_gettid);
dragon_draw_raw(
subrange.begin(),
subrange.end(),
dragon,
dragon_width,
dragon_height,
limits,
id
);
});
If the task_scheduler_init instance was not the first call to TBB on this thread, it explains why you see such a behavior. [Almost] any call to TBB initiates auto-initialization and the number of worker threads is fixed at this time, any further call to task_scheduler_init means basically nothing except additional reference to the internal task scheduler object.
If you are in control of the entire application and can make the call to task_scheduler_init object constructor the first call to TBB, it will fix your problem. But if you are writing a component or a library, look at task_arena feature, it allows to control concurrency limits regardless of the current settings.