scikit-learn · thomasjpfan · Nov 13, 2020 · Nov 12, 2020 · Nov 12, 2020 · Nov 12, 2020
diff --git a/.github/workflows/wheels.yml b/.github/workflows/wheels.yml
@@ -83,6 +83,7 @@ jobs:
                             OMP_NUM_THREADS=2
                             OPENBLAS_NUM_THREADS=2
                             SKLEARN_SKIP_NETWORK_TESTS=1
+                            SKLEARN_BUILD_PARALLEL=3
           CIBW_BUILD: cp${{ matrix.python }}-${{ matrix.platform_id }}
           CIBW_TEST_REQUIRES: pytest pandas threadpoolctl
           # Test that there are no links to system libraries

diff --git a/build_tools/azure/install.sh b/build_tools/azure/install.sh
@@ -133,19 +133,19 @@ try:
 except ImportError:
     print('pandas not installed')
 "
-python -m pip list
+# Set parallelism to 3 to overlap IO bound tasks with CPU bound tasks on CI
+# workers with 2 cores when building the compiled extensions of scikit-learn.
+export SKLEARN_BUILD_PARALLEL=3
 
+python -m pip list
 if [[ "$DISTRIB" == "conda-pip-latest" ]]; then
-    # Check that pip can automatically install the build dependencies from
-    # pyproject.toml using an isolated build environment:
+    # Check that pip can automatically build scikit-learn with the build
+    # dependencies specified in pyproject.toml using an isolated build
+    # environment:
     pip install --verbose --editable .
 else
     # Use the pre-installed build dependencies and build directly in the
     # current environment.
-    # Use setup.py instead of `pip install -e .` to be able to pass the -j flag
-    # to speed-up the building multicore CI machines.
-    python setup.py build_ext --inplace -j 3
     python setup.py develop
 fi
-
 ccache -s
diff --git a/build_tools/circle/build_doc.sh b/build_tools/circle/build_doc.sh
@@ -183,8 +183,9 @@ source activate testenv
 pip install sphinx-gallery
 pip install numpydoc
 
-# Build and install scikit-learn in dev mode
-python setup.py build_ext --inplace -j 3
+# Set parallelism to 3 to overlap IO bound tasks with CPU bound tasks on CI
+# workers with 2 cores when building the compiled extensions of scikit-learn.
+export SKLEARN_BUILD_PARALLEL=3
 python setup.py develop
 
 export OMP_NUM_THREADS=1

diff --git a/build_tools/circle/build_test_pypy.sh b/build_tools/circle/build_test_pypy.sh
@@ -32,8 +32,11 @@ export CCACHE_COMPRESS=1
 export PATH=/usr/lib/ccache:$PATH
 export LOKY_MAX_CPU_COUNT="2"
 export OMP_NUM_THREADS="1"
+# Set parallelism to 3 to overlap IO bound tasks with CPU bound tasks on CI
+# workers with 2 cores when building the compiled extensions of scikit-learn.
+export SKLEARN_BUILD_PARALLEL=3
 
-python setup.py build_ext --inplace -j 3
+# Build and install scikit-learn in dev mode
 pip install --no-build-isolation -e .
 
 # Check that Python implementation is PyPy

diff --git a/doc/developers/advanced_installation.rst b/doc/developers/advanced_installation.rst
@@ -439,3 +439,17 @@ Before using ICC, you need to set up environment variables::
 Finally, you can build scikit-learn. For example on Linux x86_64::
 
     python setup.py build_ext --compiler=intelem -i build_clib --compiler=intelem
+
+Parallel builds
+===============
+
+It is possible to build scikit-learn compiled extensions in parallel by setting
+and environment variable as follows before calling the ``pip install`` or
+``python setup.py build_ext`` commands::
+
+    export SKLEARN_BUILD_PARALLEL=3
+    pip install --verbose --no-build-isolation --editable .
+
+On a machine with 2 CPU cores, it can be beneficial to use a parallelism level
+of 3 to overlap IO bound tasks (reading and writing files on disk) with CPU
+bound tasks (actually compiling).
diff --git a/setup.py b/setup.py
@@ -108,13 +108,27 @@ def run(self):
 
 cmdclass = {'clean': CleanCommand, 'sdist': sdist}
 
-# custom build_ext command to set OpenMP compile flags depending on os and
-# compiler
+# Custom build_ext command to set OpenMP compile flags depending on os and
+# compiler. Also makes it possible to set the parallelism level via
+# and environment variable (useful for the wheel building CI).
 # build_ext has to be imported after setuptools
 try:
     from numpy.distutils.command.build_ext import build_ext  # noqa
 
     class build_ext_subclass(build_ext):
+
+        def finalize_options(self):
+            super().finalize_options()
+            if self.parallel is None:
+                # Do not override self.parallel if already defined by
+                # command-line flag (--parallel or -j)
+
+                parallel = os.environ.get("SKLEARN_BUILD_PARALLEL")
+                if parallel:
+                    self.parallel = int(parallel)
+            if self.parallel:
+                print("setting parallel=%d " % self.parallel)
+
         def build_extensions(self):
             from sklearn._build_utils.openmp_helpers import get_openmp_flag