RE: 3D versus 2D Indexing and the Speed Thereof

[Luis Ortiz]

JWE:

Just out of curiousity, what system/compiler were you using?
I was on an EM64T Opteron 8xx using GCC 3.3.5 .  I suspect it
of being worse than usual because of the horrid inlining behaviour that
GCC had back then.

LFO


-----Original Message-----
From: John W. Eaton [mailto:address@hidden
Sent: Tue 11/21/2006 3:16 AM
To: Luis Ortiz
Cc: address@hidden
Subject: 3D versus 2D Indexing and the Speed Thereof
 
BTW, on my system, I see the following results.  Not great, but not a
factor of 22 either.

  octave:4> for i = 1:3
  >   x = zeros (n, 50, 100);
  >   t = cputime ();
  >   for i = 1:100
  >     z = x(:,:,i);
  >   end
  >   t1 = cputime () - t;
  >   x = zeros (50*n, 100);
  >   t = cputime ();
  >   for i = 1:100
  >     z = x(:,i);
  >   end
  >   t2 = cputime () - t;
  >   fprintf ("%6d %f %f %f\n", n, t1, t2, t1/t2);
  >   n = n * 10;
  > end
      10 0.068005 0.012000 5.667083
     100 0.604037 0.100006 6.040008
    1000 5.996376 1.036065 5.787645

OTOH, if Octave's Array class knew how to do slices and if we
recognized these operations as slices, then the time should be
constant here, not proportional to the number of elements copied.

jwe