Re: [Xenomai-help] Huge clock drift

[Jonas Witt <jonas.witt@domain.hid>]

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Am 27.05.2011 08:40, schrieb Gilles Chanteperdrix:
> On 05/26/2011 07:28 PM, Jonas Witt wrote:
>> Hi all,
>>
>> i am having a problem concerning the clock drift under load:
>>
>> # /usr/xenomai/bin/clocktest
>> == Tested clock: 0 (CLOCK_REALTIME)
>> CPU      ToD offset [us] ToD drift [us/s]      warps max delta [us]
>> --- -------------------- ---------------- ---------- --------------
>>     0          775571614.0       166776.858          0            0.0
>>
>>
>> It remains in the hundreds of MILLIseconds, changing constantly. My
>> setup consists of an embedded Intel Atom board (1.6GHz Z530 processor)
>> with a 2.6.32.7 kernel and Xenomai 2.5.2.
> Hi Jonas,
>
> Could you try and see if 2.5.6 with latest I-pipe patches has the same
> behaviour?
>
>> Latencies under load are
>> reasonable. Mean latency<  10us. Maximum latency<  40us.
>>
>> Without load the ToD offset is approximately constant over time with a
>> ToD drift in the range of 10 microseconds (strangely after a while this
>> settles in a range of 2 microseconds). Does anyone have an idea how this
>> can be caused?
> First, I am not sure clocktest is meant to be use under load. Second,
> does your system uses ntp?
>
>> As a workaround I currently use rt_timer_read() in all
>> relevant programs (also the non-realtime ones), since I need consistent
>> timestamps between realtime and non-realtime tasks.
> In order to solve this particular issue, we have a solution, but not yet
> in stable released versions.
>
>> One other (maybe unrelated) strange behavior is occasional secondary
>> mode switches when calling rt_queue_read(...).
> For this error, we need more details, such as a simple test case
> allowing to reproduce the issue, and again, you need to be sure to
> reproduce the issue on latest stable release with latest I-pipe patches.
>
> Regards.

Hi Gilles,

thanks for your input. I will try Xenomai 2.5.6 soon. In the meantime I 
wrote a simple load simulator to reproduce the issue with my more 
complex program. The clock drift only occurs with a load of more than 
30%. Below that the clock is fine. So you may have to adjust the 
attached code (e.g. change the 2000 value in the for-loop to something 
larger) to stress your system to that level where the processing time is 
more than 2000 microseconds.

Cheers,
Jonas

[-- Attachment #2: CMakeLists.txt --]
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cmake_minimum_required(VERSION 2.6)

project(XenoLoadSimulator)

link_directories(/usr/xenomai/lib)
include_directories(/usr/xenomai/include)

add_executable(${PROJECT_NAME} main.cpp)
target_link_libraries(${PROJECT_NAME}  xenomai rtdm native)

[-- Attachment #3: main.cpp --]
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#include <sys/mman.h>
#include <csignal>
#include <native/queue.h>
#include <native/timer.h>
#include <native/task.h>
#include <cmath>

bool running = true;
RT_QUEUE queue;

void exitSignal (int param)
{
	printf ("Exiting program...\n");
	running = false;
}

void mainLoop(void* dummy)
{	
	int status;
	printf("Main loop...\n");
		
	RTIME currentTime, lastTime;
	
	double result = 7.0;
	double a = 1000.;
	double b = 333.;
	unsigned int counter = 0;
	float periodTime = 500.0f;
	float processingTime = 0.0f;
	double position[3];
	
	while(running)
	{
		rt_task_wait_period(NULL);
		
		RTIME startTime = rt_timer_read();
		for(unsigned long long int i=0; i < ((unsigned long)result) % 5 + 3000; ++i)
		{
			a += sin((double)i / 500.);
			b = sqrt(a);
			result += ((double)i / 3.0 + 3.1423) * (double) i - a * b;
		}
		currentTime = rt_timer_read();
		
		processingTime = 0.9f * processingTime + 0.1f * (float)(currentTime - startTime) / 1000.f;
		periodTime = 0.9f * periodTime + 0.1f * (float)(currentTime - lastTime) / 1000.f;
		lastTime = currentTime;
		
		/*if(rt_queue_read(&queue, (void*)position, 3*sizeof(double), TM_NONBLOCK) > 0)
		{	
			printf("X: %.1f  Y: %.1f  Z: %.1f\n", position[0], position[1], position[2]);
		}*/
		
		if(counter++ > 100)
		{
			counter = 0;
			printf("Period Time: %.1fus      Result: %.1f\n", periodTime, result);
			printf("Processing Time: %.1fus\n", processingTime);
		}
	}	
}


int main (int argc , char * argv []) 
{
	mlockall(MCL_CURRENT | MCL_FUTURE);
	
	signal (SIGINT, exitSignal);
	printf("Initialization...");
	
	int status;
								
	/*if(rt_queue_bind(&queue, "camPosQueue", TM_NONBLOCK) != 0)
	{
		return 0;
	}*/
	
	rt_timer_set_mode(TM_ONESHOT);
	
	RT_TASK task;
	if(rt_task_create(&task, NULL, 0, 80, T_JOINABLE) != 0)
	{
		printf("Creation of Xenomai Task failed!\n");
		return 0;
	}
	
	rt_task_set_periodic 	(&task, TM_NOW,	4000000);

	printf("done!\n");
	
	rt_task_start(&task, &mainLoop, (void*)NULL);
  	
	
	rt_task_join(&task);	
	
	mainLoop(NULL);
	
	printf("CleanUp...");
	
	//rt_queue_unbind(&queue); 
 	
	printf("done!\n");
	return 0;
}