Re: Consistency problem on IPF

[David Mosberger <davidm@napali.hpl.hp.com>]

>>>>> On Mon, 12 Jul 2004 14:01:15 +0200, Marc Gonzalez-Sigler <marc.gonzalez-sigler@inria.fr> said:

  Marc> (For the record, I tried gcc-3.3.4 and orcc-2.1)

For floating-point stuff, you'll definitely want to try the Intel compiler.
It's a free download for the unsupported/non-commercial version.

As others have pointed out, for matrix multiply, you'll definitely
want to use a hand-tuned version as is available in several math
libraries (yeah, I realize you are not really after matrix multiply).

  Marc> Perhaps I was not clear enough. I used matrix-matrix multiply
  Marc> only as an example. My real problem is the non-deterministic
  Marc> behavior.

Page-coloring can make things more deterministic, at the expense of making
_everything_ go slower.  If you search the net, you should be able to
find a page-coloring module.  We have experimented with it in the past
and it did its job, but it's overall impact was to slow things down, so
it's not a great solution.

The other thing you could do on Linux is use huge pages.  That will
mitigate/eliminate the effect of page coloring (and also reduce TLB
pressure).

  Marc> Say I tile the main loop nest. I want to compare the execution
  Marc> time of the original, untiled program and the execution time
  Marc> of the modified, tiled program.

  Marc> If the original version completes in 1 second 80% of the time,
  Marc> and the modified version completes in 0.5 seconds 80% of the
  Marc> time, but 2 seconds 20% of the time, then, if I am unlucky, I
  Marc> might eliminate an excellent candidate. This is why I need the
  Marc> execution times of a given program to be consistent.

I think you have to allow for the fact that modern CPUs (and OSes) do
not really offer deterministic performance.  And don't expect things
to get better.  Dynamic power throttling, multi-threading, etc., will
make performance analysis very "interesting".

	--david