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Recipe 3.9 High-Resolution Timers

3.9.1 Problem

You need to measure time with a finer granularity than the full seconds that time returns.

3.9.2 Solution

The Time::HiRes module, which is included standard starting with the v5.8 release of Perl, encapsulates this functionality for most systems:

use Time::HiRes qw(gettimeofday);
$t0 = gettimeofday( );    
## do your operation here
$t1 = gettimeofday( );
$elapsed = $t1 - $t0;
# $elapsed is a floating point value, representing number
# of seconds between $t0 and $t1

3.9.3 Discussion

Here's some code that uses Time::HiRes to time how long the user takes to press the Return key:

use Time::HiRes qw(gettimeofday);
print "Press return when ready: ";
$before = gettimeofday( );
$line = <STDIN>;
$elapsed = gettimeofday( ) - $before;
print "You took $elapsed seconds.\n";

Press return when ready: 
You took 0.228149 seconds.

The module's gettimeofday function returns a two-element list representing seconds and microseconds when called in list context, or a single floating-point number combining the two when called in scalar context. You can also import its time function to replace the standard core version by that name; this always acts like scalar gettimeofday.

The module also provides usleep and ualarm functions, which are alternate versions of the standard Perl sleep and alarm functions that understand granularities of microseconds instead of just seconds. They take arguments in microseconds; alternatively, you can import the module's sleep and alarm functions, which take floating-point arguments in seconds, to replace the standard versions, which take integer arguments in seconds. For access to your system's low-level itimer routines (if you have them), setitimer and getitimer are also provided.

If your system doesn't support that module, you might try to poke around by hand using syscall. Compare Time::HiRes to the equivalent syscall code. (This example is included principally so that you can see an example of Perl's abstruse and archaic syscall function.)

require 'sys/';

# initialize the structures returned by gettimeofday
$done = $start = pack($TIMEVAL_T, (0,0));

# prompt
print "Press return when ready: ";

# read the time into $start
syscall(&SYS_gettimeofday, $start, 0) != -1
           || die "gettimeofday: $!";

# read a line
$line = <>;

# read the time into $done
syscall(&SYS_gettimeofday, $done, 0) != -1
       || die "gettimeofday: $!";

# expand the structure
@start = unpack($TIMEVAL_T, $start);
@done  = unpack($TIMEVAL_T, $done);

# fix microseconds
for ($done[1], $start[1]) { $_ /= 1_000_000 }

# calculate time difference
$delta_time = sprintf "%.4f", ($done[0]  + $done[1]  )
                              ($start[0] + $start[1] );

print "That took $delta_time seconds\n";
Press return when ready: 
That took 0.3037 seconds

It's longer because it's doing system calls in Perl, whereas Time::HiRes does them in C providing a single function. It's complex because directly accessing system calls peculiar to your operating system requires understanding the underlying C structures that the system call takes and returns. Some programs that come with the Perl distribution try to automatically calculate the formats to pack and unpack for you, if fed the appropriate C header file. In the example, sys/ is a Perl library file generated with h2ph, which converts the sys/syscall.h header file into sys/, defining (among other things) &SYS_gettimeofday as a subroutine that returns the system call number of gettimeofday.

Here's another example of Time::HiRes, showing how you could use it to benchmark a sort (if you didn't care to use the standard Benchmark module):

use Time::HiRes qw(gettimeofday);
# take mean sorting time
$size = 2000;
$number_of_times = 100;
$total_time = 0;

for ($i = 0; $i < $number_of_times; $i++) {
    my (@array, $j, $begin, $time);
    # populate array
    @array = ( );
    for ($j=0; $j < $size; $j++) { push(@array, rand) }

    # sort it
    $begin = gettimeofday;
    @array = sort { $a <=> $b } @array;
    $time = gettimeofday-$begin;
    $total_time += $time;

printf "On average, sorting %d random numbers takes %.5f seconds\n",
    $size, ($total_time/$number_of_times);
On average, sorting 2000 random numbers takes 0.01033 seconds

3.9.4 See Also

The documentation for the Time::HiRes and Benchmark modules; the syscall function in perlfunc(1) and Chapter 29 of Programming Perl; your system's syscall(2) manpage

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