Dask options for Iris processing (#2511)

Dask opts controlled by simple function

lib/iris/_lazy_data.py

@@ -23,11 +23,37 @@
 from __future__ import (absolute_import, division, print_function)
 from six.moves import (filter, input, map, range, zip)  # noqa
 
+import dask
 import dask.array as da
+import dask.context
 import numpy as np
 import numpy.ma as ma
 
 
+def _iris_dask_defaults():
+    """
+    Set dask defaults for Iris. The current default dask operation mode for
+    Iris is running single-threaded using `dask.async.get_sync`. This default
+    ensures that running Iris under "normal" conditions will not use up all
+    available computational resource.
+
+    Otherwise, by default, `dask` will use a multi-threaded scheduler that uses
+    all available CPUs.
+
+    .. note::
+        We only want Iris to set dask options in the case where doing so will
+        not change user-specified options that have already been set.
+
+    """
+    if 'pool' not in dask.context._globals and \
+            'get' not in dask.context._globals:
+        dask.set_options(get=dask.async.get_sync)
+
+
+# Run this at import time to set dask options for Iris.
+_iris_dask_defaults()
+
+
 def is_lazy_data(data):
     """
     Return whether the argument is an Iris 'lazy' data array.

lib/iris/tests/unit/lazy_data/test_iris_dask_defaults.py

@@ -0,0 +1,63 @@
+# (C) British Crown Copyright 2017, Met Office
+#
+# This file is part of Iris.
+#
+# Iris is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Lesser General Public License as published by the
+# Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# Iris is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with Iris.  If not, see <http://www.gnu.org/licenses/>.
+"""
+Test :func:`iris._lazy data._iris_dask_defaults` function.
+
+"""
+
+from __future__ import (absolute_import, division, print_function)
+from six.moves import (filter, input, map, range, zip)  # noqa
+
+# Import iris.tests first so that some things can be initialised before
+# importing anything else.
+import iris.tests as tests
+
+import dask.context
+from iris._lazy_data import _iris_dask_defaults
+
+
+class Test__iris_dask_defaults(tests.IrisTest):
+    def setUp(self):
+        set_options = 'dask.set_options'
+        self.patch_set_options = self.patch(set_options)
+        get_sync = 'dask.async.get_sync'
+        self.patch_get_sync = self.patch(get_sync)
+
+    def test_no_user_options(self):
+        self.patch('dask.context._globals', {})
+        _iris_dask_defaults()
+        self.patch_set_options.assert_called_once_with(get=self.patch_get_sync)
+
+    def test_user_options__pool(self):
+        self.patch('dask.context._globals', {'pool': 5})
+        _iris_dask_defaults()
+        self.assertEqual(self.patch_set_options.call_count, 0)
+
+    def test_user_options__get(self):
+        self.patch('dask.context._globals', {'get': 'threaded'})
+        _iris_dask_defaults()
+        self.assertEqual(self.patch_set_options.call_count, 0)
+
+    def test_user_options__wibble(self):
+        # Test a user-specified dask option that does not affect Iris.
+        self.patch('dask.context._globals', {'wibble': 'foo'})
+        _iris_dask_defaults()
+        self.patch_set_options.assert_called_once_with(get=self.patch_get_sync)
+
+
+if __name__ == '__main__':
+    tests.main()