[ale] tech "magic"

[Michael H. Warfield]

On Wed, 2009-05-27 at 09:18 -0400, Geoffrey wrote:
> Rev. Johnny Healey wrote:
> > There's a great wikipedia article on unusual software bugs:
> > 
> > http://en.wikipedia.org/wiki/Unusual_software_bug
> > 
> > I actually ran into a schroedinbug in an apache config file today.
> 
> I actually identified a bug in an old c compiler on SVR3 years ago that 
> would be defined as a heisenbug.  A piece of code was dropping core at a 
> particular point.  I put in a printf and a comment above it, recompiled 
> and it ran fine.  Remove the printf and comment, drops core.  We finally 
> figured out that we could remove the printf and leave the comment in the 
> code and the code would run.  Remove the comment and it would drop core. 
>   We eventually simply placed a comment that said something like:

> /* don't remove this comment, the code will drop core with out it */

	Sounds like an optimizer problem.  Use to run into those all quite
often at high optimizer settings.  Sometimes entire loops would get
optimized into non-existence and volatile variables were a laugh,
especially in system/kernel code.  I forget what compiler it was but I
remember reading the documentation and the optimizer when from 0 (no
optimization) through several levels that included in-line optimization
and unrolling loops and speed vs size.  The highest level of
optimization was described in the documentation as "Dearly beloved we
are gathered here...".

	Mike

> > -Johnny
> > 
> > On Tue, May 26, 2009 at 10:07 PM, Jim Kinney <jim.kinney at gmail.com> wrote:
> >> Everyone here has had it happen. Something fails and you get called to
> >> "take a look at it". You get there. take it and it isn't working. So
> >> you methodically take it apart, inspecting everything alonmg the way
> >> looking for failure points and find
> >>
> >>
> >> nothing
> >>
> >>
> >>
> >> it all looks fine
> >>
> >>
> >>
> >> So you start testing a few thing as you put it back together.
> >>
> >>
> >>
> >> all the parts work correctly even though 20 minutes ago they were all
> >> totally dead
> >>
> >>
> >>
> >> Finally you plug in the reassembled device and it comes up normally
> >> and works perfectly.
> >>
> >>
> >>
> >> The 3Com switch I noted last Thursday that likely had a dead power
> >> supply just ran me through that exact process. It works fine now.
> >> Power supply puts out a rock solid 14V (no load) and 12.01V at 10A
> >> load. All the fans work just fine.
> >>
> >> At least I don't have to explain "what I fixed" in an invoice...
> >>
> >> --
> >> --
> >> James P. Kinney III
> >> Actively in pursuit of Life, Liberty and Happiness
> >> _______________________________________________
> >> Ale mailing list
> >> Ale at ale.org
> >> http://mail.ale.org/mailman/listinfo/ale
> >>
> > _______________________________________________
> > Ale mailing list
> > Ale at ale.org
> > http://mail.ale.org/mailman/listinfo/ale
> > 
> 

-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
   /\/\|=mhw=|\/\/          | (678) 463-0932 |  http://www.wittsend.com/mhw/
   NIC whois: MHW9          | An optimist believes we live in the best of all
 PGP Key: 0xDF1DD471        | possible worlds.  A pessimist is sure of it!

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