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
@@ -335,7 +335,8 @@
         params = userParams,
         config_prefix = opts.configPrefix,
         maxIndividualTestTime = maxIndividualTestTime,
-        maxFailures = opts.maxFailures)
+        maxFailures = opts.maxFailures,
+        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
@@ -177,9 +177,15 @@
         self.tests = tests
 
     def execute_test(self, test):
+        pg = test.config.parallelism_group
+        if callable(pg): pg = pg(test)
+
         result = None
-        start_time = time.time()
+        semaphore = None
         try:
+            if pg: semaphore = self.parallelism_semaphores[pg]
+            if semaphore: semaphore.acquire()
+            start_time = time.time()
             result = test.config.test_format.execute(test, self.lit_config)
 
             # Support deprecated result from execute() which returned the result
@@ -189,6 +195,8 @@
                 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 +206,8 @@
             output += traceback.format_exc()
             output += '\n'
             result = lit.Test.Result(lit.Test.UNRESOLVED, output)
-        result.elapsed = time.time() - start_time
+        finally:
+            if semaphore: semaphore.release()
 
         test.setResult(result)
 
@@ -231,6 +240,7 @@
             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 +252,13 @@
         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)