Re: is 'my' that much faster than 'local'?

But they often can achieve the same result. And usually people use them without even noticing the difference... And ther is a difference. . . a quick example:

use Time::HiRes qw(gettimeofday);
$start_time = gettimeofday();
$global_variable = "hello!";

for ($i = 0; $i < 10000000; $i++) {
        # replace this with "my"
        local $global_variable = "redefined. . ";
}
print gettimeofday - $start_time." seconds\n";
print "$global_variable\n";
[download]

outputs

31.4016380310059 seconds
hello!
[download]

and

14.1771370172501 seconds
hello!
[download]

when using "my" instead.

Comment on Re: is 'my' that much faster than 'local'? Select or Download Code

Replies are listed 'Best First'.
Re: Re: is 'my' that much faster than 'local'? by sierrathedog04 (Hermit) on Mar 27, 2001 at 03:05 UTC
Using twerq's original script, I got similar results on an ActiveState Perl/Win32 machine: my was more than twice as fast as local. However, I modified twerq's script so that each variable is only declared once. Here is the modified script, which measures time to access or change variables rather than to declare them: `# replace local with "my" local $start_time = time; local $global_variable = "hello!"; for (local $i = 0; $i < 5000000; $i++) { $global_variable = "redefined. . "; } print time - $start_time." seconds\n"; print "$global_variable\n";` [download] The program now takes 24 seconds when local is used and 18 when my is used. So it appears that local can introduce significant overhead into a program even when the program only declares variables a few times but accesses them over and over.	[reply] [d/l]
Re: is 'my' that much faster than 'local'? by Dominus (Parson) on Mar 27, 2001 at 21:32 UTC
Says sierrathedog04: So it appears that `local` can introduce significant overhead into a program even when the program only declares variables a few times but accesses them over and over. Sure. Global variables must be looked up in the symbol table. This is essentially a hash lookup. Lexical variables are retrieved from the scratchpad, which is an array. Arrays are always going to beat hashes for speed. But I still think my main point is correct: It doesn't matter which one is faster, because they do different things. You use `local` when it is appropriate, and `my` when you need `my`, not because it is faster. Comparing them for speed doesn't make sense. What would you think if someone asked you whether `open` was faster or slower than `getprotoent`? You can write all the benchmarks you want to compare `open` with `getprotoent`, and none of them are going to mean a thing.	[reply]