[RL] fast weight update with zmq + ipc handles

examples/offline_inference/rlhf_colocate.py

@@ -28,12 +28,15 @@
 https://docs.ray.io/en/latest/placement-groups.html
 """
 
+import gc
 import os
 
 import ray
 import torch
+import zmq
 from ray.util.placement_group import placement_group
 from ray.util.scheduling_strategies import PlacementGroupSchedulingStrategy
+from torch.multiprocessing.reductions import reduce_tensor
 
 from vllm import LLM
 
@@ -86,20 +89,72 @@ def __init__(self):
         from vllm.platforms import current_platform
 
         self.device_uuid = current_platform.get_device_uuid(0)
+        self.zmq_context = zmq.Context()
+        self.zmq_address_counter = 0
+        self.zmq_handle = None
 
     def report_device_id(self) -> str:
         return self.device_uuid
 
-    def get_weight_ipc_handles(self):
-        from torch.multiprocessing.reductions import reduce_tensor
+    def get_zmq_handles(self) -> dict[str, str]:
+        suffix = f"{self.device_uuid}-{self.zmq_address_counter}"
+        self.zmq_handle = f"ipc:///tmp/rl-colocate-zmq-{suffix}.sock"
+        self.zmq_address_counter += 1
+        return {self.device_uuid: self.zmq_handle}
 
-        data = {}
-        for name, p in self.model.named_parameters():
-            # A training actor might hold only a subset of the weights and may
-            # need to gather weights from other actors. For demonstration
-            # purposes, each training actor owns the full weight set.
-            data[name] = reduce_tensor(p.detach())
-        return {self.device_uuid: data}
+    def update_weights(self):
+        # align size to avoid misaligned address
+        align_size = 256
+
+        def get_size(p: torch.Tensor) -> int:
+            return (p.nbytes + align_size - 1) // align_size * align_size
+
+        named_parameters: dict[str, torch.nn.Parameter] = dict(
+            self.model.named_parameters()
+        )
+        max_tensor_size = max(get_size(p) for p in named_parameters.values())
+        # use max_tensor_size * 2 as buffer size
+        buffer = torch.empty(max_tensor_size * 2, dtype=torch.uint8, device="cuda:0")
+        s = self.zmq_context.socket(zmq.REQ)
+        s.bind(self.zmq_handle)
+        handle = reduce_tensor(buffer)
+
+        offset = 0
+        buckets: list[tuple[list[dict], list[torch.Tensor]]] = []
+        named_tensors: list[dict] = []
+        real_tensors: list[torch.Tensor] = []
+        for name, p in named_parameters.items():
+            size = get_size(p)
+            if offset + size > buffer.numel():
+                buckets.append((named_tensors, real_tensors))
+                named_tensors, real_tensors = [], []
+                offset = 0
+            # assume tensors are contiguous
+            named_tensors.append(
+                {"name": name, "dtype": p.dtype, "shape": p.shape, "offset": offset}
+            )
+            real_tensors.append(p)
+            offset += size
+        if named_tensors:
+            buckets.append((named_tensors, real_tensors))
+        s.send_pyobj(handle)
+        s.recv()
+        for named_tensors, real_tensors in buckets:
+            offset = 0
+            for p in real_tensors:
+                buffer[offset : offset + p.nbytes].data.copy_(
+                    p.data.view(-1).view(dtype=torch.uint8), non_blocking=True
+                )
+                offset += get_size(p)
+            torch.cuda.synchronize()
+            s.send_pyobj(named_tensors)
+            s.recv()
+        s.send_pyobj(None)
+        s.recv()
+        s.close()
+        del buffer
+        gc.collect()
+        torch.cuda.empty_cache()
 
 
 # Ray manages four GPUs.
@@ -175,18 +230,22 @@ def get_weight_ipc_handles(self):
 # the second inference engine.
 assert training_actor_device_ids[2:] == inference_engine_device_ids[1]
 
-print("Gather all the IPC handles from the training actors.")
-ipc_handles = {}
+print("Gather all the ZMQ handles from the training actors.")
+zmq_handles = {}
 for actor in training_actors:
-    ipc_handles.update(ray.get(actor.get_weight_ipc_handles.remote()))
+    zmq_handles.update(ray.get(actor.get_zmq_handles.remote()))
+
+print(f"ZMQ handles: {zmq_handles}")
 
 print("Update the weights of the inference engines.")
-for llm in inference_engines:
-    ray.get(
-        llm.collective_rpc.remote(
-            "update_weights_from_ipc_handles", args=(ipc_handles,)
-        )
-    )
+ray.get(
+    [actor.update_weights.remote() for actor in training_actors]
+    + [
+        llm.collective_rpc.remote("update_weights_from_ipc", args=(zmq_handles,))
+        for llm in inference_engines
+    ]
+)
+
 print("Check if the weights are updated.")
 for llm in inference_engines:
     assert ray.get(llm.collective_rpc.remote("check_weights_changed", args=tuple()))

examples/offline_inference/rlhf_utils.py

@@ -1,6 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+import gc
+from typing import Callable, Optional, TypedDict
+
 import torch
+import zmq
 
 
 def stateless_init_process_group(master_address, master_port, rank, world_size, device):
@@ -66,6 +70,27 @@ def check_weights_changed(self):
         return weights_updated
 
 
+def rebuild_ipc(
+    handle: tuple[Callable, tuple], device_id: Optional[int] = None
+) -> torch.Tensor:
+    func, args = handle
+    list_args = list(args)
+    if device_id is not None:
+        # the key is to change device id to the current device id
+        # in case two processes have different CUDA_VISIBLE_DEVICES
+        list_args[6] = device_id
+    buffer = func(*list_args)
+    return buffer
+
+
+class FlattenedTensorMetadata(TypedDict):
+    name: str
+    shape: torch.Size
+    dtype: torch.dtype
+    # specify the start offset of this tensor in shared ipc_buffer tensor
+    offset: int
+
+
 class ColocateWorkerExtension:
     """
     The class for vLLM's worker to inherit from, in the colocate setting.
@@ -76,27 +101,62 @@ class ColocateWorkerExtension:
     should pass the full qualified name as `worker_extension_cls` argument.
     """
 
+    def update_weights_from_ipc(self, zmq_handles: dict[str, str]):
+        from vllm.model_executor.model_loader.utils import process_weights_after_loading
+
+        assert self.device is not None
+        if not hasattr(self, "_zmq_ctx") or self._zmq_ctx is None:
+            self._zmq_ctx = zmq.Context()
+        socket = self._zmq_ctx.socket(zmq.REP)
+        socket.connect(zmq_handles[self.report_device_id()])
+        buffer: Optional[torch.Tensor] = None
+        while True:
+            payload: tuple[Callable, tuple] | list[FlattenedTensorMetadata] | None = (
+                socket.recv_pyobj()
+            )
+            if payload is None:
+                # means the update is done
+                process_weights_after_loading(
+                    self.model_runner.model, self.model_config, self.device
+                )
+                torch.cuda.synchronize()
+                socket.send(b"")
+                break
+            if isinstance(payload, tuple):
+                # an ipc handle that vLLM can use `func, args = handle`
+                # and `func(*args)` to rebuild GPU tensor.
+                buffer = rebuild_ipc(payload, self.device.index)
+                assert buffer.dtype == torch.uint8
+                socket.send(b"")
+                continue
+            assert isinstance(payload, list)
+            assert buffer is not None
+            weights = []
+            for item in payload:
+                shape = item["shape"]
+                if isinstance(shape, (list, tuple)):
+                    shape = torch.Size(shape)
+                assert isinstance(shape, torch.Size)
+                dtype, offset = item["dtype"], item["offset"]
+                size = dtype.itemsize * shape.numel()
+                tensor = buffer[offset : offset + size].view(dtype=dtype).view(shape)
+                weights.append((item["name"], tensor))
+            self.model_runner.model.load_weights(weights=weights)
+            del weights
+            torch.cuda.synchronize()
+            socket.send(b"")
+
+        socket.close()
+        del buffer
+        gc.collect()
+        torch.cuda.empty_cache()
+
     def report_device_id(self) -> str:
         from vllm.platforms import current_platform
 
         self.device_uuid = current_platform.get_device_uuid(self.device.index)
         return self.device_uuid
 
-    def update_weights_from_ipc_handles(self, ipc_handles):
-        handles = ipc_handles[self.device_uuid]
-        device_id = self.device.index
-        weights = []
-        for name, handle in handles.items():
-            func, args = handle
-            list_args = list(args)
-            # the key is to change device id to the current device id
-            # in case two processes have different CUDA_VISIBLE_DEVICES
-            list_args[6] = device_id
-            tensor = func(*list_args)
-            weights.append((name, tensor))
-        self.model_runner.model.load_weights(weights=weights)
-        torch.cuda.synchronize()
-
     def check_weights_changed(self):
         """
         Check if the weights are updated to 0.