Thanks, but using low-level commands is faster. For us speed (and thus accuracy) is very important.

I have tested your code on my computer and the average difference from a second is +0.00006222222222222222, which again is more by two decimal places than the difference of the negative_timer (which is -0.000000676723442).

EV is an interface to the lower-level libev, and AFAIK it's fairly performant. Its "periodic" function claims to work with wall clock time, so you shouldn't have any drift relative to the system clock. That the system clock can drift relative to real time, and that it can be adjusted in jumps, are whole different issues. I'm not sure if the default ntpd supports smooth adjustments to the system clock (Update: apparently, it does), butand I believe chrony does. (I have several systems with a GPS receiver that supports the PPS signal, and I feed that into chronyd to provide a system time that should be quite accurate. I've published some info on that here.)

use warnings;
use strict;
use Math::BigFloat;
use Time::HiRes qw/gettimeofday/;
use EV;

my $w1 = EV::signal( 'INT', sub { EV::break } );

my ($cnt, $avg, $prev) = (0);
my $w2 = EV::periodic( 0, 1, 0, sub {
    my $cur = Math::BigFloat->new( sprintf("%d.%06d",gettimeofday) );
    if ( defined $prev ) {
        my $delta = $cur - $prev;
        $avg = defined $avg ? ( $avg + $delta ) / 2.0 : $delta->copy;
        $cnt++;
    }
    $prev = $cur->copy;
} );

EV::run;

print "\nAverage over $cnt deltas: $avg\n";
[download]

Sample output from different runs on my system:

Average over 67 deltas: 1.000005597916695584862263807609389365538
Average over 76 deltas: 1.00012161307435464691799175142220832736
Average over 137 deltas: 0.9997675784785176200109895891945901313775
Average over 442 deltas: 0.9999437250604076389815434390305220372265
[download]

This one looks promising. :)

It is less punctual than the original code I posted, but I might do.

But I got something strange when I replaced time() in my original code with gettimeofday: exactly precise seconds (to 5 decimal places). I am not quite sure if that is actually correct–is that just calculated to be precise or it the measurement actually precise? I believe that my measurements are not as precise as yours. :)

I tried to simplify the test using my $w2 = EV::periodic( 0, 1, 0, sub { print gettimeofday, "\n";} );, but the result was quite unprecise; here’s the output:

15748078781259
15748078791348
15748078801032
15748078811008
1574807882999
1574807883541
1574807884744
15748078851030
15748078861002
15748078871099
1574807888937
15748078892034
15748078901001
15748078912030
15748078921019
1574807893339M
[download]

Do I do something wrong?

Anyway, I use chrony, too, but my original timer function (from time to time) still complains about negative time not invented yet. Although, I use default settings of CentOS 7 for chrony–do I need to adjust something to use those smooth adjustments?