Feature request: -j100%

[Frank Heckenbach]

GNU parallel has a "-j" option quite like GNU make. But it also
allows to specify the number of jobs relative to the number of
CPU cores present:

  --jobs N
  -j N     Number of jobslots on each machine. Run up to N jobs in parallel.  0 
means as many as possible.
           Default is 100% which will run one job per CPU core on each machine.

In my experience, that's what I almost always do when running
parallel jobs (either 100%, or even something like 150% or 200% for
I/O intensive and not very memory hungry jobs).

Of couse, with make I can compute the number of cores myself (e.g.
in a wrapper script by reading /proc/cpuinfo), but that's not very
portable and rather cumbersome to do in various places, so it would
seem useful for make to support this in a centralized location and
more portable than anyone's individual solution.

The implementation is hopefully not too difficult. Under Linux you
can use "sysconf (_SC_NPROCESSORS_ONLN)", and other systems probably
have similar methods. The implemention of GNU parallel can serve as
an example. In the worst case, it can always default to 1.

Regards,
Frank