[Xenomai] big deviation on clock_nanosleep wait time

[Xenomai] big deviation on clock_nanosleep wait time

[Thierry Bultel]

Hi,

My configuration is
xenomai-2.6.3 + ipipe-1.18-13, freescale kernel rel_imx_3.0.35_4.0.0, on
the IMX6Q.

I want to use CONFIG_HIGH_RES_TIMERS and CONFIG_HZ=100, which is the
default value for the IMX.

The described issue does not happen when CONFIG_IPIPE is not set.
This happens with a linux task, no other xenomai task is running.

The following linux program performs 1000 calls to clock_nanosleep and
calculates some stats at the end.

When sleeping for delays around 10 to 30 ms the results are pretty good,
the error is pretty much constant and around 0.1 %
but when waiting for 100 to 500 microseconds things get bad, with
sometimes, wake up that happen 40 ms after.

Average 247.354000, error 0.247354%
Deviation 44240.660581, average error 147.354000, max_err 40781.000000


Looking at the log shows that there are about 7 occurrences where the
sleep takes between 3 and 40 ms instead of 0.1 ms .

sample program:

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <time.h>
#include <sched.h>
#include <math.h>

#define TIMESPEC2USEC(t1, t2) \
    ( (unsigned long)(t2.tv_sec * 1000000 + t2.tv_nsec / 1000) - \
      (unsigned long)(t1.tv_sec * 1000000 + t1.tv_nsec / 1000) )

#define NB 1000

int main(int argc, char * argv[]) {

	struct timespec res;
	struct timespec delay, time1, time2;

    	int default_priority;
	sigset_t mask;
	int ix;

	unsigned long total = 0;
	int sleep_time = 100 * 1000; // 0.1 ms

	unsigned long vals[NB];
	double  max_err = 0.0;


	struct sched_param p;

	default_priority = ( sched_get_priority_min(SCHED_FIFO) +
sched_get_priority_max(SCHED_FIFO) ) / 2;

	p.sched_priority = default_priority;

	if ( sched_setscheduler( 0, SCHED_FIFO, &p) != 0 )
	{
        	perror( "sched_setscheduler" );
	        exit(-1);
	}


	if (clock_getres (CLOCK_REALTIME, &res) == -1)
	        perror ("clock_getres: CLOCK_REALTIME");
	else
        	printf ("CLOCK_REALTIME: %ld s, %ld ns\n", res.tv_sec,
res.tv_nsec);

	if (clock_getres (CLOCK_MONOTONIC, &res) == -1)
		perror ("clock_getres: CLOCK_MONOTONIC");
	else
		printf ("CLOCK_MONOTONIC: %ld s, %ld ns\n", res.tv_sec, res.tv_nsec);

	if (clock_getres (CLOCK_PROCESS_CPUTIME_ID, &res) == -1)
	        perror ("clock_getres: CLOCK_PROCESS_CPUTIME_ID");
	else
		printf ("CLOCK_PROCESS_CPUTIME_ID: %ld s, %ld ns\n", res.tv_sec,
res.tv_nsec);

	if (clock_getres (CLOCK_THREAD_CPUTIME_ID, &res) == -1)
		perror ("clock_getres: CLOCK_THREAD_CPUTIME_ID");
	else
		printf ("CLOCK_THREAD_CPUTIME_ID: %ld s, %ld ns\n", res.tv_sec,
res.tv_nsec);

	clock_gettime(CLOCK_MONOTONIC, &time1);

	for (ix=0; ix < NB; ix++)
	{

		delay.tv_sec = 0;
		delay.tv_nsec = sleep_time; // 20 ms

		if (clock_nanosleep(CLOCK_MONOTONIC, /*TIMER_ABSTIME */ 0, &delay,
NULL) != 0)
		{
			perror("clock_nanosleep\n");
			exit(0);
		}

		if (clock_gettime(CLOCK_MONOTONIC, &time2) !=0)
		{
			perror("clock_gettime\n");
			exit(0);
		}

	
		vals[ix] = TIMESPEC2USEC(time1,time2);

		printf("Slept %ld usecs\n", vals[ix]);
		total += vals[ix];

		time1 = time2;
	}

	double average = ((double)total) / NB;

	printf("Average %f, error %f%%\n", average, 100*average/sleep_time);

	// calculates deviation

	double dev = 0.0;
	double err, av_err = 0.0;

	for (ix = 0 ; ix < NB; ix++)
	{
		double diff = vals[ix]-average;
		dev += (diff*diff);

		err = abs(vals[ix]-(sleep_time/1000.0));
		av_err+=err;

		if (max_err < err)
			max_err = err;
	}

	printf("Deviation %f, average error %f, max_err %f\n",
		sqrt(dev), av_err/NB, max_err);

	return 0;
}


Best regards
Thierry

Re: [Xenomai] big deviation on clock_nanosleep wait time

[Gilles Chanteperdrix]

On 11/04/2013 05:16 PM, Thierry Bultel wrote:
> Hi,
> 
> My configuration is
> xenomai-2.6.3 + ipipe-1.18-13, freescale kernel rel_imx_3.0.35_4.0.0, on
> the IMX6Q.
> 
> I want to use CONFIG_HIGH_RES_TIMERS and CONFIG_HZ=100, which is the
> default value for the IMX.

Do you observe the same issue without CONFIG_HIGH_RES_TIMERS?


-- 
                                                                Gilles.

Re: [Xenomai] big deviation on clock_nanosleep wait time

[Gilles Chanteperdrix]

On 11/04/2013 05:34 PM, Thierry Bultel wrote:
> Le 04/11/2013 17:11, Gilles Chanteperdrix a écrit :
>> On 11/04/2013 05:16 PM, Thierry Bultel wrote:
>>> Hi,
>>>
>>> My configuration is
>>> xenomai-2.6.3 + ipipe-1.18-13, freescale kernel rel_imx_3.0.35_4.0.0, on
>>> the IMX6Q.
>>>
>>> I want to use CONFIG_HIGH_RES_TIMERS and CONFIG_HZ=100, which is the
>>> default value for the IMX.
>>
>> Do you observe the same issue without CONFIG_HIGH_RES_TIMERS?
>>
>>
> 
> Without CONFIG_HIGH_RES_TIMERS, the wait precision is dictated by
> CONFIG_HZ, in other words 10 ms.
> 
> FYI as I needed to wait 30ms with a 1 ms precision, I formerly had to
> hack Kconfig to allow CONFIG_HZ=1000 (and it worked fine).

It appeared to work fine, however Linux is not an RTOS, it can not
guarantee a 1ms latency. Why not using Xenomai for precise sleeps? Other
than that, I have no idea, I would use the I-pipe tracer to understand
what is going on in between the time when the timer ticks and the time
when the task is woken up.

-- 
                                                                Gilles.

Re: [Xenomai] big deviation on clock_nanosleep wait time

[Gilles Chanteperdrix]

On 11/04/2013 05:16 PM, Thierry Bultel wrote:
> Hi,
> 
> My configuration is
> xenomai-2.6.3 + ipipe-1.18-13, freescale kernel rel_imx_3.0.35_4.0.0, on
> the IMX6Q.

Do you have the -pre and -post patches applied or do you still run
without them?

-- 
                                                                Gilles.