[TPU] Support collective communications in XLA devices (vllm-project#6813)

Signed-off-by: LeiWang1999 <[email protected]>

Author: WoosukKwon

vllm/distributed/device_communicators/tpu_communicator.py

@@ -0,0 +1,30 @@
+import torch
+import torch.distributed as dist
+from torch.distributed import ProcessGroup
+
+from vllm.platforms import current_platform
+
+if current_platform.is_tpu():
+    import torch_xla.core.xla_model as xm
+    from torch_xla._internal import pjrt
+
+
+class TpuCommunicator:
+
+    def __init__(self, group: ProcessGroup):
+        if not current_platform.is_tpu():
+            self.disabled = True
+            return
+        self.disabled = False
+
+        local_rank = dist.get_rank(group)
+        world_size = dist.get_world_size(group)
+        pjrt.initialize_multiprocess(local_rank, world_size)
+        xm._init_world_size_ordinal()
+
+    def all_reduce(self, x: torch.Tensor) -> torch.Tensor:
+        return xm.all_reduce(xm.REDUCE_SUM, x)
+
+    def all_gather(self, x: torch.Tensor, dim: int = -1) -> torch.Tensor:
+        assert dim == -1, "TPUs only support dim=-1 for all-gather."
+        return xm.all_gather(x, dim=dim)

vllm/distributed/parallel_state.py

@@ -133,6 +133,7 @@ def __init__(
         torch_distributed_backend: Union[str, Backend],
         use_pynccl: bool,
         use_custom_allreduce: bool,
+        use_tpu_communicator: bool,
         use_message_queue_broadcaster: bool = False,
     ):
 
@@ -164,6 +165,7 @@ def __init__(
 
         self.use_pynccl = use_pynccl
         self.use_custom_allreduce = use_custom_allreduce
+        self.use_tpu_communicator = use_tpu_communicator
 
         # lazy import to avoid documentation build error
         from vllm.distributed.device_communicators.custom_all_reduce import (
@@ -190,6 +192,12 @@ def __init__(
         else:
             self.ca_comm = None
 
+        from vllm.distributed.device_communicators.tpu_communicator import (
+            TpuCommunicator)
+        self.tpu_communicator: Optional[TpuCommunicator]
+        if use_tpu_communicator and self.world_size > 1:
+            self.tpu_communicator = TpuCommunicator(group=self.cpu_group)
+
         from vllm.distributed.device_communicators.shm_broadcast import (
             MessageQueue)
         self.mq_broadcaster: Optional[MessageQueue] = None
@@ -289,6 +297,12 @@ def all_reduce(self, input_: torch.Tensor) -> torch.Tensor:
         # Bypass the function if we are using only 1 GPU.
         if self.world_size == 1:
             return input_
+
+        # For TPUs, use TPU communicator.
+        tpu_comm = self.tpu_communicator
+        if tpu_comm is not None and not tpu_comm.disabled:
+            return tpu_comm.all_reduce(input_)
+
         if ca_comm is not None:
             out = ca_comm.custom_all_reduce(input_)
             if out is not None:
@@ -310,6 +324,12 @@ def all_gather(self, input_: torch.Tensor, dim: int = -1) -> torch.Tensor:
             return input_
         assert -input_.dim() <= dim < input_.dim(), (
             f"Invalid dim ({dim}) for input tensor with shape {input_.size()}")
+
+        # For TPUs, use TPU communicator.
+        tpu_comm = self.tpu_communicator
+        if tpu_comm is not None and not tpu_comm.disabled:
+            return tpu_comm.all_gather(input_, dim)
+
         if dim < 0:
             # Convert negative dim to positive.
             dim += input_.dim()
@@ -727,6 +747,7 @@ def init_world_group(ranks: List[int], local_rank: int,
         torch_distributed_backend=backend,
         use_pynccl=False,
         use_custom_allreduce=False,
+        use_tpu_communicator=False,
     )
 
 
@@ -745,6 +766,7 @@ def init_model_parallel_group(
         torch_distributed_backend=backend,
         use_pynccl=True,
         use_custom_allreduce=use_custom_allreduce,
+        use_tpu_communicator=True,
         use_message_queue_broadcaster=use_message_queue_broadcaster,
     )
 

vllm/lora/layers.py

@@ -1067,6 +1067,10 @@ def scale(self):
     def soft_cap(self):
         return self.base_layer.soft_cap
 
+    @property
+    def use_gather(self):
+        return self.base_layer.use_gather
+
     @property
     def org_vocab_size(self):
         return self.base_layer.org_vocab_size

vllm/model_executor/layers/logits_processor.py

@@ -5,10 +5,12 @@
 import torch
 import torch.nn as nn
 
-from vllm.distributed import tensor_model_parallel_gather
+from vllm.distributed import (tensor_model_parallel_all_gather,
+                              tensor_model_parallel_gather)
 from vllm.model_executor.layers.vocab_parallel_embedding import (
     VocabParallelEmbedding)
 from vllm.model_executor.sampling_metadata import SamplingMetadata
+from vllm.platforms import current_platform
 
 
 class LogitsProcessor(nn.Module):
@@ -39,6 +41,8 @@ def __init__(self,
         self.org_vocab_size = org_vocab_size or vocab_size
         # Soft cap the logits. Used in Gemma 2.
         self.soft_cap = soft_cap
+        # Whether to use gather or all-gather to gather the logits.
+        self.use_gather = not current_platform.is_tpu()
 
     def forward(
         self,
@@ -76,7 +80,15 @@ def _get_logits(self, hidden_states: torch.Tensor,
         logits = lm_head.linear_method.apply(lm_head,
                                              hidden_states,
                                              bias=embedding_bias)
-        logits = tensor_model_parallel_gather(logits)
+        if self.use_gather:
+            logits = tensor_model_parallel_gather(logits)
+        else:
+            # Gather is not supported for some devices such as TPUs.
+            # Use all-gather instead.
+            # NOTE(woosuk): Here, the outputs of every device should not be None
+            # because XLA requires strict SPMD among all devices. Every device
+            # should execute the same operations after gathering the logits.
+            logits = tensor_model_parallel_all_gather(logits)
         # Remove paddings in vocab (if any).
         if logits is not None:
             logits = logits[:, :self.org_vocab_size]