Add init_system API for multi-host GPU.

CHANGELOG.md

@@ -14,6 +14,8 @@ PLEASE REMEMBER TO CHANGE THE '..main' WITH AN ACTUAL TAG in GITHUB LINK.
 * [GitHub
   commits](https://github.com/google/jax/compare/jax-v0.2.24...main).
 
+* New features:
+  * (Experimental) `jax.distributed.initialize` exposes multi-host GPU backend.
 * Breaking changes
   * Moved `jax.experimental.stax` to `jax.example_libraries.stax`
   * Moved `jax.experimental.optimizers` to `jax.example_libraries.optimizers`

jax/__init__.py

@@ -120,6 +120,7 @@
 # jax and rely on the names imported above.
 from . import abstract_arrays as abstract_arrays
 from . import api_util as api_util
+from . import distributed as distributed
 from . import dtypes as dtypes
 from . import errors as errors
 from . import image as image

jax/_src/distributed.py

@@ -0,0 +1,59 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import functools
+
+from absl import logging
+from jax._src.lib import xla_bridge
+from jax._src.lib import xla_client
+from jax._src.lib import xla_extension
+
+_service = None
+def initialize(coordinator_address: str, num_processes: int, process_id: int):
+  """Initialize distributed system for topology discovery.
+
+  Currently, calling ``initialize`` sets up the multi-host GPU backend, and
+  is not required for CPU or TPU backends.
+
+  Args:
+    coordinator_address: IP address of the coordinator.
+    num_processes: Number of processes.
+    process_id: Id of the current processe.
+
+  Example:
+
+  Suppose there are two GPU hosts, and host 0 is the designated coordinator
+  with address '10.0.0.1:1234', to initialize the GPU cluster, run the
+  following commands before anything else.
+
+  On host 0
+  >>> jax.distributed.initialize('10.0.0.1:1234', 2, 0)  # doctest: +SKIP
+
+  On host 1
+  >>> jax.distributed.initialize('10.0.0.1:1234', 2, 1)  # doctest: +SKIP
+  """
+  if process_id == 0:
+    global _service
+    assert _service is None, 'initialize should be called once only'
+    logging.info('Starting JAX distributed service on %s', coordinator_address)
+    _service = xla_extension.get_distributed_runtime_service(coordinator_address,
+                                                             num_processes)
+
+  client = xla_extension.get_distributed_runtime_client(coordinator_address,
+                                                        process_id)
+  logging.info('Connecting to JAX distributed service on %s', coordinator_address)
+  client.connect()
+
+  factory = functools.partial(xla_client.make_gpu_client, client, process_id)
+  xla_bridge.register_backend_factory('gpu', factory, priority=300)

jax/_src/lib/xla_bridge.py

@@ -170,8 +170,15 @@ def _log_warning():
 # example, there could be multiple backends that provide the same kind of
 # device.
 _backend_factories = {}
+_default_backend = None
+_backends : Dict[str, Any] = {}
+_backends_errors : Dict[str, str] = {}
+_backend_lock = threading.Lock()
 
 def register_backend_factory(name, factory, *, priority=0):
+  with _backend_lock:
+    if name in _backends:
+      raise RuntimeError(f"Backend {name} already initialized")
   _backend_factories[name] = (factory, priority)
 
 
@@ -187,11 +194,6 @@ def register_backend_factory(name, factory, *, priority=0):
 register_backend_factory(
   'tpu', partial(tpu_client_timer_callback, timer_secs=60.0), priority=300)
 
-_default_backend = None
-_backends : Dict[str, Any] = {}
-_backends_errors : Dict[str, str] = {}
-_backend_lock = threading.Lock()
-
 
 def backends():
   global _backends

jax/distributed.py

@@ -0,0 +1,16 @@
+# Copyright 2021 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     https://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# flake8: noqa: F401
+from jax._src.distributed import initialize