python3Packages.dask: limit processes on tests

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

@@ -32,8 +32,6 @@ buildPythonPackage rec {
     pytest_xdist # takes >10mins to run single-threaded
   ];
 
-  pytestFlagsArray = [ "-n $NIX_BUILD_CORES" ];
-
   dontUseSetuptoolsCheck = true;
 
   propagatedBuildInputs = [
@@ -56,6 +54,13 @@ buildPythonPackage rec {
       --replace "cmdclass=versioneer.get_cmdclass()," ""
   '';
 
+  # dask test suite with consistently fail when using high core counts
+  preCheck = ''
+    NIX_BUILD_CORES=$((NIX_BUILD_CORES > 8 ? 8 : NIX_BUILD_CORES))
+  '';
+
+  pytestFlagsArray = [ "-n $NIX_BUILD_CORES" ];
+
   disabledTests = [
     "test_argwhere_str"
     "test_count_nonzero_str"