Re: Please no science on an Intel Mac

[Valdis.Kletnieks () vt edu]

On Thu, 20 Sep 2007 16:49:47 EDT, rms () computerbytesman com said:
> The same problem should exist on a regular PC running on a Core 2
> processor with XP.  XP running natively on an Intel Mac really shouldn't
> be any different than XP running on regular PC hardware.

This would, of course, require that the Intel Mac BIOS (or what passes for
one) initialize the system the same way a regular PC BIOS does, and/or that
XP guarantees the floating-point hardware is set to a specific known state
when a process is started, and/or that SPSS is careful to reset the state
itself because it doesn't trust the BIOS or operating system.

Phrased differently - there's lots of ways to screw up setting things like
rounding modes and the like.

Favorite bug I've seen - when I was still at my previous university, one of the
professors had a huge numerical simulation that he would test-run on our IBM
4341, and then run on the big 3090-600J's down at the Cornell Theory Center.
(Particularly ugly math - it was basically solving a multiple integral of a
multiply-discontinuous function, between endpoints which were *themselves* the
values of integrals of discontinuous functions).

He ran into one test case that would crash if run during the day, but would run
just fine if run overnight.  After a *LOT* of time on his part, and the
Theory Center's part, and IBM engineers part, they found it:

When run at night, his code would get onto the vector processors in the 3090
and get *lots* of iterations done between the relatively infrequent I/O
interrupts that would happen at night, so he was *effectively* getting about
132 bits of precision (128 for the official plus 4 for a guard digit in the
hardware).  However, during the day, interrupts happened a lot more because
people were logged on and editing and compiling and the like, and when the
operating system did save/restore of the vector registers, they only saved 128
bits.  And the function was numerically unstable enough that it needed 130 or
so bits....

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