php time() and microtime() do sometimes not concord

Question

While logging some data using microtime() (using PHP 5), I encountered some values that seemed slightly out of phase in respect to the timestamp of my log file, so I just tried to compare the output of time() and microtime() with a simple script (usleep is just here in order to limit the data output):

<?php
for($i = 0; $i < 500; $i++) {
    $microtime = microtime();
    $time = time();
    list($usec, $sec) = explode(" ", $microtime);
    if ((int)$sec > $time) {
        echo $time . ' : ' . $microtime . '<br>';
    }
    usleep(50000);
}
?>

Now, as $microtime is declared before $time, I expect it to be smaller, and nothing should ever be output; however, this obviously is not the case, and every now and then, $time is smaller than the seconds returned from microtime(), as in this example (truncated) output:

1344536674 : 0.15545100 1344536675
1344536675 : 0.15553900 1344536676
1344536676 : 0.15961000 1344536677
1344536677 : 0.16758900 1344536678

Now, this is just a small gap; however, I have observed some series where the difference is (quite) more than a second... so, how is this possible?

Groth

Could you tell me the outcome of some big upcoming matches? I'm want to place some bets ;) – Honky

Can you tell us what OS you are using, what version of PHP this is happening in, and what version of libc/glibc PHP was compiled with? microtime works differently on Windows than it does on Linux, but both functions on Linux rely on C functions to get this info so likely this isn't really a PHP issue. I ran your code on PHP 5.4.5 and no time values were output (meaning time was always greater than or equal to microtime [as would be expected]). – Simon

Answer 1

If you look at the implementations of time and microtime, you see they're radically different:

• time just calls the C time function.
• microtime has two implementations: If the C function gettimeofday is available (which it should be on a Linux system), it is called straight-forward. Otherwise they pull of some acrobatics to use rusage to calculate the time.

Since the C time call is only precise up to a second, it may also intentionally use a low-fidelity time source.

Furthermore, on modern x86_64 systems, both C functions can be implemented without a system call by looking into certain CPU registers. If you're on a multi-core system, these registers may not exactly match across cores, and that could be a reason.

Another potential reason for the discrepancies is that NTPd(a time-keeping daemon) or some other user-space process is changing the clock. Normally, these kinds of effects should be avoided by adjtime.

All of these rationales are pretty esoteric. To further debug the problem, you should:

• Determine OS and CPU architecture _{(hint: and tell us!)}
• Try to force the process on one core
• Stop any programs that are (or may be) adjusting the system time.

Highstrung

Ok, thanks for the detailed hints. This definitely seems to be related to the OS, as it does occur on the two windows 7 machines I have at hand, but apparently not on the linux box of my hoster... As this might be a rather difficult problem to address, I guess I will just try to find some alternative to the usage of microtime... – Groth

Answer 2

This could be due to the fact that microtime() uses floating point numbers, and therefore rounding errors may occur.

You can specify floating point numbers' precision in php.ini

Scorpius