Re: number of processors

[Philip Willoughby]

Today, Philipp Gortan wrote:

>case "$MACHTYPE" in
>~     i686-*-linux)    test -f /proc/cpuinfo && \
>                       PROC_NR=`grep processor /proc/cpuinfo | \
>                       wc -l | awk '{print $1}'`
>       ;;

This should work for *-*-linux* not just i686s.

>the script works fine on all machines, but i'm not especially satisfied,
>as for each new machine added, the script has to be manually modified.

All recent POSIX platforms should respond to

sysconf(_SC_NPROCESSORS_CONF)

although

sysconf(_SC_NPROCESSORS_ONLN)

is better in case you're running on a box with some processors offline e.g.
a mid-upgrade sun E10K or a S/390 or somesuch.

I'd suggest the following C code be used:

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
  long nprocs;
  nprocs = sysconf(_SC_NPROCESSORS_ONLN);
  if (nprocs < 1)
    nprocs = 1;
  printf ("%ld\n",nprocs);
  exit (EXIT_SUCCESS);
}

If you just run this it will print out the number of cpus if available or 1
if not. If it doesn't compile, assume 1.

It'd be nice if your macro took the load average into account -- flooring
the 1-minute load average and subtracting that from the number of
processors, and setting the result to 1 if it's less than one would be
good.

Even better -- If you're using GNU make, make -j -l <NUM_CPUs> is the best
thing to do, but I dunno if this is portable.

HTH

Regards,

Philip Willoughby

PS the above code works on Solaris 8 and on Linux 2.4.x, tested.

Systems Programmer, Department of Computing, Imperial College, London, UK
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