Greetings,
I seems to me that at least 2.4.8 & 2.4.14 have miscalculation
bugs beneath the sys_nanosleep() call. Calls to nanosleep() on
my Mandrake 8.1 based Dell 450 MHz PII produce poor results
even when called with a resolution of 1/10 of a second.
Is this a) a 2.4 kernel bug, b) a glibc 2.2.4 bug, c) due to
a 450MHz PII being "underpowered", or d) the nature of scheduling
under Linux? I tried to investigate a), but between all the
jiffies and x86 assembly I got lost.
Look at these results. Each value should be 1.000 second.
The included program loops N times with a 1/N second sleep,
using three different methods. Using select(), nanosleep(),
and usleep().
Sleeping 1000 times for one millisecond should take 1 second
of wall clock, yet it takes 20 seconds! Try the program and
see for yourself.
Thank you for any insights!!
Mike Arras
mkarras110@yahoo.com
My typical results on a quiet machine:
1x 1/1 of a second...
Select 0.997
Nanosleep 1.010
Usleep 1.010
10x 1/10 of a second...
Select 1.000
Nanosleep 1.100
Usleep 1.100
50x 1/50 of a second...
Select 1.000
Nanosleep 1.500
Usleep 1.500
100x 1/100 of a second...
Select 1.001
Nanosleep 2.000
Usleep 2.000
200x 1/200 of a second...
Select 2.000
Nanosleep 4.000
Usleep 4.000
500x 1/500 of a second...
Select 5.000
Nanosleep 10.000
Usleep 10.000
1000x 1/1000 of a second...
Select 10.000
Nanosleep 20.000
Usleep 20.000
#include <sys/time.h>
#include <time.h>
#include <stdio.h>
#include <unistd.h>
double precTime(void)
{
struct timeval tv;
gettimeofday(&tv, 0);
return tv.tv_sec + tv.tv_usec / 1000000.0;
}
static void runtimer(int fraction)
{
struct timeval tm;
struct timespec ts;
double startTime, endTime;
int i;
printf("%dx 1/%d of a second...\n", fraction, fraction);
startTime = precTime();
for (i = 0; i < fraction; i++)
{
tm.tv_sec = 0;
tm.tv_usec = 1000000 / fraction;
select(0, 0, 0, 0, &tm);
}
endTime = precTime();
printf("Select %1.3f\n", endTime - startTime);
startTime = precTime();
for (i = 0; i < fraction; i++)
{
ts.tv_sec = 0;
ts.tv_nsec = 1000000000 / fraction - 1;
nanosleep(&ts, 0);
}
endTime = precTime();
printf("Nanosleep %1.3f\n", endTime - startTime);
startTime = precTime();
for (i = 0; i < fraction; i++)
{
usleep(1000000 / fraction);
}
endTime = precTime();
printf("Usleep %1.3f\n\n", endTime - startTime);
}
int main(void)
{
runtimer(1);
runtimer(10);
runtimer(50);
runtimer(100);
runtimer(200);
runtimer(500);
runtimer(1000);
return 0;
}
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