nanosleep with small value

Dag Nygren
2005-11-17T16:30:49Z

Hi,

seeing a strange thing happening here:
using nanosleep() with a smallish value gives me a very long sleeptime?

Is this because of a context switch being forced?
Shouldn't the scheduler change affect that?

The test program:
===================================
#include <time.h>
#include <sched.h>
#include <stdio.h>

void delay_ns(unsigned long dly)
{
static struct timespec time;
int err;
{
time.tv_sec = 0;
time.tv_nsec = dly;
err = nanosleep(&time, NULL);
if (err) {
perror( "nanosleep failed" );
}
}
}

main()
{
int i;

struct sched_param mysched;
int err;

if ( sched_getparam( 0, &mysched ) != 0 )
perror( "" );
else {
mysched.sched_priority = sched_get_priority_max(SCHED_FIFO);
err = sched_setscheduler(0, SCHED_FIFO, &mysched);
if( err != 0 ) {
fprintf (stderr,"sched_setscheduler returned: %d\n",
err );
perror( "" );
}
}

for (i=0; i < 1000; i++)
delay_ns(1000UL);
}
==================================
The result running this is:
% time ./tst

real 0m8.000s
user 0m0.000s
sys 0m0.000s

I would have expected about 1000 * 1 us + overhead,
but 8 seconds ????

Noticed this when trying to debug a PIC-programming
software where the delay_ns() routine is used.

Increasing the nanosleep() argument to something more than HZ
will give me the expected sleep times.

Best

--
Dag Nygren email: dag@newtech.fi
Oy Espoon NewTech Ab phone: +358 9 8024910
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FINLAND