>> Respectfully, the assertion that it is "wrong" to use characteristic
>> of the hardware such as number of CPU's (and cache sizes, etc.) to
>> tune their compilation strikes me as naive. Sure, it would be nice
>> in theory if there existed some magic way to write code that could run
>> optimally on all possible numbers of CPU's (processor flags, cache
>> sizes, etc.), but I think that's basically impossible with present
>> technology for nontrivial algorithms.
>>
>> -- William
>
> Some things like cache sizes, I would agree are probably non-trivial get to at
> run time,
>
> But the number of processors should be trivial to determine at run-time.
> Wolfram
> Research do it in Mathematica. Here's their code for Linux, Solaris and HP-UX.
>
>
> if [ "${SystemID}" = "Linux" -o "${SystemID}" = "Linux-x86-64" -o
> "${SystemID}" = "Linux-IA64" ]; then
> if [ -z "${OMP_NUM_THREADS}" ]; then
> OMP_NUM_THREADS=`cat /proc/cpuinfo | grep processor | wc -l | tr
> -d
> ' '`;
> export OMP_NUM_THREADS;
> fi
> fi
> if [ "${SystemID}" = "Solaris-SPARC" -o "${SystemID}" = "Solaris-x86-64"
> ];
> then
> if [ -z "${OMP_NUM_THREADS}" ]; then
> OMP_NUM_THREADS=`/usr/sbin/psrinfo | wc -l | tr -d ' '`;
> export OMP_NUM_THREADS;
> fi
> fi
> if [ "${SystemID}" = "HPUX-PA64" -o "${SystemID}" = "HP-RISC" ]; then
> if [ -z "${MLIB_NUMBER_OF_THREADS}" ]; then
> MLIB_NUMBER_OF_THREADS=`/usr/sbin/ioscan -k -C processor | grep
> processor | wc -l`;
> export MLIB_NUMBER_OF_THREADS;
> fi
> fi
> One might say their solution is not optimal, given the multi-core CPUs in
> existence. But it does seem a bit silly determining such things at compile
> time,
> if, as that person on the HP-UX newsgroup said, this could be done at runtime
> using Open MP, which libm4ri uses anyway.
>
> Given that:
>
> 1) The autoconf developers say that those two macros (one for number of CPUs,
> the other for cache size) in the autoconf achieve are are badly written.
>
> 2) The autoconf developers say the macros will only work for a subset of x86
> CPUs.
>
> 3) Neither macro works on Solaris.
>
> 4) Neither macro works on HP-UX
>
> 5) The macro for determining cache size crashes on HP-UX
>
> There must surely be a better way.
>
> I would have thought that the performance impact for assuming too many CPUs
> would be smaller than the performance impact for underestimating the number of
> CPUs. If you have quad processors and run assuming only one, their could be
> upto
> a factor of 4 peformance degradation. I suspect assuming 4 CPUs, with only 1
> present would in general for multi-threaded code, have a far smaller impact.
>
> I was going to look at fixing those autoconf macros, but from comments on both
> the HP-UX newsgroup and the autoconf mailing list, have convinced me that not
> only is their implementation poor, but the use of them is suspect.
>
Well irregardless of anything we're discussing, you're right that we
definitely need to find a fix, and anything you can do toward that
will be appreciated!
William
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