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Re^2: Why does Perl get slower when building a larger hash? (Not due to the memory swapping)

by chialingh (Initiate)
on Mar 01, 2013 at 16:21 UTC ( #1021292=note: print w/ replies, xml ) Need Help??


in reply to Re: Why does Perl get slower when building a larger hash? (Not due to the memory swapping)
in thread Why does Perl get slower when building a larger hash? (Not due to the memory swapping)

Yes, what I did in the code is measuring the time taken to compare every file with every other file. However, the time log I showed is the running time divided by the number of file pairs, i.e. $time/$x

In that case, the time is the unit time for each comparison and it shouldn't go up with the number of files, right?

Sorry for the misleading code, I'll revise it. :)


Comment on Re^2: Why does Perl get slower when building a larger hash? (Not due to the memory swapping)
Re^3: Why does Perl get slower when building a larger hash? (Not due to the memory swapping)
by tmharish (Friar) on Mar 04, 2013 at 08:56 UTC

    Dividing by $x ( number of files ) does not help cause you still have a O( log(N) ) in there ( the inner loop )

    As a demonstration, consider the following code that benchmarks hash access times:

    use strict ; use warnings ; use Time::HiRes qw( time ) ; my @bench_mark_points = ( 1_000, 10_000, 100_000, 1_000_000, 10_000_00 +0, 20_000_000, 30_000_000, 40_000_000, 50_000_000 ) ; my $bench_mark_point = 0 ; my %large_hash ; my @keys ; for( my $elem = 0 ; $elem < 50_000_001; $elem++ ) { my $key = rand( $elem ) . "hello" . rand( $elem ); $large_hash{ $key } = 1 ; push @keys, $key ; if( $elem == $bench_mark_points[ $bench_mark_point ] ) { _bench_mark( $bench_mark_points[ $bench_mark_point ] ) ; $bench_mark_point++ ; } } sub _bench_mark { my $benchmark_point = shift ; my @benchmark_keys = map( { $keys[ int( rand( $benchmark_point ) +) ] } ( 0 .. 1_000_000 ) ) ; my $total_time = 0 ; foreach my $key ( @benchmark_keys ) { my $start_time = time; my $val = $large_hash{ $key } ; my $end_time = time ; $total_time += ( $end_time - $start_time ) ; } print "Benchmarked Hash access of size $benchmark_point \t -> Acce +ss time for 1_000_000 keys: " . $total_time . "\n"; return ; }

    Output

    Benchmarked Hash access of size 1000    	 -> Access time for 1_000_000 keys: 0.11689305305481
    Benchmarked Hash access of size 10000   	 -> Access time for 1_000_000 keys: 0.121062278747559
    Benchmarked Hash access of size 100000   	 -> Access time for 1_000_000 keys: 0.125393152236938
    Benchmarked Hash access of size 1000000 	 -> Access time for 1_000_000 keys: 0.116819381713867
    Benchmarked Hash access of size 10000000 	 -> Access time for 1_000_000 keys: 0.118601083755493
    Benchmarked Hash access of size 20000000 	 -> Access time for 1_000_000 keys: 0.117170572280884
    

    You will notice that the time to access a million keys is nearly always the same - The memory on the other hand shoots up dramatically reaching 7.8 GB at around 25 Million entries ( which is why my bench-marking stops there )

    Perl is working really hard in the background to ensure that it builds structures that can ensure that finding an element ( which should be O(N) ) is closing constant time here.

    UPDATE: Here is the output again with the CPU throttled so the difference can actually be seen:

    Benchmarked Hash access of size 1000    	 -> Access time for 1_000_000 keys: 0.438439846038818
    Benchmarked Hash access of size 10000   	 -> Access time for 1_000_000 keys: 0.467595815658569
    Benchmarked Hash access of size 100000  	 -> Access time for 1_000_000 keys: 0.615071773529053
    Benchmarked Hash access of size 1000000 	 -> Access time for 1_000_000 keys: 0.804181814193726
    Benchmarked Hash access of size 10000000 	 -> Access time for 1_000_000 keys: 0.873048782348633
    Benchmarked Hash access of size 20000000 	 -> Access time for 1_000_000 keys: 0.910530567169189
    

    So the time does go up - and thats expected. You cannot find an element in less than O( logN ); The speed here is because Perl optimizes this at a low level, but eventually the effect adds up.

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