[Nix-dev] make -jx within builds?

[Marc Weber]

>   buildInParallel = makeSetupHook ...
>     ''
>       nrCores=$(cat /proc/cpuinfo | grep 'processor.*:' | wc -l)
>       makeFlags="$makeFlags -j -l $nrCores"

-j -l ? Agreed. This is superior.

However I don't know whether we want to depend on Makefile. I can't
think about other systems which do concurrent builds in nixpkgs right
now though.

We still have to find a way to mark makefiles as concurrency unsafe so
that -j1 is used on them.

Probably -j -l does not work nicely with

buildImportant -j4 &
nice -n10 buildLessImportant -j4 &
nice -n20 buildLeastImportart -j4 &
wait

because less and least important would run one thread causing
buildImportant to spawn less threads.

So maybe we want to use  $( .. |wc -l) * 0.9 for less important jobs
such as stdenv-updates so that it spawns less processes :)

Marc Weber