(super!) linear slowdown of parallel builds on 40 core machine

I'm seeing slowdowns in parallel builds of a (simple!!) 6 module project when I build it on a 40 core server that I'm using for work. For any given ghc invocation with -jn, once n>10, I start to see a super-linear slow down as a function of n,

here are some basic numbers,

	compile time
-j1	0m2.693s
-j4	0m2.507s
-j10	0m2.763s
-j25	0m12.634s
-j30	0m39.154s
-j40	0m57.511s
-j60	2m21.821s

These timings are another 2-4x worse if ghc is invoked indirectly via cabal-install / Setup.hs

According to the linux utility latencytop, 100% of ghc's cpu time was spent on user-space lock contention when I did the -j40 invocation.

The timing in the -j40 case stayed the same even when ghc was also passed -O0 (and -fforce-recomp to ensure it did the same )

A bit of experimentation makes me believe that *any* cabalized project on a 40 core machine will exhibit this performance issue.

cabal clean
cabal configure --ghc-options="-j"
cabal build -j1

should be enough to trigger the lock contention.

That said, I'll try to cook up a minimal repro that i can share the source for post haste.