latency error (~2ms) with nanosleep

quade
2005-06-13T13:31:06Z

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Playing around with the (simple) measurement of latency-times
I noticed, that the systemcall "nanosleep" has always a minimal
latency from about ~2ms (haven't run it all night, so...). It
seems to be a systematical error.

A short investigation shows, that "sys_nanosleep()" uses
schedule_timeout(), but schedule_timeout() is working exactly
as expected. Therefore I think it has something to do with
the scheduling?

Has someone an explanation for the ~2ms error?
If it is indeed a systematical error, does it make sense to
"adjust" (correct) this error in the systemcall "sys_nanosleep()"?

Find attached my small test program.

Juergen.

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#include <stdio.h>
#include <time.h>
#include <sched.h>
#include <sys/time.h>

#define TAKTFREQ 600

static inline unsigned long long int rdtsc()
{
unsigned long long int x;
__asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
return x;
}

// XXX - I know, there can be an overrun ...
static inline unsigned long time_in_usec( struct timeval *tv )
{
return (tv->tv_sec*1000000)+tv->tv_usec;
}

int main( int argc, char **argv )
{
int i;
struct timespec delay;
struct timeval tvstart, tvend;
unsigned long start, end, timediff, maxdiff, shouldbetime;
unsigned long mindiff;
#if 1
struct sched_param SchedulingParameter;

SchedulingParameter.sched_priority = 50;
if( sched_setscheduler( 0, SCHED_RR, &SchedulingParameter )!= 0 ) {
perror( "Set Scheduling Priority" );
return -1;
}
#endif

for( shouldbetime=1000; shouldbetime<51000;shouldbetime+=1000 ) {
mindiff = 0xffffffff;
maxdiff = 0;
for( i=0; i<30; i++ ) {
delay.tv_sec = 0;
delay.tv_nsec = shouldbetime*1000; // in nsec
gettimeofday(&tvstart,NULL);
start = rdtsc();
nanosleep( &delay, NULL );
end = rdtsc();
gettimeofday(&tvend,NULL);
#if 0
printf("timdiff: %ld - %ld\n",
(end-start)/TAKTFREQ, // 600 MHz - in usec
time_in_usec(&tvend)-time_in_usec(&tvstart));
#endif
timediff = time_in_usec(&tvend)-time_in_usec(&tvstart);
timediff -= shouldbetime;
if( (timediff > maxdiff)&&i>0 )
maxdiff = timediff;
if( (timediff < mindiff)&&i>0 )
mindiff = timediff;
}
//printf("%7.1ld-> max diff: %ld\n", shouldbetime, maxdiff );
printf("%7.1ld %ld %ld\n", shouldbetime, maxdiff, mindiff );
}
return 0;
}