[Python-Dev] Numerical robustness, IEEE etc.

Nick Maclaren nmm1 at cus.cam.ac.uk
Fri Jun 23 14:50:36 CEST 2006

Michael Hudson <mwh at python.net> wrote:
> But, a floating point exception isn't a machine check interrupt, it's
> a program interrupt...

For reasons that I could go into, but are irrelevant, almost all
modern CPU architectures have one ONE interrupt mechanism, and use
it for both of those.  It is the job of the interrupt handler (i.e.
FLIH, first-level interrupt handler, usually in Assembler) to
classify those, get into the appropriate state and call the interrupt
handling code.

Now, this is a Bad Idea, but separating floating-point exceptions
from machine checks at the hardware level died with mainframes, as
far as I know.  The problem with the current approach is that it
makes it very hard for the operating system to allow the application
to handle the former.  And the problem with most modern operating
systems is that don't even do what they could do at all well, because
THAT died with the mainframes, too :-(

The impact of all this mess on things like Python is that exception
handling is a nightmare area, especially when you bring in threading
(i.e. hardware threading with multiple cores, or genuinely parallel
threading on a single core).  Yes, I have brought a system down by
causing too many floating-point exceptions in all threads of a
highly parallel program on a large SMP ....

> See, that wasn't so hard!  We'd have saved a lot of heat and light if
> you'd said that at the start (and if you think you'd made it clear
> already: you hadn't).

I thought I had.  I accept your statement that I hadn't.  Sorry.

Nick Maclaren,
University of Cambridge Computing Service,
New Museums Site, Pembroke Street, Cambridge CB2 3QH, England.
Email:  nmm1 at cam.ac.uk
Tel.:  +44 1223 334761    Fax:  +44 1223 334679

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