python3Packages.cupy: parallel build

pkgs/development/python-modules/cupy/default.nix

@@ -19,8 +19,15 @@ buildPythonPackage rec {
     sha256 = "sha256-5ovvA76QGOsOnVztMfDgLerks5nJrKR08rLc+ArmWA8=";
   };
 
+  # See https://docs.cupy.dev/en/v10.2.0/reference/environment.html. Seting both
+  # CUPY_NUM_BUILD_JOBS and CUPY_NUM_NVCC_THREADS to NIX_BUILD_CORES results in
+  # a small amount of thrashing but it turns out there are a large number of
+  # very short builds and a few extremely long ones, so setting both ends up
+  # working nicely in practice.
   preConfigure = ''
     export CUDA_PATH=${cudatoolkit}
+    export CUPY_NUM_BUILD_JOBS="$NIX_BUILD_CORES"
+    export CUPY_NUM_NVCC_THREADS="$NIX_BUILD_CORES"
   '';
 
   nativeBuildInputs = [