Index: llvm/trunk/utils/lit/lit/LitConfig.py
===================================================================
--- llvm/trunk/utils/lit/lit/LitConfig.py
+++ llvm/trunk/utils/lit/lit/LitConfig.py
@@ -24,7 +24,8 @@
                  noExecute, debug, isWindows,
                  params, config_prefix = None,
                  maxIndividualTestTime = 0,
-                 maxFailures = None):
+                 maxFailures = None,
+                 parallelism_groups = []):
         # The name of the test runner.
         self.progname = progname
         # The items to add to the PATH environment variable.
@@ -62,6 +63,7 @@
 
         self.maxIndividualTestTime = maxIndividualTestTime
         self.maxFailures = maxFailures
+        self.parallelism_groups = parallelism_groups
 
     @property
     def maxIndividualTestTime(self):
Index: llvm/trunk/utils/lit/lit/TestingConfig.py
===================================================================
--- llvm/trunk/utils/lit/lit/TestingConfig.py
+++ llvm/trunk/utils/lit/lit/TestingConfig.py
@@ -106,7 +106,7 @@
                  environment, substitutions, unsupported,
                  test_exec_root, test_source_root, excludes,
                  available_features, pipefail, limit_to_features = [],
-                 is_early = False):
+                 is_early = False, parallelism_group = ""):
         self.parent = parent
         self.name = str(name)
         self.suffixes = set(suffixes)
@@ -125,6 +125,7 @@
         self.limit_to_features = set(limit_to_features)
         # Whether the suite should be tested early in a given run.
         self.is_early = bool(is_early)
+        self.parallelism_group = parallelism_group
 
     def finish(self, litConfig):
         """finish() - Finish this config object, after loading is complete."""
Index: llvm/trunk/utils/lit/lit/main.py
===================================================================
--- llvm/trunk/utils/lit/lit/main.py
+++ llvm/trunk/utils/lit/lit/main.py
@@ -313,6 +313,13 @@
 
     isWindows = platform.system() == 'Windows'
 
+    parallelism_groups = {}
+    if platform.system() == 'Darwin':
+      # Only run up to 3 64-bit sanitized processes simultaneously on Darwin.
+      # Using more scales badly and hogs the system due to inefficient handling
+      # of large mmap'd regions (terabytes) by the kernel.
+      parallelism_groups["darwin-64bit-sanitizer"] = 3
+
     # Create the global config object.
     litConfig = lit.LitConfig.LitConfig(
         progname = os.path.basename(sys.argv[0]),
@@ -327,7 +334,8 @@
         params = userParams,
         config_prefix = opts.configPrefix,
         maxIndividualTestTime = maxIndividualTestTime,
-        maxFailures = opts.maxFailures)
+        maxFailures = opts.maxFailures,
+        parallelism_groups = parallelism_groups)
 
     # Perform test discovery.
     run = lit.run.Run(litConfig,
Index: llvm/trunk/utils/lit/lit/run.py
===================================================================
--- llvm/trunk/utils/lit/lit/run.py
+++ llvm/trunk/utils/lit/lit/run.py
@@ -175,9 +175,14 @@
     def __init__(self, lit_config, tests):
         self.lit_config = lit_config
         self.tests = tests
+        self.parallelism_semaphores = ""
 
     def execute_test(self, test):
+        pg = test.config.parallelism_group
+        if callable(pg): pg = pg(test)
+
         result = None
+        if pg: self.parallelism_semaphores[pg].acquire()
         start_time = time.time()
         try:
             result = test.config.test_format.execute(test, self.lit_config)
@@ -189,6 +194,8 @@
                 result = lit.Test.Result(code, output)
             elif not isinstance(result, lit.Test.Result):
                 raise ValueError("unexpected result from test execution")
+
+            result.elapsed = time.time() - start_time
         except KeyboardInterrupt:
             raise
         except:
@@ -198,7 +205,8 @@
             output += traceback.format_exc()
             output += '\n'
             result = lit.Test.Result(lit.Test.UNRESOLVED, output)
-        result.elapsed = time.time() - start_time
+        finally:
+            if pg: self.parallelism_semaphores[pg].release()
 
         test.setResult(result)
 
@@ -231,6 +239,7 @@
             try:
                 task_impl = multiprocessing.Process
                 queue_impl = multiprocessing.Queue
+                sem_impl = multiprocessing.Semaphore
                 canceled_flag =  multiprocessing.Value('i', 0)
                 consumer = MultiprocessResultsConsumer(self, display, jobs)
             except:
@@ -242,9 +251,13 @@
         if not consumer:
             task_impl = threading.Thread
             queue_impl = queue.Queue
+            sem_impl = threading.Semaphore
             canceled_flag = LockedValue(0)
             consumer = ThreadResultsConsumer(display)
 
+        self.parallelism_semaphores = {k: sem_impl(v)
+            for k, v in self.lit_config.parallelism_groups.items()}
+
         # Create the test provider.
         provider = TestProvider(queue_impl, canceled_flag)
         handleFailures(provider, consumer, self.lit_config.maxFailures)