Add support for NCCL symmetric memory for TP allreduces (#8238)

docs/backend/server_arguments.md

@@ -251,6 +251,7 @@ Please consult the documentation below and [server_args.py](https://github.com/s
 | `--disable-cuda-graph-padding` | Disable cuda graph when padding is needed. Still uses cuda graph when padding is not needed. | False |
 | `--enable-profile-cuda-graph` | Enable profiling of cuda graph capture. | False |
 | `--enable-nccl-nvls` | Enable NCCL NVLS for prefill heavy requests when available. | False |
+| `--enable-symm-mem` | Enable NCCL symmetric memory for fast collectives. | False |
 | `--enable-tokenizer-batch-encode` | Enable batch tokenization for improved performance when processing multiple text inputs. Do not use with image inputs, pre-tokenized input_ids, or input_embeds. | False |
 | `--disable-outlines-disk-cache` | Disable disk cache of outlines to avoid possible crashes related to file system or high concurrency. | False |
 | `--disable-custom-all-reduce` | Disable the custom all-reduce kernel and fall back to NCCL. | False |

python/sglang/srt/distributed/device_communicators/pynccl.py

@@ -75,6 +75,7 @@ class PyNcclCommunicator:
         self.available = True
         self.disabled = False
 
+        self.nccl_version = self.nccl.ncclGetRawVersion()
         if self.rank == 0:
             logger.info("sglang is using nccl==%s", self.nccl.ncclGetVersion())
 
@@ -259,6 +260,12 @@ class PyNcclCommunicator:
             cudaStream_t(stream.cuda_stream),
         )
 
+    def register_comm_window_raw(self, ptr: int, size: int):
+        return self.nccl.ncclCommWindowRegister(self.comm, buffer_type(ptr), size, 1)
+
+    def deregister_comm_window(self, window):
+        return self.nccl.ncclCommWindowDeregister(self.comm, window)
+
     @contextmanager
     def change_state(
         self, enable: Optional[bool] = None, stream: Optional[torch.cuda.Stream] = None

python/sglang/srt/distributed/device_communicators/pynccl_allocator.py

+import tempfile
+
+import torch
+from packaging import version
+from torch.cuda.memory import CUDAPluggableAllocator
+
+from sglang.srt.distributed.parallel_state import GroupCoordinator
+from sglang.srt.managers.schedule_batch import global_server_args_dict
+
+nccl_allocator_source = """
+#include <nccl.h>
+extern "C" {
+
+void* nccl_alloc_plug(size_t size, int device, void* stream) {
+  void* ptr;
+  ncclResult_t err = ncclMemAlloc(&ptr, size);
+  return ptr;
+
+}
+
+void nccl_free_plug(void* ptr, size_t size, int device, void* stream) {
+  ncclResult_t err = ncclMemFree(ptr);
+}
+
+}
+"""
+
+_allocator = None
+_mem_pool = None
+_registered_base_addrs = set()
+_graph_pool_id = None
+
+
+def is_symmetric_memory_enabled():
+    return global_server_args_dict["enable_symm_mem"]
+
+
+def set_graph_pool_id(graph_pool_id):
+    global _graph_pool_id
+    _graph_pool_id = graph_pool_id
+
+
+def get_nccl_mem_pool():
+    global _allocator, _mem_pool
+    if _mem_pool is None:
+        out_dir = tempfile.gettempdir()
+        nccl_allocator_libname = "nccl_allocator"
+        torch.utils.cpp_extension.load_inline(
+            name=nccl_allocator_libname,
+            cpp_sources=nccl_allocator_source,
+            with_cuda=True,
+            extra_ldflags=["-lnccl"],
+            verbose=True,
+            is_python_module=False,
+            build_directory=out_dir,
+        )
+        _allocator = CUDAPluggableAllocator(
+            f"{out_dir}/{nccl_allocator_libname}.so",
+            "nccl_alloc_plug",
+            "nccl_free_plug",
+        ).allocator()
+        _mem_pool = torch.cuda.MemPool(_allocator)
+    return _mem_pool
+
+
+class use_symmetric_memory:
+    def __init__(self, group_coordinator: GroupCoordinator):
+        if not is_symmetric_memory_enabled():
+            self.group_coordinator = None
+            self._mem_pool_ctx = None
+            self.is_graph_capture = None
+            self.device = None
+            self.pre_2_8_0 = None
+        else:
+            self.group_coordinator = group_coordinator
+            self._mem_pool_ctx = torch.cuda.use_mem_pool(get_nccl_mem_pool())
+            self.is_graph_capture = torch.cuda.is_current_stream_capturing()
+            self.device = torch.cuda.current_device()
+            self.pre_2_8_0 = version.parse(torch.__version__) < version.parse("2.8.0")
+
+    def __enter__(self):
+        if not is_symmetric_memory_enabled():
+            return self
+        assert (
+            self.group_coordinator.pynccl_comm is not None
+        ), f"Symmetric memory requires pynccl to be enabled in group '{self.group_coordinator.group_name}'"
+        assert (
+            self.group_coordinator.pynccl_comm.nccl_version >= 22703
+        ), "NCCL version 2.27.3 or higher is required for NCCL symmetric memory"
+        if self.is_graph_capture:
+            assert (
+                _graph_pool_id is not None
+            ), "graph_pool_id is not set under graph capture"
+            # Pause graph memory pool to use symmetric memory with cuda graph
+            if self.pre_2_8_0:
+                torch._C._cuda_endAllocateCurrentStreamToPool(
+                    self.device, _graph_pool_id
+                )
+            else:
+                torch._C._cuda_endAllocateToPool(self.device, _graph_pool_id)
+        self._mem_pool_ctx.__enter__()
+        return self
+
+    def tag(self, tensor: torch.Tensor):
+        if not is_symmetric_memory_enabled():
+            return
+        tensor.symmetric_memory = True
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if not is_symmetric_memory_enabled():
+            return
+        global _registered_base_addrs
+        self._mem_pool_ctx.__exit__(exc_type, exc_val, exc_tb)
+        for segment in get_nccl_mem_pool().snapshot():
+            if segment["address"] not in _registered_base_addrs:
+                if segment["stream"] == 0 and self.pre_2_8_0:
+                    # PyTorch version < 2.8.0 has a multi-thread MemPool bug
+                    # See https://github.com/pytorch/pytorch/issues/152861
+                    # Fixed at https://github.com/pytorch/pytorch/commit/f01e628e3b31852983ab30b25bf251f557ba9c0b
+                    # WAR is to skip allocations on the default stream since the forward_pass thread always runs on a custom stream
+                    continue
+                self.group_coordinator.pynccl_comm.register_comm_window_raw(
+                    segment["address"], segment["total_size"]
+                )
+                _registered_base_addrs.add(segment["address"])
+
+        if self.is_graph_capture:
+            if self.pre_2_8_0:
+                torch._C._cuda_beginAllocateToPool(self.device, _graph_pool_id)
+            else:
+                torch._C._cuda_beginAllocateCurrentThreadToPool(
+                    self.device, _graph_pool_id
+                )

python/sglang/srt/distributed/device_communicators/pynccl_wrapper.py

@@ -67,6 +67,7 @@ def find_nccl_library() -> str:
 
 ncclResult_t = ctypes.c_int
 ncclComm_t = ctypes.c_void_p
+ncclWindow_t = ctypes.c_void_p
 
 
 class ncclUniqueId(ctypes.Structure):
@@ -279,6 +280,23 @@ class NCCLLibrary:
         Function("ncclCommDestroy", ncclResult_t, [ncclComm_t]),
     ]
 
+    exported_functions_symm_mem = [
+        # ncclResult_t ncclCommWindowRegister(ncclComm_t comm, void* buff, size_t size, ncclWindow_t* win, int winFlags);
+        Function(
+            "ncclCommWindowRegister",
+            ncclResult_t,
+            [
+                ncclComm_t,
+                buffer_type,
+                ctypes.c_size_t,
+                ctypes.POINTER(ncclWindow_t),
+                ctypes.c_int,
+            ],
+        ),
+        # ncclResult_t ncclCommWindowDeregister(ncclComm_t comm, ncclWindow_t win);
+        Function("ncclCommWindowDeregister", ncclResult_t, [ncclComm_t, ncclWindow_t]),
+    ]
+
     # class attribute to store the mapping from the path to the library
     # to avoid loading the same library multiple times
     path_to_library_cache: Dict[str, Any] = {}
@@ -312,7 +330,10 @@ class NCCLLibrary:
 
         if so_file not in NCCLLibrary.path_to_dict_mapping:
             _funcs: Dict[str, Any] = {}
-            for func in NCCLLibrary.exported_functions:
+            exported_functions = NCCLLibrary.exported_functions
+            if hasattr(self.lib, "ncclCommWindowRegister"):
+                exported_functions.extend(NCCLLibrary.exported_functions_symm_mem)
+            for func in exported_functions:
                 f = getattr(self.lib, func.name)
                 f.restype = func.restype
                 f.argtypes = func.argtypes
@@ -328,10 +349,14 @@ class NCCLLibrary:
             error_str = self.ncclGetErrorString(result)
             raise RuntimeError(f"NCCL error: {error_str}")
 
-    def ncclGetVersion(self) -> str:
+    def ncclGetRawVersion(self) -> int:
         version = ctypes.c_int()
         self.NCCL_CHECK(self._funcs["ncclGetVersion"](ctypes.byref(version)))
-        version_str = str(version.value)
+        # something like 21903
+        return version.value
+
+    def ncclGetVersion(self) -> str:
+        version_str = str(self.ncclGetRawVersion())
         # something like 21903 --> "2.19.3"
         major = version_str[0].lstrip("0")
         minor = version_str[1:3].lstrip("0")
@@ -460,6 +485,20 @@ class NCCLLibrary:
     def ncclCommDestroy(self, comm: ncclComm_t) -> None:
         self.NCCL_CHECK(self._funcs["ncclCommDestroy"](comm))
 
+    def ncclCommWindowRegister(
+        self, comm: ncclComm_t, buff: buffer_type, size: int, win_flags: int
+    ) -> ncclWindow_t:
+        window = ncclWindow_t()
+        self.NCCL_CHECK(
+            self._funcs["ncclCommWindowRegister"](
+                comm, buff, size, ctypes.byref(window), win_flags
+            )
+        )
+        return window
+
+    def ncclCommWindowDeregister(self, comm: ncclComm_t, window: ncclWindow_t) -> None:
+        self.NCCL_CHECK(self._funcs["ncclCommWindowDeregister"](comm, window))
+
 
 __all__ = [
     "NCCLLibrary",

python/sglang/srt/distributed/parallel_state.py

@@ -497,6 +497,17 @@ class GroupCoordinator:
         if self.npu_communicator is not None and not self.npu_communicator.disabled:
             return self.npu_communicator.all_reduce(input_)
 
+        if (
+            self.pynccl_comm is not None
+            and hasattr(input_, "symmetric_memory")
+            and input_.symmetric_memory
+        ):
+            with self.pynccl_comm.change_state(
+                enable=True, stream=torch.cuda.current_stream()
+            ):
+                self.pynccl_comm.all_reduce(input_)
+                return input_
+
         outplace_all_reduce_method = None
         if (
             self.qr_comm is not None

python/sglang/srt/entrypoints/engine.py

@@ -623,7 +623,8 @@ class Engine(EngineBase):
 def _set_envs_and_config(server_args: ServerArgs):
     # Set global environments
     os.environ["TF_CPP_MIN_LOG_LEVEL"] = "3"
-    os.environ["NCCL_CUMEM_ENABLE"] = "0"
+    os.environ["NCCL_CUMEM_ENABLE"] = str(int(server_args.enable_symm_mem))
+    if not server_args.enable_symm_mem:
         os.environ["NCCL_NVLS_ENABLE"] = str(int(server_args.enable_nccl_nvls))
     os.environ["TORCH_NCCL_AVOID_RECORD_STREAMS"] = "1"
     os.environ["CUDA_DEVICE_MAX_CONNECTIONS"] = "4"

python/sglang/srt/layers/linear.py

@@ -13,10 +13,14 @@ from sglang.srt.distributed import (
     divide,
     get_tensor_model_parallel_rank,
     get_tensor_model_parallel_world_size,
+    parallel_state,
     split_tensor_along_last_dim,
     tensor_model_parallel_all_gather,
     tensor_model_parallel_all_reduce,
 )
+from sglang.srt.distributed.device_communicators.pynccl_allocator import (
+    use_symmetric_memory,
+)
 from sglang.srt.layers.parameter import (
     BasevLLMParameter,
     BlockQuantScaleParameter,
@@ -1292,7 +1296,9 @@ class RowParallelLinear(LinearBase):
         # Only fuse bias add into GEMM for rank 0 (this ensures that
         # bias will not get added more than once in TP>1 case)
         bias_ = None if (self.tp_rank > 0 or self.skip_bias_add) else self.bias
+        with use_symmetric_memory(parallel_state.get_tp_group()) as sm:
             output_parallel = self.quant_method.apply(self, input_parallel, bias=bias_)
+            sm.tag(output_parallel)
         if self.reduce_results and self.tp_size > 1 and not can_fuse_mlp_allreduce:
             output = tensor_model_parallel_all_reduce(output_parallel)
         else:

python/sglang/srt/layers/moe/fused_moe_triton/layer.py

@@ -14,8 +14,12 @@ from sglang.srt.distributed import (
     get_moe_expert_parallel_world_size,
     get_moe_tensor_parallel_rank,
     get_moe_tensor_parallel_world_size,
+    get_tp_group,
     tensor_model_parallel_all_reduce,
 )
+from sglang.srt.distributed.device_communicators.pynccl_allocator import (
+    use_symmetric_memory,
+)
 from sglang.srt.eplb.expert_location import get_global_expert_location_metadata
 from sglang.srt.layers.moe.topk import StandardTopKOutput
 from sglang.srt.layers.quantization.base_config import (
@@ -626,6 +630,7 @@ class FusedMoE(torch.nn.Module):
             )
 
         # Matrix multiply.
+        with use_symmetric_memory(get_tp_group()) as sm:
             final_hidden_states = self.quant_method.apply(
                 layer=self,
                 x=hidden_states,
@@ -640,10 +645,12 @@ class FusedMoE(torch.nn.Module):
                         ep_rank=self.moe_ep_rank,
                         ep_size=self.moe_ep_size,
                     )
-                if self.quant_method.__class__.__name__ == "ModelOptNvFp4FusedMoEMethod"
+                    if self.quant_method.__class__.__name__
+                    == "ModelOptNvFp4FusedMoEMethod"
                     else {}
                 ),
             )
+            sm.tag(final_hidden_states)
 
         if self.reduce_results and (self.moe_tp_size > 1 or self.moe_ep_size > 1):
             final_hidden_states = tensor_model_parallel_all_reduce(final_hidden_states)

python/sglang/srt/layers/vocab_parallel_embedding.py

@@ -11,8 +11,12 @@ from sglang.srt.distributed import (
     divide,
     get_tensor_model_parallel_rank,
     get_tensor_model_parallel_world_size,
+    parallel_state,
     tensor_model_parallel_all_reduce,
 )
+from sglang.srt.distributed.device_communicators.pynccl_allocator import (
+    use_symmetric_memory,
+)
 from sglang.srt.layers.amx_utils import PackWeightMethod
 from sglang.srt.layers.dp_attention import get_attention_tp_rank, get_attention_tp_size
 from sglang.srt.layers.parameter import BasevLLMParameter
@@ -464,7 +468,9 @@ class VocabParallelEmbedding(torch.nn.Module):
         else:
             masked_input = input_
         # Get the embeddings.
+        with use_symmetric_memory(parallel_state.get_tp_group()) as sm:
             output_parallel = self.quant_method.embedding(self, masked_input.long())
+            sm.tag(output_parallel)
         # Mask the output embedding.
         if self.tp_size > 1:
             output_parallel.masked_fill_(input_mask.unsqueeze(-1), 0)

python/sglang/srt/managers/schedule_batch.py

@@ -108,6 +108,7 @@ GLOBAL_SERVER_ARGS_KEYS = [
     "weight_loader_disable_mmap",
     "enable_triton_kernel_moe",
     "enable_multimodal",
+    "enable_symm_mem",
 ]
 
 # Put some global args for easy access

python/sglang/srt/model_executor/cuda_graph_runner.py

@@ -29,6 +29,9 @@ from torch.profiler import ProfilerActivity, profile
 
 from sglang.srt.custom_op import CustomOp
 from sglang.srt.distributed import get_tensor_model_parallel_rank
+from sglang.srt.distributed.device_communicators.pynccl_allocator import (
+    set_graph_pool_id,
+)
 from sglang.srt.distributed.parallel_state import GroupCoordinator, graph_capture
 from sglang.srt.layers.dp_attention import DPPaddingMode, get_attention_tp_size
 from sglang.srt.layers.logits_processor import LogitsProcessorOutput
@@ -643,11 +646,15 @@ class CudaGraphRunner:
 
             run_once()
 
-        global global_graph_memory_pool
-        with torch.cuda.graph(graph, pool=global_graph_memory_pool, stream=stream):
+        if get_global_graph_memory_pool() is None:
+            set_global_graph_memory_pool(torch.cuda.graph_pool_handle())
+        # Set graph pool id globally to be able to use symmetric memory
+        set_graph_pool_id(get_global_graph_memory_pool())
+        with torch.cuda.graph(
+            graph, pool=get_global_graph_memory_pool(), stream=stream
+        ):
             out = run_once()
 
-        global_graph_memory_pool = graph.pool()
         return graph, out
 
     def recapture_if_needed(self, forward_batch: ForwardBatch):

python/sglang/srt/models/deepseek_v2.py

@@ -34,6 +34,9 @@ from sglang.srt.distributed import (
     parallel_state,
     tensor_model_parallel_all_reduce,
 )
+from sglang.srt.distributed.device_communicators.pynccl_allocator import (
+    use_symmetric_memory,
+)
 from sglang.srt.eplb.expert_distribution import get_global_expert_distribution_recorder
 from sglang.srt.eplb.expert_location import ModelConfigForExpertLocation
 from sglang.srt.eplb.expert_location_dispatch import ExpertLocationDispatchInfo
@@ -481,7 +484,11 @@ class DeepseekV2MoE(nn.Module):
             if not _is_cuda:
                 final_hidden_states *= self.routed_scaling_factor
         current_stream.wait_stream(self.alt_stream)
-        final_hidden_states += shared_output
+        with use_symmetric_memory(parallel_state.get_tp_group()) as sm:
+            final_hidden_states_out = torch.empty_like(final_hidden_states)
+        torch.add(final_hidden_states, shared_output, out=final_hidden_states_out)
+        final_hidden_states = final_hidden_states_out
+        sm.tag(final_hidden_states)
         if self.tp_size > 1 and not can_fuse_mlp_allreduce:
             final_hidden_states = tensor_model_parallel_all_reduce(final_hidden_states)
         return final_hidden_states
@@ -507,7 +514,11 @@ class DeepseekV2MoE(nn.Module):
             # fused in biased_grouped_topk so we can skip here
             final_hidden_states *= self.routed_scaling_factor
         if shared_output is not None:
-            final_hidden_states = final_hidden_states + shared_output
+            with use_symmetric_memory(parallel_state.get_tp_group()) as sm:
+                final_hidden_states_out = torch.empty_like(final_hidden_states)
+            torch.add(final_hidden_states, shared_output, out=final_hidden_states_out)
+            final_hidden_states = final_hidden_states_out
+            sm.tag(final_hidden_states)
         if self.tp_size > 1 and not can_fuse_mlp_allreduce:
             final_hidden_states = tensor_model_parallel_all_reduce(final_hidden_states)
         return final_hidden_states

python/sglang/srt/server_args.py

@@ -218,6 +218,7 @@ class ServerArgs:
     enable_profile_cuda_graph: bool = False
     enable_cudagraph_gc: bool = False
     enable_nccl_nvls: bool = False
+    enable_symm_mem: bool = False
     enable_tokenizer_batch_encode: bool = False
     disable_outlines_disk_cache: bool = False
     disable_custom_all_reduce: bool = False
@@ -1599,6 +1600,11 @@ class ServerArgs:
             action="store_true",
             help="Enable NCCL NVLS for prefill heavy requests when available.",
         )
+        parser.add_argument(
+            "--enable-symm-mem",
+            action="store_true",
+            help="Enable NCCL symmetric memory for fast collectives.",
+        )
         parser.add_argument(
             "--enable-tokenizer-batch-encode",
             action="store_true",