Re^5: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" (7000x faster)

Curiouser and curiouser!

When I run the code under 5.10, I get the sort of timings I posted above:

C:\test>1176081 -WIDTH=1000 -HEIGHT=1000
yr() took 330.252300
buk() took 1.584859
buk2() took 1.277809
buk3() took 0.226701
[download]

But if I run it under 5.22, your sub runs very much faster* and buk3() doesn't terminate at all:

which is weird and indicates (IMO) a bug in the later versions

(*I have an idea about the cause of the slowness in 5.10; I'll need to think of a way to verify it.)

Modifying buk3() along the lines of your modification, but taking an lvalue ref outside the while loop and using it within the loop, allows it to work again:

sub buk3{
    my $str = shift;
    my @b = map[ ( 1e99, 0 ) x 2 ], 1 .. 256;

    for my $y ( 0 .. $HEIGHT-1 ) {
        my $ref = \substr( $$str, $y * $WIDTH, $WIDTH );
        while( $$ref  =~ m[((.)\2*)]sg ) {
            my $c = ord($1); 
            $b[ $c ][ LEFT   ] = $-[0]   if  $-[0]    < $b[ $c ][ LEFT
+   ];
            $b[ $c ][ RIGHT  ] = $+[0]-1 if  $+[0]-1  > $b[ $c ][ RIGH
+T  ];
            $b[ $c ][ TOP    ] = $y      if        $y < $b[ $c ][ TOP 
+   ];
            $b[ $c ][ BOTTOM ] = $y      if        $y > $b[ $c ][ BOTT
+OM ];
        }
    }
    return \@b;
}

C:\test>\perl22\bin\perl 1176081.pl -WIDTH=1000 -HEIGHT=1000
yr() took 2.438320
buk() took 1.068071
buk2() took 0.681810
buk3() took 0.160666
[download]

Which is okay, but a strange difference.

It kinda takes the steam out of my amazing speedup figures -- 15x instead of 7000x -- but the thrill is transitory anyway :)

And now I can run your sub on a larger image, even that gain is far less:

C:\test>\perl22\bin\perl 1176081.pl -WIDTH=10000 -HEIGHT=10000
yr() took 12.714030
buk() took 110.152981
buk2() took 70.100982
buk3() took 8.158657
[download]

With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.

"Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity.

In the absence of evidence, opinion is indistinguishable from prejudice.

Comment on Re^5: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" (7000x faster) Select or Download Code

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Re^6: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" (7000x faster) by dbuckhal (Chaplain) on Nov 22, 2016 at 03:51 UTC
(*I have an idea about the cause of the slowness in 5.10; I'll need to think of a way to verify it.) I came across this article a little while back. It might help explain the speed difference. I certain have seen a difference running a Perl application between 5.1x and 5.22. How We Spent Two Days Making Perl Faster Hope you find it interesting!	[reply]
Re^7: PDL: Looking for efficient way to extract sub-images, by finding bounding boxes of "objects" (7000x faster) by BrowserUk (Patriarch) on Nov 22, 2016 at 08:39 UTC
If this difference in performance between versions was due to optimisations of the internals between those versions, then: Wouldn't you expect it to apply pretty evenly to all the subroutines rather than just one of them? If they'd succeeded in making Perl run 135x faster, I'm pretty sure we'd all have heard about it. The idea I had about a possible cause of the pathological behaviour of yr() code is my memory of this Windows memory allocation bug What could cause excessive page faults? that I discovered 7 years ago that, under some very specific circumstances, caused the CRT code underlying Perl to generate millions of page faults. But that is not the cause here. With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". The enemy of (IT) success is complexity. In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]