find_compilers has been parallelized using ThreadPool

There should be no issues with the GIL, as the functions that are
invoked in parallel spawn a subprocess to start a system call and
then wait for I/O.

lib/spack/llnl/util/multiproc.py

@@ -36,18 +36,20 @@ def invoke(f):
     return f()
 
 
-def execute(command_list, executor=map):
+def execute(command_list, map_fn=map, transformation=invoke):
     """Execute a list of packaged commands and return their result.
 
     Args:
         command_list: list of commands to be executed
-        executor: object that execute each command. Must have the
+        map_fn: object that execute each command. Must have the
-            same semantic as ``map``.
+            same semantic as ``map``
+        transformation: callable invoked on each item to construct
+            the output list
 
     Returns:
         List of results
     """
-    return executor(invoke, command_list)
+    return map_fn(transformation, command_list)
 
 
 class Barrier:

lib/spack/spack/architecture.py

@@ -60,7 +60,6 @@
 import inspect
 import itertools
 
-import llnl.util.multiproc
 import llnl.util.tty as tty
 from llnl.util.lang import memoized, list_modules, key_ordering
 
@@ -255,17 +254,6 @@ def search_compiler_commands(self, *path_hints):
             )
         return commands
 
-    def find_compilers(self, *path_hints):
-        """
-        Return a list of compilers found in the supplied paths.
-        This invokes the find() method for each Compiler class,
-        and appends the compilers detected to a list.
-        """
-        commands = self.search_compiler_commands(*path_hints)
-        compilers = llnl.util.multiproc.execute(commands)
-        compilers = spack.compiler.discard_invalid(compilers)
-        return spack.compiler.make_compiler_list(compilers)
-
     def to_dict(self):
         return {
             'name': self.name,

lib/spack/spack/compiler.py

@@ -375,7 +375,8 @@ def detect_version_command(
         return None
 
 
-def discard_invalid(compilers):
+def _discard_invalid(compilers):
+    """Removes invalid compilers from the list"""
     # Remove search with no results
     compilers = filter(None, compilers)
 
@@ -389,6 +390,8 @@ def has_known_version(compiler_entry):
 
 
 def make_compiler_list(compilers):
+    compilers = _discard_invalid(compilers)
+
     # Group by (os, compiler type, version), (prefix, suffix), language
     def sort_key_fn(item):
         key, _ = item
@@ -396,6 +399,10 @@ def sort_key_fn(item):
                (key.prefix, key.suffix), key.language
 
     compilers_s = sorted(compilers, key=sort_key_fn)
+    # This dictionary is needed because a class (NOT an instance of it)
+    # doesn't have __lt__ or other similar functions defined. Therefore
+    # we sort on its string representation and need to maintain the map
+    # to the class here
     cmp_cls_d = {str(key.cmp_cls): key.cmp_cls for key, _ in compilers_s}
 
     compilers_d = {}

lib/spack/spack/compilers/__init__.py

@@ -7,8 +7,10 @@
 system and configuring Spack to use multiple compilers.
 """
 import itertools
+import multiprocessing.pool
 import os
 
+
 import llnl.util.multiproc
 from llnl.util.lang import list_modules
 
@@ -193,10 +195,10 @@ def find_compilers(*paths):
     search_commands = itertools.chain.from_iterable(
         o.search_compiler_commands(*paths) for o in all_os_classes()
     )
-    # TODO: activate multiprocessing
+
-    # with multiprocessing.Pool(processes=None) as p:
+    with multiprocessing.pool.ThreadPool() as tp:
-    compilers = llnl.util.multiproc.execute(search_commands, executor=map)
+        compilers = llnl.util.multiproc.execute(search_commands, map_fn=tp.map)
-    compilers = spack.compiler.discard_invalid(compilers)
+
     return spack.compiler.make_compiler_list(compilers)