[tune] Allow 0 CPU head bundles in for placement group factories (#15338)

Author: krfricke

python/ray/tune/tests/test_trial_runner_pg.py

@@ -231,6 +231,20 @@ def testPlacementGroupDistributedTrainingWithActorReuse(self):
         self.testPlacementGroupDistributedTraining(reuse_actors=True)
 
 
+class PlacementGroupNoAutoSetupTest(unittest.TestCase):
+    def testPlacementGroupNoCPUDriver(self):
+        """Bundles with only GPU:1 but no CPU should work"""
+        ray.init(num_gpus=1, num_cpus=1)
+
+        pgf = PlacementGroupFactory([{"GPU": 1, "CPU": 0}, {"CPU": 1}])
+
+        def train(config):
+            time.sleep(1)
+            return 5
+
+        tune.run(train, resources_per_trial=pgf)
+
+
 if __name__ == "__main__":
     import pytest
     sys.exit(pytest.main(["-v", __file__]))

python/ray/tune/utils/placement_groups.py

@@ -118,6 +118,10 @@ def __init__(self,
 
         self._bind()
 
+    @property
+    def head_cpus(self):
+        return self._bundles[0].get("CPU", None)
+
     def _bind(self):
         sig = signature(placement_group)
         try:
@@ -400,6 +404,11 @@ def get_full_actor_cls(self, trial: "Trial",
         # Only custom resources remain in `first_bundle`
         resources = first_bundle or None
 
+        if num_cpus is None:
+            # If the placement group specifically set the number
+            # of CPUs to 0, use this.
+            num_cpus = pgf.head_cpus
+
         logger.debug(f"For trial {trial} use pg {pg.id}")
 
         return actor_cls.options(