parallelizing ghc

[Evan Laforge]

> I'm slightly surprised by this - in my experience parallel builds beat
> --make as long as the parallelism is a factor of 2 or more.  Is your
> dependency graph very narrow, or do you have lots of very small modules?

I get full parallelism, 4 threads at once on a 2 core machine * 2
hyperthread/whatever core i5, and SSD.  Maybe I should try with just 2
threads.  I only ever get 200% CPU at most, so it seems like the
hyperthreads are not really much like a whole core.

The modules are usually around 150-250 lines.  Here are the timings
for an older run:

    from scratch (191 modules):
    runghc Shake/Shakefile.hs build/debug/seq  128.43s user 20.04s system 178%
cpu 1:23.01 total
    no link: runghc Shake/Shakefile.hs build/debug/seq  118.92s user 19.21s sys
tem 249% cpu 55.383 total
    make -j3 build/seq  68.81s user 9.98s system 98% cpu 1:19.60 total

    modify nothing:
    runghc Shake/Shakefile.hs build/debug/seq  0.65s user 0.10s system 96% cpu
0.780 total
    make -j3 build/seq  6.05s user 1.21s system 85% cpu 8.492 total

    modify one file:
    runghc Shake/Shakefile.hs build/debug/seq  19.50s user 2.37s system 94% cpu
 23.166 total
    make -j3 build/seq  12.81s user 1.85s system 94% cpu 15.586 total