Re: What use is the Schwartzian Transform?

[mjd-perl-pm@plover.com]

> I got a much larger differential than 5%; see below.

Something is obviously wrong with your benchmarks.  To start with,
they claim that on your computer, Perl can sort 9952 items in 3.85
microseconds flat.  I find that impossible to believe.

Moreover:

> 	'CODE A' => '@sorted = sort { -M $b <=> -M $a } @filenames;',

This code is supposed to be sorting a list of 9952 filenames.  You
can't do that with fewer than 9951 calls to the comparator, and
therefore with fewer than 19,902 -M operations.  For a typical set of
random data, the comparator is called 120214 times (put '$count++'
into the comparator to see this) therefore making 240428 -M
operations.

Now, according to your benchmark, 

>     CODE A: 39 wallclock secs 
>         (38.50 usr +  0.00 sys = 38.50 CPU) @ 259740.26/s (n000000)


the system has taken 0.00 seconds to perform these 19,902 -M's, or
maybe it is more like 240428 -M's.

That's bullshit.

> Note further that, as you predicted, Code D was fractionally faster than 
> Code B.  However, contrary to your prediction, Code B was more than 
> twice as fast as Code C -- not just 5% faster.

No credit or blame attaches to me for predicting or failing to predict
the results of your absurd benchmarks.

Here's the code I used; the array had 125 entries.

        $IT = shift || 1000;
        opendir D, "/tmp" or die;
        @files = map "/tmp/$_", readdir D;
        closedir D;

        print "NULL LOOP\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          1;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;

        print "ST\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          my @sorted = map {$_->[0]} sort { $b->[1] <=> $a->[1] }
            map { [$_, -M $_] } @files;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;

        print "ST (noblock)\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          my @sorted = map $_->[0], sort { $b->[1] <=> $a->[1] }
            map [$_, -M $_], @files;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;

        print "ST (hash)\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          my @sorted = map {$_->{name}} sort { $b->{date} <=> $a->{date} }
            map { {name => $_, date => -M $_} } @files;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;

        print "Aux. hash\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          my %date;
          $date{$_} = -M $_ for @files;
          my @sorted = sort {$date{$b} <=> $date{$a}} @files;
          undef %date;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;

        print "Naive\n";
        my ($u1, $s1) = times();
        my $t1 = $u1 + $s1;
        for (1..$IT) {
          my @sorted = sort {-M $b <=> -M $a} @files;
        }
        my ($u2, $s2) = times();
        my $t2 = $u2 + $s2;
        my ($ue, $se, $te) = ($u2-$u1, $s2-$s1, $t2-$t1);
        printf "Elapsed: %6.2fu %6.2fs %6.2ftotal\n", $ue, $se, $te;



Here's a typical output:

        NULL LOOP
        Elapsed:   0.00u   0.00s   0.00total
        ST
        Elapsed:   8.93u   1.54s  10.47total
        ST (noblock)
        Elapsed:   9.08u   1.34s  10.42total
        ST (hash)
        Elapsed:  11.14u   2.01s  13.15total
        Aux. hash
        Elapsed:   9.77u   1.80s  11.57total
        Naive
        Elapsed:   7.19u  12.88s  20.07total
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