what is that configures the clock_gettime resolution? - c++

Question

I'm experimenting with the time-management of linux on a raspberry pi. For that I'm looking at clock_gettime (and clock_getres) for CLOCK_REALTIME.
I noticed that when I check clock_getres, it always says nanoseconds resolution (it returns 0 for tv_sec and 1 for tv_nsec) and also the values returned by clock_gettime point in that direction (e.g. I get values like 1642070078.415542996) but only if the system is fully booted with systemd etc running! When I put my test-program as a replacement for init (e.g. init=/test-clock in cmdline.txt), then suddenly getres still returns 1 for tv_nsec but all values returned by clock_gettime are with microsecond resolution!
test-program:
#include <stdint.h>
#include <stdio.h>
#include <time.h>
void test_clock(const clockid_t id, const char *const name)
{
struct timespec ts { 0, 0 };
uint64_t n = 0;
struct timespec start { 0, 0 }, end { 0, 0 };
clock_gettime(id, &start);
uint64_t start_ts = start.tv_sec * uint64_t(1000000000) + start.tv_nsec, end_ts = 0;
do {
clock_gettime(id, &end);
n++;
end_ts = end.tv_sec * uint64_t(1000000000) + end.tv_nsec;
}
while(end_ts - start_ts < 2500000000);
struct timespec now { 0, 0 };
int ns = 0;
for(int i=0; i<1000; i++) {
clock_gettime(id, &now);
if (now.tv_nsec % 1000)
ns++;
}
if (clock_getres(id, &ts))
fprintf(stderr, "clock_getres(%d) failed (%s)\n", id, name);
else
printf("%s:\t%ld.%09ld\t%f/s\t%d\t%ld.%09ld\n", name, ts.tv_sec, ts.tv_nsec, n / 2.5, ns, now.tv_sec, now.tv_nsec);
}
int main(int argc, char *argv[])
{
printf("clock\tresolution\tcalls/s\t# with ns\tnow\n");
test_clock(CLOCK_REALTIME, "CLOCK_REALTIME");
test_clock(CLOCK_TAI, "CLOCK_TAI");
test_clock(CLOCK_MONOTONIC, "CLOCK_MONOTONIC");
test_clock(CLOCK_MONOTONIC_RAW, "CLOCK_MONOTONIC_RAW");
return 0;
}
Compile with:
g++ -Ofast -ggdb3 -o test-clock test-clock.cpp -lrt
Expected output:
clock resolution calls/s # with ns now
CLOCK_REALTIME: 0.000000001 48881594.400000/s 1000 1642071062.213603835
CLOCK_TAI: 0.000000001 49500959.200000/s 1000 1642071101.713668922
CLOCK_MONOTONIC: 0.000000001 49248353.200000/s 1000 2402707.303582035
CLOCK_MONOTONIC_RAW: 0.000000001 47072281.600000/s 1000 2402705.604860726
What I see when starting test-clock as init replacement:
clock resolution calls/s # with ns now
CLOCK_REALTIME: 0.000000001 853001.200000/s 0 19.216404000
CLOCK_TAI: 0.000000001 736536.000000/s 0 21.718848000
CLOCK_MONOTONIC: 0.000000001 853367.200000/s 0 24.220166000
CLOCK_MONOTONIC_RAW: 0.000000001 855598.800000/s 0 26.721360000
The 4th column tells me that no readings were with nanosecond resolution.
So what I would like to know is: how can I configure the kernel/glibc/whatever so that it gives me nanosecond resolution at boot as well?
Any ideas?