Benchmarking functions

[Anon]

Hello,

I'm attempting to time functions in PHP, but I keep getting
negative values. I use the following function to get the time
before and after the function I want to time has been called:

function time_now(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);
}

The timing code looks something like this:

for ($i = 1; $i <= $count; $i++) {

$t_start = time_now(); // Start Time
// Some action goes here
$t_end = time_now(); // End Time

$t[$i] = $t_end - $t_start; // Total time passed

}

This give me an array with $count elements containing the total time elapsed for each run. How is it possible that roughly 1/2 of the result times turn up negative? Is there some caching going on?

Cilanntrus

[[deleted]]

Are you sure that the $t array is not affected by what's in "some action goes here"?

[Anon]

I am positive that $t is not affected.

I plugged away at this problem for a while and figured out that the problem was the original time_now() function (which was copied verbatim from the PHP docs -- albeit with a changed name).

I traced the problem back to the original microtime function:

code:

function time_now(){
list($usec, $sec) = explode(" ",microtime());
return ((float)$usec + (float)$sec);

}

$sec is the amount of seconds (a whole number) $usec is the amount of time below 1 second (a decimal number)

For some reason, occationally $usec would "turn-over" (ie: 0.998, 0.999, 0.000) but $sec would not increase by one at the same time.

So, when I ran this code:

code:

$t_start = time_now(); // Start Time
func_to_time(); // Some action
$t_end = time_now(); // End Time

$t_total = $t_end - $t_start; // Total time passed

$t_start would actually be greater than $t_end .

Anyhoo, I fixed this by making my own time_now() function:

code:

function time_now(){
$time = gettimeofday();
return ((float)$time['usec']/1000000);

}

...which returns values similar to 0.32101234. My version of time_now() is good for timing up 0.999999999 seconds, which is way more than enough for the way I am benchmarking.