Re^5: The most precise second (timer)

nanosleep (1000000000);

I think that with a fixed time like that, you'll probably still see a cumulative error if you run it for a longer time, I don't think 5 minutes is enough.

However, I only now realized that I overlooked a solution that should have been staring me in the face: clock_nanosleep(2), which is also provided by Time::HiRes. Try running the following over a longer time (several hours):

use warnings;
use strict;
use Time::HiRes qw/ clock_gettime clock_nanosleep
    CLOCK_REALTIME TIMER_ABSTIME /;
die "Don't have nanosleep" unless Time::HiRes::d_nanosleep();

my $run = 1; $SIG{INT} = sub { $run = 0 };
while ($run) {
    my $now_s = int(clock_gettime(CLOCK_REALTIME));
    clock_nanosleep(CLOCK_REALTIME, ($now_s+1)*1e9, TIMER_ABSTIME);
    last unless $run;
    print clock_gettime(CLOCK_REALTIME), "\n";
}
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Re^6: The most precise second (timer) by tukusejssirs (Beadle) on Nov 28, 2019 at 11:33 UTC
Okay, I your point with the fixed times. I like the way you made the `clock_nanosleep(CLOCK_REALTIME, ($now_s+1)1e9, TIMER_ABSTIME);`. In short tests (like for 10-20 s), the average diff is always `1.00000000000000000000`, at least with the 5 decimal places the `clock_gettime(CLOCK_REALTIME)` outputs. Anyway, I am in the middle of 5 h run of the script; I’ll report the results when it is finished. Again, thanks for your time! Update:* I have found out (using `printf`) that ~~`0001511573791503906250`~~ `printf ("%.40f\n\n", clock_gettime(CLOCK_REALTIME));` outputs floats with 21 to 22 decimal places, therefore I’ll print the time out using `printf ("%.22f\n", clock_gettime(CLOCK_REALTIME));`. This might slow the execution of the script and thus the measurement won’t be that accurate. Or even worse: the extra time (decimal places) are only the time required to execute the extra stuff, I don’t know. Update 2: I have the results of the 5-hour run of your script. I have calculated the difference as follows, but my calculations might be wrong, therefore here is the list of 18,000 time values. Note that the machine was not running only this script, because the timer (periodic runner) won’t be run as the only program on the server in production. `average diff between times : +0.999999998888827179577276638156035915 +0171 average diff from second : +0.000000001111172842935718417798556472 +2552` [download] This is quite nice. I has already surpassed my assumptions. I’ve read the Time::HiRes readme, so I do not expect `clock_nanosleep()` to be exact down to one nanosecond, but the average results above tells me that that it is as least _nearly_ that accurate (at least on my system and its configuration). hawkex, I can’t thank you enough! You’re great!	[reply] [d/l] [select]

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Re^6: The most precise second (timer)
by tukusejssirs (Beadle) on Nov 28, 2019 at 11:33 UTC

Okay, I your point with the fixed times.

I like the way you made the clock_nanosleep(CLOCK_REALTIME, ($now_s+1)*1e9, TIMER_ABSTIME);. In short tests (like for 10-20 s), the average diff is always 1.00000000000000000000, at least with the 5 decimal places the clock_gettime(CLOCK_REALTIME) outputs.

Anyway, I am in the middle of 5 h run of the script; I’ll report the results when it is finished.

Again, thanks for your time!

Update: I have found out (using printf) that ~~0001511573791503906250~~ printf ("%.40f\n\n", clock_gettime(CLOCK_REALTIME)); outputs floats with 21 to 22 decimal places, therefore I’ll print the time out using printf ("%.22f\n", clock_gettime(CLOCK_REALTIME));. This might slow the execution of the script and thus the measurement won’t be that accurate. Or even worse: the extra time (decimal places) are only the time required to execute the extra stuff, I don’t know.

Update 2: I have the results of the 5-hour run of your script. I have calculated the difference as follows, but my calculations might be wrong, therefore here is the list of 18,000 time values. Note that the machine was not running only this script, because the timer (periodic runner) won’t be run as the only program on the server in production.

average diff between times   : +0.999999998888827179577276638156035915
+0171
average diff from second     : +0.000000001111172842935718417798556472
+2552
[download]

This is quite nice. I has already surpassed my assumptions. I’ve read the Time::HiRes readme, so I do not expect clock_nanosleep() to be exact down to one nanosecond, but the average results above tells me that that it is as least _nearly_ that accurate (at least on my system and its configuration).

hawkex, I can’t thank you enough! You’re great!

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