I have a situation here in my code where all tasks are running with same priority based on round robin (with fixed time slice of 50ms) scheduling algorithm. Now I want to run one particular task say Task A, exactly within a time period of 10ms to update some communication db. Since,current scheduling is based on round robin with fixed time slice of 50ms due to that the Task A is not able to get called exactly in 10ms. I am not getting any solution to cope up with the current problem.
Please do provide your valuable suggestion & advice.
Thanks in advance,
Vijay Khaitan
Not exactly sure what you are asking here. If you do not want Task A to run longer than 10ms, and you know that you will return from your communication functions in less than that, you can take a time reading at the beginning of Task A, and call osThreadYield() from Task A after you hit 10ms (busy loop).
If you are somewhere in Task B, and need to call Task A in exactly 10ms, it becomes a bit more complicated, since you don't know what thread can preempt your Task B at that time. What you can try, is in Task B, keep a handle to Task A. Then when you are ready to wait 10ms, do the following:
osThreadId id;
id = osThreadGetId (); // id for the currently running thread
osThreadSetPriority(id, osPriorityRealtime); // Make sure we get back here quickly
osWait(10); // Wait 10ms
osThreadSetPriority(id, osPriorityNormal); // Go back to normal
// If you need to create Task A, do so here, otherwise you can
// use osSignalSet here and osSignalWait in Task A
You can also call directly create Task A, set its priority to osPriorityRealtime, yield from Task B, and have the first method in Task A be osWait(10). As soon as you return, set its priority back to normal.
Related
I am trying to simulate a customer by using agents. In the statechart I built, I would like to apply to each individual agent the waiting time spent in the system by defining different variables:
WatingTimeStart, WaitingTimeEnd and WaitingTime
In order to assign the waiting time to each agent I am using the following command in the transition prior to the state I would like to apply the condition:
this.WaitingTimeStart=time();
In the next State I am then using the following:
this.WaitingTimeEnd=time();
this.WaitingTime=this.WaitingTimeEnd-this.WaitingTimeStart;
Followed by the next transition with the condition (TolerarableWaitingTime is a pre-defined Variable)
this.WaitingTime>TolerarableWaitingTime;
My Problem is the transition does not accept the condition and is not processing the agents to the next state.
I probably make a mistake in:
assinging the variable WaitingTime to each agent
applying the condition correcly
Thanks a lot for any thoughts.
Bastian
It was difficult to understand your question, but here it goes: first, you don't need to use "this", you can just do in the transition previous to the state in question:
WaitingTimeStart=time();
also by convention your variables should start with a low case letter, so it should be waitingTimeStart.
But you don't really even need that code and you are overcomplicating yourself... if you want to apply a waiting time (or a delay) you don't need a conditional transition, you can just use a timeout transition instead, where the timeout time is equal to TolerarableWaitingTime
I am currently reading the Programming Erlang Second Edition Writing Software for a concurrent world written by Joe Armstrong and I have the following assignment :
Write a function start(AnAtom, Fun) to register AnAtom as spawn(Fun). Make sure your program works correctly in the case when two parallel processes simultaneously evaluate start/2. In this case you must guarantee that one succeeds and the other fails.
I understand the first bit. I need to register the process of Fun to the AnAtom. However what does the second part want me to do?
If two processes call start/2 at the same time then one of them must fail? Why? Given that the AnAtom is different to any others (which will be done inside the body of start/2 why would I want to fail one of the processes?
From what I can understand so far we have:
a = spawn(process1).
b = spawn(process2).
a ! {self(), registerProcess} //which should call the start/2
b ! {self(), registerProcess} //which should call the start/2
What is the problem here? Two processes will evaluate start/2. Why fail one of them? I'm probably missing the logic here or what I understood so far is completely wrong. Can anybody explain this in easier terms so I can get my head around it?
I believe the exercise is asking you to think about what happens when two parallel process evaluate start/2 using the SAME atom as the first parameter. When start(a, MyFunction) completes, there should be a spawned function (running MyFunction) associated with the name (atom) a.... what happens if
start(cool, MyFun1) and
start(cool, MyFun2)
are both executed simultaneously? How do you guarantee that one succeeds and the other fails.... does this help?
EDIT: I think you are not understanding the register process part of the assignment. When you get done with start(name, MyFun), doing a whereis(name) from the repl should return the process identifier of the process that got created.
This is not about sending the process a message to give it a name, it is about registering the process your created under the name passed in as the first parameter to start/2
An example of this problem is when a user creates a resource and deletes a resource. We will perform the operation and also increment (decrement) a counter cache.
In testing, there is sometimes a race condition where the counter cache has not been updated by the go routine.
EDIT: Sorry about the confusion, to clarify: the counter cache is not in memory, it is actually a field in the database. The race condition is not to a variable in memory, it is actually that the goroutine might be slow to write into the database itself!
I currently use a 1 second sleep after the operation to ensure that the counter cache has been updated before testing the counter cache. Is there another way to test go routine without the arbitrary 1 second sleep to wait for the go routine to finish?
Cheers
In testing, there is sometimes a race condition where the counter cache has not been updated by the go routine. I currently use a 1 second sleep after the operation to ensure that the counter cache has been updated before testing the counter cache.
Yikes, I hate to say it, but you're doing it wrong. Go has first-class features to make concurrency easy! If you use them correctly, it's impossible to have race conditions.
In fact, there's a tool that will detect races for you. I'll bet it complains about your program.
One simple solution:
Have the main routine create a goroutine for keeping track of the counter.
the goroutine will just do a select and get a message to increment/decrement or read the counter. (If reading, it will be passed in a channel to return the number)
when you create/delete resources, send an appropriate message to the goroutine counter via it's channel.
when you want to read the counter, send a message for read, and then read the return channel.
(Another alternative would be to use locks. It would be a tiny bit more performant, but much more cumbersome to write and ensure it's correct.)
One solution is to make to let your counter offer a channel which is updated as soon as the value
changes. In go it is common practice to synchronize by communicating the result. For example your
Couter could look like this:
type Counter struct {
value int
ValueChange chan int
}
func (c *Counter) Change(n int) {
c.value += n
c.ValueChange <- c.value
}
Whenever Change is called, the new value is passed through the channel and whoever is
waiting for the value unblocks and continues execution, therefore synchronizing with the
counter. With this code you can listen on ValueChange for changes like this:
v := <-c.ValueChange
Concurrently calling c.Change is no problem anymore.
There is a runnable example on play.
I have a machine which uses an NTP client to sync up to internet time so it's system clock should be fairly accurate.
I've got an application which I'm developing which logs data in real time, processes it and then passes it on. What I'd like to do now is output that data every N milliseconds aligned with the system clock. So for example if I wanted to do 20ms intervals, my oututs ought to be something like this:
13:15:05:000
13:15:05:020
13:15:05:040
13:15:05:060
I've seen suggestions for using the stopwatch class, but that only measures time spans as opposed to looking for specific time stamps. The code to do this is running in it's own thread, so should be a problem if I need to do some relatively blocking calls.
Any suggestions on how to achieve this to a reasonable (close to or better than 1ms precision would be nice) would be very gratefully received.
Don't know how well it plays with C++/CLR but you probably want to look at multimedia timers,
Windows isn't really real-time but this is as close as it gets
You can get a pretty accurate time stamp out of timeGetTime() when you reduce the time period. You'll just need some work to get its return value converted to a clock time. This sample C# code shows the approach:
using System;
using System.Runtime.InteropServices;
class Program {
static void Main(string[] args) {
timeBeginPeriod(1);
uint tick0 = timeGetTime();
var startDate = DateTime.Now;
uint tick1 = tick0;
for (int ix = 0; ix < 20; ++ix) {
uint tick2 = 0;
do { // Burn 20 msec
tick2 = timeGetTime();
} while (tick2 - tick1 < 20);
var currDate = startDate.Add(new TimeSpan((tick2 - tick0) * 10000));
Console.WriteLine(currDate.ToString("HH:mm:ss:ffff"));
tick1 = tick2;
}
timeEndPeriod(1);
Console.ReadLine();
}
[DllImport("winmm.dll")]
private static extern int timeBeginPeriod(int period);
[DllImport("winmm.dll")]
private static extern int timeEndPeriod(int period);
[DllImport("winmm.dll")]
private static extern uint timeGetTime();
}
On second thought, this is just measurement. To get an action performed periodically, you'll have to use timeSetEvent(). As long as you use timeBeginPeriod(), you can get the callback period pretty close to 1 msec. One nicety is that it will automatically compensate when the previous callback was late for any reason.
Your best bet is using inline assembly and writing this chunk of code as a device driver.
That way:
You have control over instruction count
Your application will have execution priority
Ultimately you can't guarantee what you want because the operating system has to honour requests from other processes to run, meaning that something else can always be busy at exactly the moment that you want your process to be running. But you can improve matters using timeBeginPeriod to make it more likely that your process can be switched to in a timely manner, and perhaps being cunning with how you wait between iterations - eg. sleeping for most but not all of the time and then using a busy-loop for the remainder.
Try doing this in two threads. In one thread, use something like this to query a high-precision timer in a loop. When you detect a timestamp that aligns to (or is reasonably close to) a 20ms boundary, send a signal to your log output thread along with the timestamp to use. Your log output thread would simply wait for a signal, then grab the passed-in timestamp and output whatever is needed. Keeping the two in separate threads will make sure that your log output thread doesn't interfere with the timer (this is essentially emulating a hardware timer interrupt, which would be the way I would do it on an embedded platform).
CreateWaitableTimer/SetWaitableTimer and a high-priority thread should be accurate to about 1ms. I don't know why the millisecond field in your example output has four digits, the max value is 999 (since 1000 ms = 1 second).
Since as you said, this doesn't have to be perfect, there are some thing that can be done.
As far as I know, there doesn't exist a timer that syncs with a specific time. So you will have to compute your next time and schedule the timer for that specific time. If your timer only has delta support, then that is easily computed but adds more error since the you could easily be kicked off the CPU between the time you compute your delta and the time the timer is entered into the kernel.
As already pointed out, Windows is not a real time OS. So you must assume that even if you schedule a timer to got off at ":0010", your code might not even execute until well after that time (for example, ":0540"). As long as you properly handle those issues, things will be "ok".
20ms is approximately the length of a time slice on Windows. There is no way to hit 1ms kind of timings in windows reliably without some sort of RT add on like Intime. In windows proper I think your options are WaitForSingleObject, SleepEx, and a busy loop.
I am using a microcontroller with a C51 core. I have a fairly timeconsuming and large subroutine that needs to be called every 500ms. An RTOS is not being used.
The way I am doing it right now is that I have an existing Timer interrupt of 10 ms. I set a flag after every 50 interrupts that is checked for being true in the main program loop. If the Flag is true the subroutine is called. The issue is that by the time the program loop comes round to servicing the flag, it is already more than 500ms,sometimes even >515 ms in case of certain code paths. The time taken is not accurately predictable.
Obviously, the subroutine cannot be called from inside the timer interrupt due to that large time it takes to execute.The subroutine takes 50ms to 89ms depending upon various conditions.
Is there a way to ensure that the subroutine is called in exactly 500ms each time?
I think you have some conflicting/not-thought-through requirements here. You say that you can't call this code from the timer ISR because it takes too long to run (implying that it is a lower-priority than something else which would be delayed), but then you are being hit by the fact that something else which should have been lower-priority is delaying it when you run it from the foreground path ('program loop').
If this work must happen at exactly 500ms, then run it from the timer routine, and deal with the fall-out from that. This is effectively what a pre-emptive RTOS would be doing anyway.
If you want it to run from the 'program loop', then you will have to make sure than nothing else which runs from that loop ever takes more than the maximum delay you can tolerate - often that means breaking your other long-running work into state-machines which can do a little bit of work per pass through the loop.
I don't think there's a way to guarantee it but this solution may provide an acceptable alternative.
Might I suggest not setting a flag but instead modifying a value?
Here's how it could work.
1/ Start a value at zero.
2/ Every 10ms interrupt, increase this value by 10 in the ISR (interrupt service routine).
3/ In the main loop, if the value is >= 500, subtract 500 from the value and do your 500ms activities.
You will have to be careful to watch for race conditions between the timer and main program in modifying the value.
This has the advantage that the function runs as close as possible to the 500ms boundaries regardless of latency or duration.
If, for some reason, your function starts 20ms late in one iteration, the value will already be 520 so your function will then set it to 20, meaning it will only wait 480ms before the next iteration.
That seems to me to be the best way to achieve what you want.
I haven't touched the 8051 for many years (assuming that's what C51 is targeting which seems a safe bet given your description) but it may have an instruction which will subtract 50 without an interrupt being possible. However, I seem to remember the architecture was pretty simple so you may have to disable or delay interrupts while it does the load/modify/store operation.
volatile int xtime = 0;
void isr_10ms(void) {
xtime += 10;
}
void loop(void) {
while (1) {
/* Do all your regular main stuff here. */
if (xtime >= 500) {
xtime -= 500;
/* Do your 500ms activity here */
}
}
}
You can also use two flags - a "pre-action" flag, and a "trigger" flag (using Mike F's as a starting point):
#define PREACTION_HOLD_TICKS (2)
#define TOTAL_WAIT_TICKS (10)
volatile unsigned char pre_action_flag;
volatile unsigned char trigger_flag;
static isr_ticks;
interrupt void timer0_isr (void) {
isr_ticks--;
if (!isr_ticks) {
isr_ticks=TOTAL_WAIT_TICKS;
trigger_flag=1;
} else {
if (isr_ticks==PREACTION_HOLD_TICKS)
preaction_flag=1;
}
}
// ...
int main(...) {
isr_ticks = TOTAL_WAIT_TICKS;
preaction_flag = 0;
tigger_flag = 0;
// ...
while (1) {
if (preaction_flag) {
preaction_flag=0;
while(!trigger_flag)
;
trigger_flag=0;
service_routine();
} else {
main_processing_routines();
}
}
}
A good option is to use an RTOS or write your own simple RTOS.
An RTOS to meet your needs will only need to do the following:
schedule periodic tasks
schedule round robin tasks
preform context switching
Your requirements are the following:
execute a periodic task every 500ms
in the extra time between execute round robin tasks ( doing non-time critical operations )
An RTOS like this will guarantee a 99.9% chance that your code will execute on time. I can't say 100% because whatever operations your do in your ISR's may interfere with the RTOS. This is a problem with 8-bit micro-controllers that can only execute one instruction at a time.
Writing an RTOS is tricky, but do-able. Here is an example of small ( 900 lines ) RTOS targeted at ATMEL's 8-bit AVR platform.
The following is the Report and Code created for the class CSC 460: Real Time Operating Systems ( at the University of Victoria ).
Would this do what you need?
#define FUDGE_MARGIN 2 //In 10ms increments
volatile unsigned int ticks = 0;
void timer_10ms_interrupt( void ) { ticks++; }
void mainloop( void )
{
unsigned int next_time = ticks+50;
while( 1 )
{
do_mainloopy_stuff();
if( ticks >= next_time-FUDGE_MARGIN )
{
while( ticks < next_time );
do_500ms_thingy();
next_time += 50;
}
}
}
NB: If you got behind with servicing your every-500ms task then this would queue them up, which may not be what you want.
One straightforward solution is to have a timer interrupt that fires off at 500ms...
If you have some flexibility in your hardware design, you can cascade the output of one timer to a second stage counter to get you a long time base. I forget, but I vaguely recall being able to cascade timers on the x51.
Ah, one more alternative for consideration -- the x51 architecture allow two levels of interrupt priorities. If you have some hardware flexibility, you can cause one of the external interrupt pins to be raised by the timer ISR at 500ms intervals, and then let the lower-level interrupt processing of your every-500ms code to occur.
Depending on your particular x51, you might be able to also generate a lower priority interrupt completely internal to your device.
See part 11.2 in this document I found on the web: http://www.esacademy.com/automation/docs/c51primer/c11.htm
Why do you have a time-critical routine that takes so long to run?
I agree with some of the others that there may be an architectural issue here.
If the purpose of having precise 500ms (or whatever) intervals is to have signal changes occuring at specific time intervals, you may be better off with a fast ISR that ouputs the new signals based on a previous calculation, and then set a flag that would cause the new calculation to run outside of the ISR.
Can you better describe what this long-running routine is doing, and what the need for the specific interval is for?
Addition based on the comments:
If you can insure that the time in the service routine is of a predictable duration, you might get away with missing the timer interrupt postings...
To take your example, if your timer interrupt is set for 10 ms periods, and you know your service routine will take 89ms, just go ahead and count up 41 timer interrupts, then do your 89 ms activity and miss eight timer interrupts (42nd to 49th).
Then, when your ISR exits (and clears the pending interrupt), the "first" interrupt of the next round of 500ms will occur about a ms later.
Given that you're "resource maxed" suggests that you have your other timer and interrupt sources also in use -- which means that relying on the main loop to be timed accurately isn't going to work, because those other interrupt sources could fire at the wrong moment.
If I'm interpretting your question correctly, you have:
a main loop
some high priority operation that needs to be run every 500ms, for a duration of up to 89ms
a 10ms timer that also performs a small number of operations.
There are three options as I see it.
The first is to use a second timer of a lower priority for your 500ms operations. You can still process your 10ms interrupt, and once complete continue servicing your 500ms timer interrupt.
Second option - doe you actually need to service your 10ms interrupt every 10ms? Is it doing anything other than time keeping? If not, and if your hardware will allow you to determine the number of 10ms ticks that have passed while processing your 500ms op's (ie. by not using the interrupts themselves), then can you start your 500ms op's within the 10ms interrupt and process the 10ms ticks that you missed when you're done.
Third option: To follow on from Justin Tanner's answer, it sounds like you could produce your own preemptive multitasking kernel to fill your requirements without too much trouble.
It sounds like all you need is two tasks - one for the main super loop and one for your 500ms task.
The code to swap between two contexts (ie. two copies of all of your registers, using different stack pointers) is very simple, and usually consists of a series of register pushes (to save the current context), a series of register pops (to restore your new context) and a return from interrupt instruction. Once your 500ms op's are complete, you restore the original context.
(I guess that strictly this is a hybrid of preemptive and cooperative multitasking, but that's not important right now)
edit:
There is a simple fourth option. Liberally pepper your main super loop with checks for whether the 500ms has elapsed, both before and after any lengthy operations.
Not exactly 500ms, but you may be able to reduce the latency to a tolerable level.