On Jun 27, 2010, at 8:57 AM, Lars Petter Mostad wrote:
I have been experimenting with nlopt to solve a simple(order 4)
constrained least squares problem using COBYLA. Running my program
on both a 32-bit Fedora 7 and a 64-bit Fedora 12 works fine, but
when I when i try to run it on the embedded STLinux(http://www.stlinux.com/
) distribution, the program prints "Floating point exception"(not a C
++ exception) and quits if i use more that about 2000 iterations.
By adding printouts at the entry and exit of my objective and
constraint functions, I can see that the exception doesn't happen
in my code. I don't know GDB, so, unfortunately, I can't be any more
specific.
I have tried both nlopt 1.2 and 2.0.2, and I get the same thing
happening. Do you have any idea about what the problem can be?
The fact that you are running COBYLA for more than 2000 iterations
with only 4 unknowns probably indicates that you haven't set an
appropriate stopping criterion....I doubt it takes that many
iterations to converge. I'm guessing that, even on the non-embedded
systems where it "works" it is returning the NLOPT_ROUNDOFF_LIMITED
error code to indicate that progress has stopped because of roundoff
errors. (Roundoff limitations mean that eventually COBYLA will end up
with a simplex so small that roundoff errors make it singular.)
Probably, you need to set larger tolerances for stopping critierion so
that it doesn't try to converge better than what machine precision
allows.
It may be that your embedded system is setting the floating-point unit
in a different mode so that it crashes on divide by zero or something
like that instead of giving infinity (NLopt checks for the infinities
creeping in and halts with NLOPT_ROUNDOFF_LIMITED if they are detected).
You can try forcing any fp exceptions to kill your program on the non-
embedded systems, too, by adding the following to the beginning of
your program:
#include <fenv.h>
then (in main):
feenableexcept(FE_DIVBYZERO | FE_UNDERFLOW | FE_OVERFLOW | FE_INVALID);
To get more information, you will have to configure NLopt with --
enable-debug, compile your progam with the -g flag, and run it under
gdb. To run it under gdb, do "gdb myprogram" then, at the gdb prompt,
do "run ....any arguments to myprogram....". Then gdb should give you
a prompt if it crashes. At the prompt, you can type "where" to get a
stack trace (tell where in the program it is) and "list" to list those
lines of code, and "print ...variable..." to print the value of a
variable.
Steven
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