[C/PyTorch] Removed MPI dependence in Userbuffers (#901)

* added DL framework callbacks for bootstrapping userbuffers without MPI
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed userbuffers availability check in TE modules since userbuffers is now always compiled
Signed-off-by: Alp Dener <adener@nvidia.com>

* added comm+GEMM overlap example with LayerNormMLP
Signed-off-by: Alp Dener <adener@nvidia.com>

* lintin and review fixes
Signed-off-by: Alp Dener <adener@nvidia.com>

* linting and review fixes
Signed-off-by: Alp Dener <adener@nvidia.com>

* added header guards
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed defunct userbuffers checks in build_utils and setup.py
Signed-off-by: Alp Dener <adener@nvidia.com>

* added exposed API in modules/base.py to __all__
Signed-off-by: Alp Dener <adener@nvidia.com>

* removed transformer_engine/CMakeLists.txt and shifted all TE/common compile into transformer_engine/common/CmakeLists.txt

Signed-off-by: Al...

build_tools/pytorch.py

@@ -11,7 +11,7 @@ import setuptools
 from .utils import (
     all_files_in_dir,
     cuda_version,
-    userbuffers_enabled,
+    cuda_path,
 )
 
 
@@ -28,6 +28,9 @@ def setup_pytorch_extension(
     sources = [
         csrc_source_files / "common.cu",
         csrc_source_files / "ts_fp8_op.cpp",
+        csrc_source_files / "userbuffers" / "ipcsocket.cc",
+        csrc_source_files / "userbuffers" / "userbuffers.cu",
+        csrc_source_files / "userbuffers" / "userbuffers-host.cpp",
     ] + all_files_in_dir(extensions_dir)
 
     # Header files
@@ -37,8 +40,12 @@ def setup_pytorch_extension(
         common_header_files / "common" / "include",
         csrc_header_files,
     ]
+
     # Compiler flags
-    cxx_flags = ["-O3"]
+    cxx_flags = [
+        "-O3",
+        "-fvisibility=hidden",
+    ]
     nvcc_flags = [
         "-O3",
         "-gencode",
@@ -67,13 +74,19 @@ def setup_pytorch_extension(
         if version >= (11, 8):
             nvcc_flags.extend(["-gencode", "arch=compute_90,code=sm_90"])
 
-    # userbuffers support
-    if userbuffers_enabled():
-        if os.getenv("MPI_HOME"):
-            mpi_home = Path(os.getenv("MPI_HOME"))
-            include_dirs.append(mpi_home / "include")
-        cxx_flags.append("-DNVTE_WITH_USERBUFFERS")
-        nvcc_flags.append("-DNVTE_WITH_USERBUFFERS")
+    # Libraries -- PyTorch CUDAExtension links to libcudart.so but not to libcuda.so
+    cuda_home, _ = cuda_path()
+    library_dirs = [ cuda_home / "compat" / "lib" ]
+    libraries = [ "cuda" ]
+    if os.getenv("UB_MPI_BOOTSTRAP"):
+        assert os.getenv("MPI_HOME") is not None, \
+            "MPI_HOME must be set when compiling with UB_MPI_BOOTSTRAP=1"
+        mpi_home = Path(os.getenv("MPI_HOME"))
+        include_dirs.append(mpi_home / "include")
+        cxx_flags.append("-DUB_MPI_BOOTSTRAP")
+        nvcc_flags.append("-DUB_MPI_BOOTSTRAP")
+        library_dirs.append(mpi_home / "lib")
+        libraries.append("mpi")
 
     # Construct PyTorch CUDA extension
     sources = [str(path) for path in sources]
@@ -82,10 +95,12 @@ def setup_pytorch_extension(
 
     return CUDAExtension(
         name="transformer_engine_torch",
-        sources=sources,
-        include_dirs=include_dirs,
+        sources=[ str(src) for src in sources ],
+        include_dirs=[ str(inc) for inc in include_dirs ],
         extra_compile_args={
             "cxx": cxx_flags,
             "nvcc": nvcc_flags,
         },
+        libraries=[ str(lib) for lib in libraries ],
+        library_dirs=[ str(lib_dir) for lib_dir in library_dirs ],
     )

build_tools/utils.py

@@ -16,15 +16,6 @@ from subprocess import CalledProcessError
 from typing import List, Optional, Tuple
 
 
-@cache
-def userbuffers_enabled() -> bool:
-    """Check if userbuffers support is enabled"""
-    if int(os.getenv("NVTE_WITH_USERBUFFERS", "0")):
-        assert os.getenv("MPI_HOME"), "MPI_HOME must be set if NVTE_WITH_USERBUFFERS=1"
-        return True
-    return False
-
-
 @cache
 def debug_build_enabled() -> bool:
     """Whether to build with a debug configuration"""

examples/pytorch/comm_gemm_overlap/ln_mlp_with_overlap.py

+#!/usr/bin/python3
+
+# Copyright (c) 2022-2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+#
+# See LICENSE for license information.
+
+import os
+import sys
+import subprocess
+import argparse
+
+import torch
+import torch.distributed as dist
+
+import transformer_engine.pytorch as te
+from transformer_engine.common.recipe import Format, DelayedScaling
+
+def parse_args(argv=None, namespace=None):
+    parser = argparse.ArgumentParser(
+        description="Test a te.LayerNormMLP module with GEMM+comm overlap via Userbuffers.")
+    parser.add_argument('-i', "--num-iters", type=int, default=5,
+                        help="Number of dummy 'training' iterations.")
+    parser.add_argument('-b', "--batch-size", type=int, default=2,
+                        help="Input batch size.")
+    parser.add_argument('-s', "--seq-length", type=int, default=2048,
+                        help="Input sequence length.")
+    parser.add_argument('-n', "--num-heads", type=int, default=64,
+                        help="Number of attention heads.")
+    parser.add_argument('-d', "--head-dim", type=int, default=128,
+                        help="Dimension of each attention head.")
+    parser.add_argument("--mlp-expansion-factor", type=int, default=4,
+                        help="MLP block intermediate size as a factor of hidden dimension.")
+    parser.add_argument("--seed", type=int, default=1234,
+                        help="RNG seed.")
+    parser.add_argument("--fp8", action="store_true", default=False,
+                        help="Enables the te.fp8_autocast() context.")
+    parser.add_argument("--no-comm-overlap", action="store_true", default=False,
+                        help="Disable the comm+GEMM overlap.")
+    parser.add_argument('-v', "--verbose", action="store_true", default=False)
+    return parser.parse_args(argv, namespace)
+
+def train(opts):
+    WORLD_RANK = int(os.getenv("RANK"))
+    WORLD_SIZE = int(os.getenv("WORLD_SIZE"))
+    def dist_print(msg, end='\n', all_ranks=False):
+        if WORLD_RANK == 0 or all_ranks:
+            print(f"[RANK-{WORLD_RANK}] {msg}", end=end)
+
+    # Seed RNG
+    torch.cuda.set_device(WORLD_RANK)
+    torch.manual_seed(opts.seed+WORLD_RANK)
+    torch.cuda.manual_seed(opts.seed+WORLD_RANK)
+
+    # Initialize torch.distributed global process group and get TP group
+    dist.init_process_group(backend="nccl",
+                            rank=WORLD_RANK,
+                            world_size=WORLD_SIZE,
+                            device_id=torch.device(f'cuda:{WORLD_RANK}'))
+    tp_group = dist.new_group(backend="nccl")
+    tp_size = dist.get_world_size(tp_group)
+
+    # Intialize userbuffers
+    ag_cfg = {  # Ring-exchange All-Gather overlap for fc1_fprop and fc2_dgrad
+        'method': 'ring_exchange',
+        'num_splits' : 8,
+        'num_sm' : 1,
+        'set_sm_margin' : False,
+    }
+    rs_cfg = {  # Reduce-scatter overlap for fc1_dgrad and fc2_fprop
+        'method': 'ring_exchange',
+        'num_splits' : 4,
+        'num_sm' : 1,
+        'set_sm_margin' : True,
+    }
+    hidden_size = opts.num_heads * opts.head_dim
+    batched_size = opts.seq_length * opts.batch_size
+    if not opts.no_comm_overlap:
+        te.initialize_ub(
+            [batched_size, hidden_size],
+            tp_group,
+            use_fp8 = opts.fp8,
+            dtype = torch.bfloat16,
+            ub_cfgs = {
+                'fc1_fprop': ag_cfg,
+                'fc1_dgrad': rs_cfg,
+                'fc2_fprop': rs_cfg,
+                'fc2_dgrad': ag_cfg,
+            },
+        )
+
+    #
+    model = te.LayerNormMLP(
+        hidden_size, opts.mlp_expansion_factor * hidden_size,
+        params_dtype = torch.bfloat16,
+        device = 'cuda',
+        tp_group = tp_group,
+        tp_size = tp_size,
+        set_parallel_mode = True,
+        sequence_parallel = True,  # this is required for comm+GEMM overlap
+        seq_length = opts.seq_length,
+        micro_batch_size = opts.batch_size,
+        ub_overlap_rs_dgrad = not opts.no_comm_overlap,
+        ub_overlap_rs = not opts.no_comm_overlap,
+        ub_overlap_ag = not opts.no_comm_overlap,
+    )
+
+    # Initialize optimizer with model parameters
+    optim = torch.optim.Adam(model.parameters(), lr=0.0001)
+
+    # Fp8 recipe setup
+    fp8_format = Format.HYBRID
+    fp8_recipe = DelayedScaling(fp8_format=fp8_format, amax_history_len=32,
+                                amax_compute_algo="max")
+
+    # Start dummy "training" iterations
+    for i in range(opts.num_iters):
+        dist_print(f"Iter {i+1}", all_ranks=opts.verbose)
+
+        dist_print("|-- Generate random input batch", all_ranks=opts.verbose)
+        x = torch.rand((opts.seq_length // tp_size, opts.batch_size, hidden_size),
+                       dtype=torch.bfloat16, device='cuda', requires_grad=True)
+
+        dist_print("|-- Forward pass", all_ranks=opts.verbose)
+        with te.fp8_autocast(enabled=opts.fp8, fp8_recipe=fp8_recipe, fp8_group=tp_group):
+            y = model(x)
+            dist_print("|-- Compute loss", all_ranks=opts.verbose)
+            loss = y.flatten().sum()
+
+        dist_print("|-- Backward pass", all_ranks=opts.verbose)
+        loss.backward()
+
+        dist_print("|-- Optimizer step", all_ranks=opts.verbose)
+        optim.step()
+
+    te.destroy_ub()
+    dist.destroy_process_group()
+
+if __name__ == "__main__":
+    if "TORCHELASTIC_RUN_ID" in os.environ.keys():
+        args = parse_args()
+        train(args)
+    else:
+        subprocess.run(
+            [
+                'torchrun', f'--nproc-per-node={torch.cuda.device_count()}',
+                *sys.argv
+            ],
+            env=os.environ,
+            check=True
+        )
+    os._exit(0)

setup.py

@@ -16,7 +16,6 @@ from build_tools.utils import (
     found_ninja,
     found_pybind11,
     remove_dups,
-    userbuffers_enabled,
     get_frameworks,
     install_and_import,
 )
@@ -41,21 +40,13 @@ CMakeBuildExtension = get_build_ext(BuildExtension)
 
 
 def setup_common_extension() -> CMakeExtension:
-    """Setup CMake extension for common library
-
-    Also builds JAX or userbuffers support if needed.
-
-    """
-    cmake_flags = []
-    if userbuffers_enabled():
-        cmake_flags.append("-DNVTE_WITH_USERBUFFERS=ON")
-
+    """Setup CMake extension for common library"""
     # Project directory root
     root_path = Path(__file__).resolve().parent
     return CMakeExtension(
         name="transformer_engine",
-        cmake_path=root_path / Path("transformer_engine"),
-        cmake_flags=cmake_flags,
+        cmake_path=root_path / Path("transformer_engine/common"),
+        cmake_flags=[],
     )
 
 

transformer_engine/CMakeLists.txt

-# Copyright (c) 2022-2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
-#
-# See LICENSE for license information.
-
-cmake_minimum_required(VERSION 3.18)
-
-if(NOT DEFINED CMAKE_CUDA_ARCHITECTURES)
-  set(CMAKE_CUDA_ARCHITECTURES 70 80 89 90)
-endif()
-
-set(CMAKE_CXX_STANDARD 17)
-set(CMAKE_CUDA_STANDARD 17)
-set(CMAKE_CUDA_STANDARD_REQUIRED ON)
-
-project(transformer_engine LANGUAGES CUDA CXX)
-
-set(CMAKE_CUDA_FLAGS "${CMAKE_CUDA_FLAGS} --threads 4")
-if (CMAKE_BUILD_TYPE STREQUAL "Debug")
-  set(CMAKE_CUDA_FLAGS_DEBUG "${CMAKE_CUDA_FLAGS_DEBUG} -G")
-endif()
-
-find_package(CUDAToolkit REQUIRED cublas nvToolsExt)
-
-# Check for cuDNN frontend API
-set(CUDNN_FRONTEND_INCLUDE_DIR
-    "${CMAKE_SOURCE_DIR}/../3rdparty/cudnn-frontend/include")
-if(NOT EXISTS "${CUDNN_FRONTEND_INCLUDE_DIR}")
-    message(FATAL_ERROR
-            "Could not find cuDNN frontend API. "
-            "Try running 'git submodule update --init --recursive' "
-            "within the Transformer Engine source.")
-endif()
-include(${CMAKE_SOURCE_DIR}/../3rdparty/cudnn-frontend/cmake/cuDNN.cmake)
-
-find_package(Python COMPONENTS Interpreter Development.Module REQUIRED)
-
-include_directories(${PROJECT_SOURCE_DIR})
-
-add_subdirectory(common)
-if(NVTE_WITH_USERBUFFERS)
-    message(STATUS "userbuffers support enabled")
-    add_subdirectory(pytorch/csrc/userbuffers)
-endif()

transformer_engine/common/CMakeLists.txt

@@ -2,6 +2,40 @@
 #
 # See LICENSE for license information.
 
+cmake_minimum_required(VERSION 3.18)
+
+if(NOT DEFINED CMAKE_CUDA_ARCHITECTURES)
+  set(CMAKE_CUDA_ARCHITECTURES 70 80 89 90)
+endif()
+
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CUDA_STANDARD 17)
+set(CMAKE_CUDA_STANDARD_REQUIRED ON)
+
+project(transformer_engine LANGUAGES CUDA CXX)
+
+set(CMAKE_CUDA_FLAGS "${CMAKE_CUDA_FLAGS} --threads 4")
+if (CMAKE_BUILD_TYPE STREQUAL "Debug")
+  set(CMAKE_CUDA_FLAGS_DEBUG "${CMAKE_CUDA_FLAGS_DEBUG} -G")
+endif()
+
+find_package(CUDAToolkit REQUIRED cublas nvToolsExt)
+
+# Check for cuDNN frontend API
+set(CUDNN_FRONTEND_INCLUDE_DIR
+    "${CMAKE_SOURCE_DIR}/../../3rdparty/cudnn-frontend/include")
+if(NOT EXISTS "${CUDNN_FRONTEND_INCLUDE_DIR}")
+    message(FATAL_ERROR
+            "Could not find cuDNN frontend API. "
+            "Try running 'git submodule update --init --recursive' "
+            "within the Transformer Engine source.")
+endif()
+include(${CMAKE_SOURCE_DIR}/../../3rdparty/cudnn-frontend/cmake/cuDNN.cmake)
+
+find_package(Python COMPONENTS Interpreter Development.Module REQUIRED)
+
+include_directories(${PROJECT_SOURCE_DIR}/..)
+
 # Configure Transformer Engine library
 set(transformer_engine_SOURCES)
 list(APPEND transformer_engine_SOURCES

transformer_engine/common/__init__.py

@@ -42,17 +42,5 @@ def _load_library():
     return ctypes.CDLL(so_path, mode=ctypes.RTLD_GLOBAL)
 
 
-def _load_userbuffers():
-    """Load shared library with userbuffers"""
-
-    so_dir = get_te_path() / "transformer_engine"
-    so_file = so_dir / f"libtransformer_engine_userbuffers.{_get_sys_extension()}"
-
-    if so_file.exists():
-        return ctypes.CDLL(so_file, mode=ctypes.RTLD_GLOBAL)
-    return None
-
-
 if "NVTE_PROJECT_BUILDING" not in os.environ:
     _TE_LIB_CTYPES = _load_library()
-    _UB_LIB_CTYPES = _load_userbuffers()

transformer_engine/pytorch/__init__.py

@@ -37,6 +37,8 @@ from transformer_engine.pytorch.module import Linear
 from transformer_engine.pytorch.module import LayerNormMLP
 from transformer_engine.pytorch.module import LayerNorm
 from transformer_engine.pytorch.module import RMSNorm
+from transformer_engine.pytorch.module import initialize_ub
+from transformer_engine.pytorch.module import destroy_ub
 from transformer_engine.pytorch.attention import DotProductAttention
 from transformer_engine.pytorch.attention import InferenceParams
 from transformer_engine.pytorch.attention import MultiheadAttention

transformer_engine/pytorch/csrc/comm_gemm_overlap.h

@@ -4,6 +4,9 @@
  * See LICENSE for license information.
  ************************************************************************/
 
+#ifndef TRANSFORMER_ENGINE_PYTORCH_CSRC_COMM_GEMM_OVERLAP_H_
+#define TRANSFORMER_ENGINE_PYTORCH_CSRC_COMM_GEMM_OVERLAP_H_
+
 #include <stdio.h>
 #include <stdlib.h>
 
@@ -20,6 +23,7 @@
 #include "common/util/logging.h"
 #include "common/util/system.h"
 #include "userbuffers/userbuffers.h"
+#include "extensions.h"
 
 #define HALF_BYTES 2
 #define UB_MAX_SM 32
@@ -36,6 +40,66 @@
 using namespace torch::indexing;
 namespace ubuf {
 
+/*
+** Static container for Python callbacks to torch.distributed collectives
+*/
+static struct TorchCallbacks : torch::CustomClassHolder {
+  bool initialized{false};
+  std::unordered_map<void *, at::Tensor> gathered_tensors;
+  std::function<at::Tensor(at::Tensor&, const std::string &)> allgather;
+  std::function<void(const std::string &)> barrier;
+  std::function<void(at::Tensor &)> free;
+} torch_callbacks;
+
+/*
+** Helper function for setting Python callbacks to torch.distributed collectives.
+*/
+void set_ubuf_bootstrap_callbacks(
+  std::function<at::Tensor(at::Tensor&, const std::string &)> allgather,
+  std::function<void(const std::string &)> barrier,
+  std::function<void(at::Tensor &)> free
+) {
+  torch_callbacks.allgather = allgather;
+  torch_callbacks.barrier = barrier;
+  torch_callbacks.free = free;
+  torch_callbacks.initialized = true;
+}
+
+/*
+** Python callback for globaldata = torch.distributed.all_gather(localdata, tp_group).
+** This *creates* a new tensor, which Userbuffers later frees with a separate callback.
+*/
+void ub_alloc_copy_allgather(void **globaldata, void *localdata, size_t localbytes, char *group) {
+  assert(torch_callbacks.initialized);
+  auto localtensor = torch::from_blob(
+    localdata, {static_cast<int64_t>(localbytes / sizeof(uint8_t))},
+    at::device(torch::kCPU).dtype(torch::kUInt8));
+  auto globaltensor = torch_callbacks.allgather(localtensor, group);
+  *globaldata = globaltensor.data_ptr();
+  torch_callbacks.gathered_tensors[*globaldata] = globaltensor;
+}
+
+/*
+** Python callback for torch.distributed.barrier(tp_group).
+*/
+void ub_barrier(char *group) {
+  assert(torch_callbacks.initialized);
+  torch_callbacks.barrier(group);
+}
+
+/*
+** Python callback for freeing up tensors created in the ub_alloc_copy_allgather(...) callback.
+*/
+void ub_free(void *ptr) {
+  assert(torch_callbacks.initialized);
+  auto i = torch_callbacks.gathered_tensors.find(ptr);
+  if (i == torch_callbacks.gathered_tensors.end())
+    return;
+  auto tensor = std::move(i->second);
+  torch_callbacks.gathered_tensors.erase(i);
+  torch_callbacks.free(tensor);
+}
+
 enum class COMM_TYPE { RS = 0, AG = 1 };
 
 enum class UBOverlapAlgo {
@@ -74,15 +138,21 @@ struct UbufCommOverlap : torch::CustomClassHolder, UbufBase {
   int use_ce;
   bool _atomic_gemm;
 
-  UbufCommOverlap(torch::Tensor sample, int rank, int tp_size, int num_comm_sm, int comm_cga_size,
-                  int num_splits, bool set_sm_margin, int num_max_streams, bool atomic_gemm,
-                  torch::Tensor empty_tensor) {
+  UbufCommOverlap(torch::Tensor sample, int rank, int world_size, int tp_rank, int tp_size,
+                  int num_comm_sm, int comm_cga_size, int num_splits, bool set_sm_margin,
+                  int num_max_streams, bool atomic_gemm, torch::Tensor empty_tensor) {
     // Initialize userbuf communicator
     if (!comm_created) {
       if (rank == 0) {
         printf("!!! [UB] Create UbufCommOverlap Communicator\n");
       }
-      create_communicator_grouped2(&_ub_comm, 1, 1, tp_size, 1);
+      if (transformer_engine::getenv<bool>("UB_MPI_BOOTSTRAP")) {
+        create_communicator_grouped2_mpi(&_ub_comm, 1, 1, tp_size, 1);
+      } else {
+        create_communicator_grouped2(&_ub_comm, rank, world_size, tp_rank, tp_size, 1, 1,
+                                   &ub_alloc_copy_allgather, &ub_barrier, &ub_free,
+                                   1, 1, tp_size, 1);
+      }
       comm_created = true;
     }
     use_ce = 0;
@@ -349,7 +419,7 @@ struct UbufCommOverlap : torch::CustomClassHolder, UbufBase {
     // Catch up the default torch stream
     at::cuda::CUDAStream stream_main = at::cuda::getCurrentCUDAStream();
     CHECK_CUDA(cudaEventRecord(_start_compute, stream_main));
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
       CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_compute[i], _start_compute, 0));
     }
     CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_comm, _start_compute, 0));
@@ -467,7 +537,7 @@ struct UbufCommOverlap : torch::CustomClassHolder, UbufBase {
         output_buf_chunk_ptr += output_chunk_size * _ubuf.element_size();
       }
     }
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
       CHECK_CUDA(
           cudaEventRecord(_stop_compute, (cudaStream_t)_stream_compute[i]));
       CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)stream_main, _stop_compute, 0));
@@ -552,15 +622,22 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
   int cga_size;
   bool _atomic_gemm;
 
-  UbufP2PCommOverlap(torch::Tensor sample, int rank, int tp_size, int num_comm_sm,
-                     int comm_cga_size, bool set_sm_margin, bool aggregate2, int num_max_streams,
-                     bool is_reduce_scatter, bool atomic_gemm, torch::Tensor empty_tensor) {
+  UbufP2PCommOverlap(torch::Tensor sample, int rank, int world_size, int tp_rank, int tp_size,
+                     int num_comm_sm, int comm_cga_size, bool set_sm_margin, bool aggregate2,
+                     int num_max_streams, bool is_reduce_scatter, bool atomic_gemm,
+                     torch::Tensor empty_tensor) {
     // Initialize userbuf communicator
     if (!comm_created) {
       if (rank == 0) {
         printf("!!! [UB] Create UbufP2PCommOverlap Communicator\n");
       }
-      create_communicator_grouped2(&_ub_comm, 1, 1, tp_size, 1);
+      if (transformer_engine::getenv<bool>("UB_MPI_BOOTSTRAP")) {
+        create_communicator_grouped2_mpi(&_ub_comm, 1, 1, tp_size, 1);
+      } else {
+        create_communicator_grouped2(&_ub_comm, rank, world_size, tp_rank, tp_size, 1, 1,
+                                     &ub_alloc_copy_allgather, &ub_barrier, &ub_free,
+                                     1, 1, tp_size, 1);
+      }
       comm_created = true;
     }
     use_ce = 1;
@@ -666,7 +743,6 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
     _ub_comm->cga_size = cga_size;
     // Get GEMM dimensions between TN and NN input layouts
     const int m = (transa) ? A.size(0) : A.size(1);
-    const int k = (transa) ? A.size(1) : A.size(0);
     const int n = _ubuf.size(0);
     const int n_chunk = n / _tp_size;
 
@@ -806,7 +882,7 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
 
     CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_send, _start_compute, 0));
     CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_recv, _start_compute, 0));
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
       CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_compute[i], _start_compute, 0));
     }
     if (_aggregate2) {
@@ -924,7 +1000,7 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
         }
       }
     }
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
       CHECK_CUDA(
           cudaEventRecord(_stop_compute, (cudaStream_t)_stream_compute[i]));
       CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)stream_main, _stop_compute, 0));
@@ -953,16 +1029,11 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
     _ub_comm->use_ce = use_ce;
     _ub_comm->sms = sms;
     _ub_comm->cga_size = cga_size;
-    int k = A.size(1);
-    int n = B.size(0);
 
     // Get communication and GEMM input chunk sizes
-    int n_chunk = n / _tp_size;
     const int comm_bytes = _ubufs[0].numel() * _ubufs[0].element_size();
-    const int input_b_chunk_bytes = n_chunk * k * B.element_size();
 
     // Get input and workspace data pointers
-    char *input_b_ptr = reinterpret_cast<char *>(B.data_ptr());
     char *workspace_ptr = reinterpret_cast<char *>(workspace.data_ptr());
     int *counter_ptr = reinterpret_cast<int *>(counter.data_ptr());
     int workspace_size_chunk = workspaceSize / _stream_compute.size();
@@ -1059,7 +1130,7 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
     CHECK_CUDA(cudaEventRecord(_start_compute, (cudaStream_t)stream_main));
     CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_send, _start_compute, 0));
     CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)_stream_recv, _start_compute, 0));
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
         CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t) _stream_compute[i], _start_compute, 0));
     }
 
@@ -1115,7 +1186,7 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
         reduce_buf_ptr, {_tp_size, _ubufs[0].size(0), _ubufs[0].size(1)}, _ubuf.options());
       torch::sum_out(rs_output, reduce_buf, 0);
     }
-    for (int i = 0; i < _stream_compute.size(); i++) {
+    for (size_t i = 0; i < _stream_compute.size(); i++) {
       CHECK_CUDA(
           cudaEventRecord(_stop_compute, (cudaStream_t)_stream_compute[i]));
       CHECK_CUDA(cudaStreamWaitEvent((cudaStream_t)stream_main, _stop_compute, 0));
@@ -1170,3 +1241,5 @@ struct UbufP2PCommOverlap : torch::CustomClassHolder, UbufBase {
 };  // UbufP2PCommOverlap
 
 }  // namespace ubuf
+
+#endif  // TRANSFORMER_ENGINE_PYTORCH_CSRC_COMM_GEMM_OVERLAP_H_

transformer_engine/pytorch/csrc/extensions.h

@@ -4,6 +4,9 @@
  * See LICENSE for license information.
  ************************************************************************/
 
+#ifndef TRANSFORMER_ENGINE_PYTORCH_CSRC_EXTENSIONS_H_
+#define TRANSFORMER_ENGINE_PYTORCH_CSRC_EXTENSIONS_H_
+
 #include "common.h"
 #include "common/common.h"
 
@@ -700,8 +703,6 @@ size_t get_cublasLt_version();
 
 size_t get_cudnn_version();
 
-bool userbuf_comm_available();
-
 void placeholder();
 
 
@@ -786,3 +787,5 @@ void multi_tensor_sgd_cuda(int chunk_size, at::Tensor noop_flag,
                            std::vector<std::vector<at::Tensor>> tensor_lists, float wd,
                            float momentum, float dampening, float lr, bool nesterov, bool first_run,
                            bool wd_after_momentum, float scale);
+
+#endif  // TRANSFORMER_ENGINE_PYTORCH_CSRC_EXTENSIONS_H_

transformer_engine/pytorch/csrc/extensions/misc.cu

@@ -17,13 +17,4 @@ size_t get_cudnn_version() {
     return cudnnGetVersion();
 }
 
-
-bool userbuf_comm_available() {  // TODO(ksivamani) check on python side
-#ifdef NVTE_WITH_USERBUFFERS
-    return true;
-#else
-    return false;
-#endif
-}
-
 void placeholder() {}  // TODO(ksivamani) clean this up

transformer_engine/pytorch/csrc/extensions/pybind.cpp

@@ -4,10 +4,10 @@
  * See LICENSE for license information.
  ************************************************************************/
 
+#include <pybind11/functional.h>
+
 #include "../extensions.h"
-#ifdef NVTE_WITH_USERBUFFERS
-#include "comm_gemm_overlap.h"
-#endif  // NVTE_WITH_USERBUFFERS
+#include "../comm_gemm_overlap.h"
 
 PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
   // Softmax functions
@@ -203,7 +203,6 @@ PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
   // Misc
   m.def("get_cublasLt_version", &get_cublasLt_version, "Get cublasLt version");
   m.def("get_cudnn_version", &get_cudnn_version, "Get cuDNN version");
-  m.def("userbuf_comm_available", &userbuf_comm_available, "If userbuf backend is available");
 
   // Support THD format for Context Parallel
   m.def("thd_read_half_tensor", &thd_read_half_tensor,
@@ -246,7 +245,9 @@ PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
     .def_readwrite("scale_inv", &transformer_engine::FP8TensorMeta::scale_inv)
     .def_readwrite("amax_history", &transformer_engine::FP8TensorMeta::amax_history);
 
-#ifdef NVTE_WITH_USERBUFFERS
+  // comm+GEMM overlap w/ userbuffers
+  m.def("set_ubuf_bootstrap_callbacks", &ubuf::set_ubuf_bootstrap_callbacks);
+
   py::enum_<ubuf::UBOverlapAlgo>(m, "UbufOverlapAlgo")
     .value("BULK_OVERLAP_AG", ubuf::UBOverlapAlgo::BULK_OVERLAP_AG)
     .value("BULK_OVERLAP_RS", ubuf::UBOverlapAlgo::BULK_OVERLAP_RS)
@@ -258,7 +259,8 @@ PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
     .value("ATOMIC_GEMM_RS_P2P", ubuf::UBOverlapAlgo::ATOMIC_GEMM_RS_P2P);
 
   py::class_<ubuf::UbufCommOverlap>(m, "UbufCommOverlap")
-    .def(py::init<torch::Tensor&, int, int, int, int, int, bool, int, bool, torch::Tensor>())
+    .def(py::init<torch::Tensor&, int, int, int, int, int, int, int, bool, int, bool,
+                  torch::Tensor>())
     .def("bulk_overlap", &ubuf::UbufCommOverlap::bulk_overlap)
     .def("split_overlap_rs", &ubuf::UbufCommOverlap::split_overlap_rs)
     .def("set_ubuf_scale_inv", &ubuf::UbufCommOverlap::set_ubuf_scale_inv)
@@ -270,7 +272,8 @@ PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
     .def("is_p2p_overlap", &ubuf::UbufCommOverlap::is_p2p_overlap);
 
   py::class_<ubuf::UbufP2PCommOverlap>(m, "UbufP2PCommOverlap")
-    .def(py::init<torch::Tensor&, int, int, int, int, bool, bool, int, bool, bool, torch::Tensor>())
+    .def(py::init<torch::Tensor&, int, int, int, int, int, int, bool, bool, int, bool, bool,
+         torch::Tensor>())
     .def("split_overlap_ag_p2p", &ubuf::UbufP2PCommOverlap::split_overlap_ag)
     .def("split_overlap_rs_p2p", &ubuf::UbufP2PCommOverlap::split_overlap_rs)
     .def("atomic_gemm_overlap_ag_p2p", &ubuf::UbufP2PCommOverlap::atomic_gemm_overlap_ag)
@@ -281,11 +284,6 @@ PYBIND11_MODULE(TORCH_EXTENSION_NAME, m) {
     .def("is_atomic_gemm", &ubuf::UbufP2PCommOverlap::is_atomic_gemm)
     .def("is_p2p_overlap", &ubuf::UbufP2PCommOverlap::is_p2p_overlap)
     .def("set_ubuf_scale_inv", &ubuf::UbufP2PCommOverlap::set_ubuf_scale_inv);
-#else  // NVTE_WITH_USERBUFFERS
-  m.def("UbufOverlapAlgo", &placeholder, "Dummy function for python side annotations");
-  m.def("UbufCommOverlap", &placeholder, "Dummy function for python side annotations");
-  m.def("UbufP2PCommOverlap", &placeholder, "Dummy function for python side annotations");
-#endif  // NVTE_WITH_USERBUFFERS
 
   py::enum_<transformer_engine::DType>(m, "DType", py::module_local())
     .value("kByte", transformer_engine::DType::kByte)

transformer_engine/pytorch/csrc/userbuffers/CMakeLists.txt

-# Copyright (c) 2022-2024, NVIDIA CORPORATION & AFFILIATES. All rights reserved.
-#
-# See LICENSE for license information.
-
-# Configure userbuffers library
-add_library(transformer_engine_userbuffers SHARED
-            userbuffers.cu
-            userbuffers-host.cpp)
-target_include_directories(transformer_engine_userbuffers PUBLIC
-                           "${CMAKE_CURRENT_SOURCE_DIR}")
-
-# Configure dependencies
-find_package(MPI REQUIRED)
-target_link_libraries(transformer_engine_userbuffers PUBLIC
-                      CUDA::cudart
-                      CUDA::cuda_driver
-                      MPI::MPI_CXX
-                      )
-target_include_directories(transformer_engine_userbuffers PRIVATE
-                           ${CMAKE_CUDA_TOOLKIT_INCLUDE_DIRECTORIES})
-
-# Compiler options
-set_source_files_properties(userbuffers.cu
-                            userbuffers-host.cpp
-                            PROPERTIES
-                            COMPILE_OPTIONS "$<$<COMPILE_LANGUAGE:CUDA>:-maxrregcount=64>")
-set(CMAKE_CUDA_FLAGS "${CMAKE_CUDA_FLAGS} --expt-relaxed-constexpr")
-set(CMAKE_CUDA_FLAGS "${CMAKE_CUDA_FLAGS} -O3")
-
-# Install library
-install(TARGETS transformer_engine_userbuffers DESTINATION .)

transformer_engine/pytorch/csrc/userbuffers/ipcsocket.cc

@@ -47,7 +47,7 @@ ncclResult_t ncclIpcSocketInit(ncclIpcSocket *handle, int rank, uint64_t hash,
   cliaddr.sun_family = AF_UNIX;
 
   // Create unique name for the socket.
-  int len =
+  size_t len =
       snprintf(temp, NCCL_IPC_SOCKNAME_LEN, NCCL_IPC_SOCKNAME_STR, rank, hash);
   if (len > (sizeof(cliaddr.sun_path) - 1)) {
     WARN("UDS: Cannot bind provided name to socket. Name too large");
@@ -76,9 +76,8 @@ ncclResult_t ncclIpcSocketInit(ncclIpcSocket *handle, int rank, uint64_t hash,
   handle->abortFlag = abortFlag;
   // Mark socket as non-blocking
   if (handle->abortFlag) {
-    int flags;
-    EQCHECK(flags = fcntl(fd, F_GETFL), -1);
-    SYSCHECK(fcntl(fd, F_SETFL, flags | O_NONBLOCK), "fcntl");
+    int flags = fcntl(fd, F_GETFL);
+    fcntl(fd, F_SETFL, flags | O_NONBLOCK);
   }
 
   return ncclSuccess;
@@ -193,7 +192,7 @@ ncclResult_t ncclIpcSocketSendMsg(ncclIpcSocket *handle, void *hdr, int hdrLen,
   bzero(&cliaddr, sizeof(cliaddr));
   cliaddr.sun_family = AF_UNIX;
 
-  int len =
+  size_t len =
       snprintf(temp, NCCL_IPC_SOCKNAME_LEN, NCCL_IPC_SOCKNAME_STR, rank, hash);
   if (len > (sizeof(cliaddr.sun_path) - 1)) {
     WARN("UDS: Cannot connect to provided name for socket. Name too large");

transformer_engine/pytorch/csrc/userbuffers/userbuffers-host.cpp

@@ -4,7 +4,6 @@
  * See LICENSE for license information.
  ************************************************************************/
 
-#include "ipcsocket.cc"
 #include "ipcsocket.h"
 #include "userbuffers.h"
 #include <assert.h>
@@ -13,29 +12,24 @@
 #include <cuda_runtime_api.h>
 #include <iostream>
 #include <math.h>
-#include <mpi.h>
 #include <sched.h>
 #include <stdio.h>
 #include <string.h>
 #include <unistd.h>
-#define MULTICAST_GB_TOTAL 512
-
-static int oob_bcast(void *comm_context, void *buf, int size, int root) {
-  MPI_Bcast(buf, size, MPI_BYTE, root,
-            (reinterpret_cast<communicator *>(comm_context))->comm_inter);
-  return 0;
-}
+#include <inttypes.h>
 
-static int oob_barrier(void *comm_context) {
-  MPI_Barrier((reinterpret_cast<communicator *>(comm_context))->comm_inter);
-  return 0;
-}
+#ifdef UB_MPI_BOOTSTRAP
+#include <mpi.h>
+static MPI_Comm EXT_COMM_WORLD = MPI_COMM_WORLD;
+static MPI_Comm EXT_COMM_INTRA;
+static MPI_Comm EXT_COMM_INTER;
+#else
+static char EXT_COMM_WORLD[] = "world";
+static char EXT_COMM_INTRA[] = "intra";
+static char EXT_COMM_INTER[] = "inter";
+#endif
 
-static int oob_gather(void *comm_context, int root, void *sbuf, void *rbuf, int len) {
-  MPI_Gather(sbuf, len, MPI_BYTE, rbuf, len, MPI_BYTE, root,
-             (reinterpret_cast<communicator *>(comm_context))->comm_inter);
-  return 0;
-}
+#define MULTICAST_GB_TOTAL 512
 
 int stringCmp(const void *a, const void *b) { return strcmp((const char *)a, (const char *)b); }
 
@@ -95,18 +89,24 @@ int pipe_rank(communicator *comm, int step) {
   return newnode * numlocal + newlocal;
 }
 
-int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenodes, int tensorgpus,
-                                 int tensornodes) {
+int create_communicator_grouped2(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free,
+  int pipegpus, int pipenodes, int tensorgpus, int tensornodes) {
   *comm = reinterpret_cast<communicator *>(malloc(sizeof(communicator)));
 
-  int myrank, nranks, cur_dev, ndev;
-  MPI_Comm_rank(MPI_COMM_WORLD, &myrank);
-  MPI_Comm_size(MPI_COMM_WORLD, &nranks);
-  (*comm)->nranks = nranks;
+  (*comm)->comm_world = EXT_COMM_WORLD;
+  (*comm)->_alloc_copy_allgather = ext_alloc_copy_allgather;
+  (*comm)->_barrier = ext_barrier;
+  (*comm)->_free = ext_free;
+  (*comm)->nranks = numranks;
   (*comm)->myrank = myrank;
   (*comm)->free_region = 0;
   (*comm)->launch_mode = NVTE_LAUNCH_GPU | NVTE_LAUNCH_CPU;
 
+  int cur_dev, ndev;
   cudaDeviceProp device_prop;
   CUDACHECK(cudaGetDevice(&cur_dev));
   CUDACHECK(cudaGetDeviceCount(&ndev));
@@ -135,34 +135,7 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
             sec_timeout, (*comm)->ub_timeout, device_clock);
   }
 
-  int ret = 0;
-  // split communicator
-  char host_name[MPI_MAX_PROCESSOR_NAME];
-  char(*host_names)[MPI_MAX_PROCESSOR_NAME];
-  int namelen, bytes, color, my_node, mylocal, numlocal, num_nodes;
-  int rank = (*comm)->myrank, size = (*comm)->nranks;
-  MPI_Get_processor_name(host_name, &namelen);
-  bytes = size * sizeof(char[MPI_MAX_PROCESSOR_NAME]);
-  host_names = (char(*)[MPI_MAX_PROCESSOR_NAME])malloc(bytes);
-  strcpy(host_names[rank], host_name);  // NOLINT(*)
-  for (int n = 0; n < size; n++)
-    MPI_Bcast(&(host_names[n]), MPI_MAX_PROCESSOR_NAME, MPI_CHAR, n, MPI_COMM_WORLD);
-  qsort(host_names, size, sizeof(char[MPI_MAX_PROCESSOR_NAME]), stringCmp);
-
-  color = 0;
-  for (int n = 0; n < size; n++) {
-    if (n > 0 && strcmp(host_names[n - 1], host_names[n]))
-      color++;
-    if (strcmp(host_name, host_names[n]) == 0)
-      break;
-  }
-  free(host_names);
-
-  MPI_Comm_split(MPI_COMM_WORLD, color, rank, &(*comm)->comm_intra);
-  // find intranode numbers and make internode communicator
-  // figure out mylocal
-  MPI_Comm_rank((*comm)->comm_intra, &mylocal);
-  MPI_Comm_size((*comm)->comm_intra, &numlocal);
+  (*comm)->comm_intra = EXT_COMM_INTRA;
   (*comm)->nvrank = mylocal;
   (*comm)->nvsize = numlocal;
 
@@ -198,7 +171,7 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
 
   if (ndev == numlocal) {  // all visible devices
     if (cur_dev != mylocal)
-      printf("%d: device used %d[%d] ,resetting device to %d\n", rank, cur_dev, ndev, mylocal);
+      printf("%d: device used %d[%d] ,resetting device to %d\n", myrank, cur_dev, ndev, mylocal);
     CUDACHECK(cudaSetDevice(mylocal));
   }
   (*comm)->mydev = cur_dev;
@@ -214,31 +187,22 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
   (*comm)->ar2_firstgpu = mylocal - mylocal % tensorgpus;
   (*comm)->ar2_nvrank = mylocal - (*comm)->ar2_firstgpu;
   // ar2 has step equal to ar_nvsize
-  int allnodes = nranks / numlocal;
-  int mynode = myrank / numlocal;
+  int allnodes = numranks / numlocal;
+  int nodeid = myrank / numlocal;
   int datanodes = allnodes / pipenodes / tensornodes;
   int pipenodegroup_id = myrank / numlocal / (datanodes * tensornodes);
 
   (*comm)->pipe_id = pipegpus * pipenodegroup_id + mylocal / (datagpus * tensorgpus);
 
-  CUDACHECK(cudaFree(0));
-  int datanodegroup_id =
-      myrank / numlocal / datanodes;  // data reduction group node belongs, equals 0 for all if both
-                                      // pipenodes=1 and tensornodes=1
-  // mpi communicator only needed for SHARP which is always
-  // allreduce1/data-parallel
-  MPI_Comm_split(MPI_COMM_WORLD, mylocal + numlocal * datanodegroup_id, rank, &(*comm)->comm_inter);
-  // different rails from same group are in different subcommunicators
-
-  MPI_Comm_size((*comm)->comm_inter, &num_nodes);
-  MPI_Comm_rank((*comm)->comm_inter, &my_node);
-  (*comm)->first_node = mynode - my_node;
-  (*comm)->num_nodes = num_nodes;
-  (*comm)->my_node = my_node;
+  (*comm)->comm_inter = EXT_COMM_INTER;
+  (*comm)->first_node = nodeid - mynode;
+  (*comm)->num_nodes = numnodes;
+  (*comm)->my_node = mynode;
 
   (*comm)->num2_nodes = tensornodes;
   (*comm)->my2_node = (mynode / datanodes) % tensornodes;
   (*comm)->first2_node = mynode - (*comm)->my2_node * datanodes;
+
   (*comm)->fifo = reinterpret_cast<ub_request *>(malloc(sizeof(ub_request) * NVTE_MAX_REQUESTS));
   (*comm)->nblocks = 8;
   (*comm)->alignblock = 1024 * 512;
@@ -262,28 +226,33 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
     mcProp.size = mc_maxsize;
     (*comm)->mc_maxsize = mc_maxsize;
 
+    // Broadcast the a POSIX file descriptor from the local root rank to other local ranks.
+    // NOTE: This cannot be done via MPI_Bcast or other external comm libraries. They mangle the
+    //       file descriptor and prevent cuMemImportFromShareableHandle() from correctly
+    //       interpreting the file. Instead, we use system socket to send/recv the file handle
+    //       without mangling.
     int fd;
     volatile uint32_t abortFlag = 0;
     struct ncclIpcSocket ipcSock = {0};
     uint64_t opId = 0xdeadcafeb000 + (*comm)->ar2_firstgpu;
     ncclResult_t ret = ncclSuccess;
     NCCLCHECK(ncclIpcSocketInit(&ipcSock, (*comm)->ar2_nvrank, (uint64_t)opId, &abortFlag));
-    MPI_Barrier(MPI_COMM_WORLD);
+    (*comm)->_barrier((*comm)->comm_world);
 
     if ((*comm)->ar2_nvrank == 0) {
       CUCHECK(cuMulticastCreate(&(*comm)->mc_handle, &mcProp));
       CUCHECK(cuMemExportToShareableHandle(&fd, (*comm)->mc_handle,
                                            CU_MEM_HANDLE_TYPE_POSIX_FILE_DESCRIPTOR, 0 /*flags*/));
       for (int p = 1; p < (*comm)->ar2_nvsize; p++) {
-        MPI_Barrier((*comm)->comm_intra);
+        (*comm)->_barrier((*comm)->comm_intra);
         NCCLCHECKGOTO(ncclIpcSocketSendFd(&ipcSock, fd, p, (uint64_t)opId), ret, error);
       }
     } else {
       for (int i = 0; i < (*comm)->ar2_nvrank; i++)
-        MPI_Barrier((*comm)->comm_intra);
+        (*comm)->_barrier((*comm)->comm_intra);
       NCCLCHECKGOTO(ncclIpcSocketRecvFd(&ipcSock, &fd), ret, error);
       for (int i = 0; i < (*comm)->ar2_nvsize - (*comm)->ar2_nvrank - 1; i++)
-        MPI_Barrier((*comm)->comm_intra);
+        (*comm)->_barrier((*comm)->comm_intra);
       CUCHECK(cuMemImportFromShareableHandle(&(*comm)->mc_handle, reinterpret_cast<void *>(fd),
                                              CU_MEM_HANDLE_TYPE_POSIX_FILE_DESCRIPTOR));
     }
@@ -303,7 +272,7 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
     CUCHECK(cuMemSetAccess(mc_va, mc_maxsize, &accessDesc, 1));
 
     (*comm)->mc_baseptr = reinterpret_cast<void *>(mc_va);
-    MPI_Barrier(MPI_COMM_WORLD);
+    (*comm)->_barrier((*comm)->comm_world);
     if (!(*comm)->myrank)
       printf("MC initialized succesfully, window size = %ld\n", mc_maxsize);
   } else {
@@ -317,12 +286,10 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
 #define LOCALSIZE 4 * (NVTE_REG0_OFFSET(*comm) + NVTE_REG0_FLAGS + NVTE_REG0_COMMBUFFER * NBUF)
   // peer pointers + op flags + comm buffer
 
-  CUDACHECK(cudaMalloc(&(*comm)->gpu_ptrs,
-                       LOCALSIZE));  // flags and pointers, no block data yet
+  CUDACHECK(cudaMalloc(&(*comm)->gpu_ptrs, LOCALSIZE));  // flags and pointers, no block data yet
   CUDACHECK(cudaMemset((*comm)->gpu_ptrs, 0, LOCALSIZE));
   CUDACHECK(cudaDeviceSynchronize());
-  register_user_buffer_collective(&((*comm)->gpu_ptrs), LOCALSIZE,
-                                  *comm);  // will use handler 0
+  register_user_buffer_collective(&((*comm)->gpu_ptrs), LOCALSIZE, *comm, false);
   CUDACHECK(cudaMalloc(&(*comm)->send_id, (*comm)->nranks * sizeof(int)));
   CUDACHECK(cudaMalloc(&(*comm)->recv_id, NVTE_MAX_REGIONS * (*comm)->nranks * sizeof(int)));
   CUDACHECK(cudaMemset((*comm)->send_id, 0, (*comm)->nranks * sizeof(int)));
@@ -336,12 +303,12 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
 #define GPU_PAGE_MASK (~GPU_PAGE_OFFSET)
 
   CUDACHECK(cudaMalloc(&(*comm)->flags, 2 * GPU_PAGE_SIZE));
-  unsigned int flag = 1;
   CUDACHECK(cudaMemset((*comm)->flags, 0, 2 * GPU_PAGE_SIZE));
   (*comm)->flags =
       reinterpret_cast<int *>(((CUdeviceptr)(*comm)->flags + GPU_PAGE_SIZE - 1) & GPU_PAGE_MASK);
 
   using namespace std;
+
   sched_param param;
   pthread_attr_t attr;
   pthread_attr_init(&attr);
@@ -353,7 +320,7 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
   if (getenv("NVTE_UBDEBUG"))
     printf("%d/%d:(%d x %d): DP %d x %d TP %d x %d, DPGROUP %dx%d TPGROUP "
            "%dx%d PIPE_ID %d/%d\n",
-           myrank, nranks, myrank / numlocal, myrank % numlocal, (*comm)->my_node,
+           myrank, numranks, myrank / numlocal, myrank % numlocal, (*comm)->my_node,
            (*comm)->ar_nvrank, (*comm)->my2_node, (*comm)->ar2_nvrank, (*comm)->num_nodes,
            (*comm)->ar_nvsize, (*comm)->num2_nodes, (*comm)->ar2_nvsize, (*comm)->pipe_id,
            pipegpus * pipenodes);
@@ -361,15 +328,141 @@ int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenode
 
   return 0;
 }
-int create_communicator_grouped(communicator **comm, int pipegpus, int pipenodes) {
-  return create_communicator_grouped2(comm, pipegpus, pipenodes, 1, 1);
+
+int create_communicator_grouped(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free,
+  int pipegpus, int pipenodes) {
+  return create_communicator_grouped2(
+    comm, myrank, numranks, mylocal, numlocal, mynode, numnodes,
+    ext_alloc_copy_allgather, ext_barrier, ext_free,
+    pipegpus, pipenodes, 1, 1);
+}
+
+int create_communicator(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free) {
+  return create_communicator_grouped2(
+    comm, myrank, numranks, mylocal, numlocal, mynode, numnodes,
+    ext_alloc_copy_allgather, ext_barrier, ext_free,
+    1, 1, 1, 1);
+}
+
+int create_communicator_grouped2_mpi(communicator **comm,
+  int pipegpus, int pipenodes, int tensorgpus, int tensornodes) {
+#ifdef UB_MPI_BOOTSTRAP
+  // get global numbers
+  int myrank, numranks;
+  MPI_Comm_rank(EXT_COMM_WORLD, &myrank);
+  MPI_Comm_size(EXT_COMM_WORLD, &numranks);
+
+  // find intranode numbers and make internode communicator
+  char host_name[MPI_MAX_PROCESSOR_NAME];
+  char(*host_names)[MPI_MAX_PROCESSOR_NAME];
+  int namelen, bytes, color;
+  int rank = (*comm)->myrank, size = (*comm)->nranks;
+  MPI_Get_processor_name(host_name, &namelen);
+  bytes = size * sizeof(char[MPI_MAX_PROCESSOR_NAME]);
+  host_names = (char(*)[MPI_MAX_PROCESSOR_NAME])malloc(bytes);
+  strcpy(host_names[rank], host_name);  // NOLINT(*)
+  for (int n = 0; n < size; n++)
+    MPI_Bcast(&(host_names[n]), MPI_MAX_PROCESSOR_NAME, MPI_CHAR, n, EXT_COMM_WORLD);
+  qsort(host_names, size, sizeof(char[MPI_MAX_PROCESSOR_NAME]), stringCmp);
+
+  color = 0;
+  for (int n = 0; n < size; n++) {
+    if (n > 0 && strcmp(host_names[n - 1], host_names[n]))
+      color++;
+    if (strcmp(host_name, host_names[n]) == 0)
+      break;
+  }
+  free(host_names);
+
+  int mylocal, numlocal;
+  MPI_Comm_split(EXT_COMM_WORLD, color, rank, &EXT_COMM_INTRA);
+  MPI_Comm_rank(EXT_COMM_INTRA, &mylocal);
+  MPI_Comm_size(EXT_COMM_INTRA, &numlocal);
+
+  // find internode numbers and make internode communicator
+  CUDACHECK(cudaFree(0));
+  int allnodes = numranks / numlocal;
+  int datanodes = allnodes / pipenodes / tensornodes;
+  // data reduction group node belongs, equals 0 for all if both pipenodes=1 and tensornodes=1
+  int datanodegroup_id = myrank / numlocal / datanodes;
+  // mpi communicator only needed for SHARP which is always allreduce1/data-parallel
+  MPI_Comm_split(EXT_COMM_WORLD, mylocal + numlocal * datanodegroup_id, rank, &EXT_COMM_INTER);
+  // different rails from same group are in different subcommunicators
+  int mynode, numnodes;
+  MPI_Comm_size(EXT_COMM_INTER, &numnodes);
+  MPI_Comm_rank(EXT_COMM_INTER, &mynode);
+
+  // finally call the abstracted constructor with MPI info
+  return create_communicator_grouped2(comm,
+    myrank, numranks, mylocal, numlocal, mynode, numnodes,
+    &ub_alloc_copy_allgather, &ub_barrier, &ub_free,
+    pipegpus, pipenodes, tensorgpus, tensornodes);
+#else
+  NVTE_UB_ERROR(std::string("Bootstrapping Userbuffers with MPI requires ") +
+                std::string("building Transformer Engine with UB_MPI_BOOTSTRAP=1"));
+#endif
+}
+
+int create_communicator_grouped_mpi(communicator **comm, int pipegpus, int pipenodes) {
+  return create_communicator_grouped2_mpi(comm, pipegpus, pipenodes, 1, 1);
 }
 
-int create_communicator(communicator **comm) {
-  return create_communicator_grouped2(comm, 1, 1, 1, 1);
+int create_communicator_mpi(communicator **comm) {
+  return create_communicator_grouped2_mpi(comm, 1, 1, 1, 1);
 }
 
 void destroy_communicator(communicator *comm) {
+  for (int hndl = 0; hndl < comm->free_region; hndl++) {
+    if (comm->mem_dealloc[hndl]) {
+      cuMemAddressFree(reinterpret_cast<CUdeviceptr>(comm->ucbase_ptr[hndl]),
+                       comm->mem_size[hndl] * comm->nvsize);
+      for (int rank = 0; rank < comm->nvsize; rank++) {
+        cuMemRelease(comm->uchandles[hndl][rank]);
+      }
+      free(reinterpret_cast<void *>(comm->uchandles[hndl]));
+    } else {
+      for (int rank = 0; rank < comm->nvsize; rank++) {
+        if (rank != comm->nvrank) {
+          cudaIpcCloseMemHandle(comm->peer_ptr[hndl][rank]);
+        } else {
+          comm->peer_ptr[hndl][rank] = nullptr;  // remove reference to external buffer
+        }
+      }
+      free(comm->peer_ptr[hndl]);
+    }
+    comm->mem_ptr[hndl] = nullptr;
+  }
+  cudaFree(reinterpret_cast<void *>(comm->flags));
+  cudaFree(reinterpret_cast<void *>(comm->recv_id));
+  cudaFree(reinterpret_cast<void *>(comm->send_id));
+  if (comm->use_mc) {
+    cuMemAddressFree(reinterpret_cast<CUdeviceptr>(comm->mc_baseptr), comm->mc_maxsize);
+    cuMemRelease(comm->mc_handle);
+  }
+  if (comm->mem_dealloc[0]) {
+    cudaFree(comm->gpu_ptrs);
+  }
+  free(comm->fifo);
+  free(comm);
+}
+
+void destroy_communicator_mpi(communicator *comm) {
+#ifdef UB_MPI_BOOTSTRAP
+  MPI_Comm_free(comm->comm_inter);
+  MPI_Comm_free(comm->comm_intra);
+  destroy_communicator(comm);
+#else
+  NVTE_UB_ERROR(std::string("Communicator is not bootstrapped with MPI and ") +
+                std::string("can only be deallocated with destroy_communicator()."));
+#endif
 }
 
 int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *comm, bool alloc) {
@@ -379,6 +472,7 @@ int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *
   comm->peer_ptr[hndl] = reinterpret_cast<void **>(malloc(sizeof(void *) * (comm->nvsize)));
   size_t aligned_size = bytes;
   comm->memflags[hndl] = 0;
+  comm->mem_dealloc[hndl] = alloc;
 
   if (alloc) {
     int nranks = comm->nvsize;  // total GPUs in NVLINK domain
@@ -420,9 +514,14 @@ int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *
     uint64_t opId = 0xdeadcafebeef;
     ncclResult_t ret = ncclSuccess;
 
+    // All-gather POSIX file descriptors across local ranks.
+    // NOTE: This cannot be done via MPI_Allgather or other external comm libraries. They mangle
+    //       the file descriptor and prevent cuMemImportFromShareableHandle() from correctly
+    //       interpreting the file. Instead, we use system socket to send/recv the file handle
+    //       without mangling.
     NCCLCHECK(ncclIpcSocketInit(&ipcSock, myrank, (uint64_t)opId, &abortFlag));
     for (int p = 1; p < nranks; p++) {
-      MPI_Barrier(comm->comm_intra);
+      comm->_barrier(comm->comm_intra);
       NCCLCHECKGOTO(
           ncclIpcSocketSendFd(&ipcSock, peerfd[myrank], (myrank + p) % nranks, (uint64_t)opId), ret,
           error);
@@ -482,18 +581,20 @@ int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *
 
   } else {
     assert(comm->nvsize <= 8);
-    cudaIpcMemHandle_t *memhndl =
-        reinterpret_cast<cudaIpcMemHandle_t *>(malloc(sizeof(cudaIpcMemHandle_t) * (comm->nvsize)));
-
-    CUDACHECK(cudaIpcGetMemHandle(&memhndl[comm->nvrank], *gpubuff));
-
-    MPI_Allgather(&memhndl[comm->nvrank], sizeof(cudaIpcMemHandle_t), MPI_BYTE, memhndl,
-                  sizeof(cudaIpcMemHandle_t), MPI_BYTE, comm->comm_intra);
-
-    for (int i = 0; i < comm->nvsize; i++)
-      if (i != comm->nvrank)
-        CUDACHECK(cudaIpcOpenMemHandle((void **)&(comm->peer_ptr[hndl][i]),  // NOLINT(*)
-                                       memhndl[i], cudaIpcMemLazyEnablePeerAccess));
+    cudaIpcMemHandle_t memhndl;
+    CUDACHECK(cudaIpcGetMemHandle(&memhndl, *gpubuff));
+
+    cudaIpcMemHandle_t *tmp;
+    comm->_alloc_copy_allgather(
+      reinterpret_cast<void **>(&tmp), reinterpret_cast<void *>(&memhndl),
+      sizeof(cudaIpcMemHandle_t), comm->comm_intra);
+
+    for (int i = 0; i < comm->nvsize; i++) {
+      if (i != comm->nvrank) {
+        CUDACHECK(cudaIpcOpenMemHandle(&(comm->peer_ptr[hndl][i]), tmp[i],   // NOLINT(*)
+                                       cudaIpcMemLazyEnablePeerAccess));
+      }
+    }
     comm->peer_ptr[hndl][comm->nvrank] = *gpubuff;
     CUDACHECK(cudaDeviceSynchronize());
 
@@ -502,7 +603,7 @@ int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *
         comm->peer_ptr[hndl], comm->nvsize * sizeof(void *), cudaMemcpyHostToDevice));
 
     CUDACHECK(cudaDeviceSynchronize());
-    free(memhndl);
+    comm->_free(tmp);
   }
   comm->mem_size[hndl] = aligned_size;
 

transformer_engine/pytorch/csrc/userbuffers/userbuffers.h

@@ -8,11 +8,56 @@
 #define TRANSFORMER_ENGINE_USERBUFFERS_H_
 
 #include <cuda.h>
-#include <mpi.h>  // TODO (tym): Removing will remove PyT extension dependence on MPI
-#include "cuda_runtime.h"
+#include <cuda_runtime.h>
 #include <pthread.h>
 #include <chrono>
 #include <stdexcept>
+#include <functional>
+
+#ifdef UB_MPI_BOOTSTRAP
+#include <stdexcept>
+#include <mpi.h>
+
+#define UB_MPI_CHECK(expr)                                                                       \
+  do {                                                                                           \
+    const int mpicode = (expr);                                                                  \
+    if (mpicode != MPI_SUCCESS) {                                                                \
+      char mpimsg[MPI_MAX_ERROR_STRING];                                                         \
+      int mpilen;                                                                                \
+      MPI_Error_string(mpicode, mpimsg, &mpilen);                                                \
+      std::vector<char> errmsg(1024);  \
+      snprintf(errmsg.data(), errmsg.size(), "%s:%s in function %s: %s",  \
+               __FILE__, __LINE__, __func__, mpimsg);  \
+      throw std::runtime_error(errmsg.data());  \
+    }                                                                                            \
+  } while (false)
+
+typedef MPI_Comm ExtComm;
+
+void ub_alloc_copy_allgather(void **globaldata, void *localdata, size_t localbytes, ExtComm comm) {
+  int myrank, nranks;
+  UB_MPI_CHECK(MPI_Comm_rank(comm, &myrank));
+  UB_MPI_CHECK(MPI_Comm_size(comm, &nranks));
+  *globaldata = malloc(nranks * localbytes);
+  UB_MPI_CHECK(MPI_Allgather(localdata,
+                             localbytes,
+                             MPI_BYTE,
+                             *globaldata,
+                             nranks * localbytes,
+                             MPI_BYTE,
+                             comm));
+}
+
+void ub_barrier(ExtComm comm) {
+  UB_MPI_CHECK(MPI_Barrier(comm));
+}
+
+void ub_free(void *ptr) {
+  free(ptr);
+}
+#else
+typedef char* ExtComm;
+#endif
 
 #define NVTE_MAX_REGIONS 16
 #define NVTE_MAX_SMS 32
@@ -99,6 +144,7 @@ struct communicator {
   CUmemGenericAllocationHandle *uchandles[NVTE_MAX_REGIONS];
   void* ucbase_ptr[NVTE_MAX_REGIONS];  // only for cuMem allocated memory
   size_t mem_size[NVTE_MAX_REGIONS];
+  bool mem_dealloc[NVTE_MAX_REGIONS];
 
   void* mc_ptr[NVTE_MAX_REGIONS];
   void* mc_baseptr;
@@ -130,9 +176,18 @@ struct communicator {
   int padding2[15];
   volatile int tail;
 
+  // Abstract communication callbacks to support external bootstrapping (e.g. DL frameworks)
+  std::function<void(void**, void*, size_t, ExtComm)> _alloc_copy_allgather;
+  std::function<void(ExtComm)> _barrier;
+  std::function<void(void*)> _free;
+
+  ExtComm comm_world,
+          comm_inter,       // reduction group communicator (subset of the nodes) along GPU rail
+          comm_intra;       // full intranode (all ndev GPUS)
+#ifdef UB_MPI_BOOTSTRAP
   MPI_Request mpihndl[NVTE_MAX_SHARP];
-  MPI_Comm comm_inter,  // reduction group communicator (subset of the nodes) along GPU rail
-      comm_intra;       // full intranode (all ndev GPUS)
+#endif
+
   int *send_id, *recv_id;
   int mydev;
   uint64_t ub_timeout;
@@ -142,18 +197,38 @@ typedef struct communicator communicator;
 void producer(void *atomic_ptr, int chunk_i, cudaStream_t stream);
 void consumer(void *atomic_ptr, int chunk_i, cudaStream_t stream);
 void consumer_batch(void *atomic_ptr, int first_chunk_i, int num_chunks, cudaStream_t stream);
-int create_communicator(communicator **comm);
+
 /*  creates communicator, allocates all internal buffers if necessary */
+int create_communicator_grouped2(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free,
+  int pipegpus, int pipenodes, int tensorgpus, int tensornodes);
+
+int create_communicator_grouped(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free,
+  int pipegpus, int pipenodes);
+
+int create_communicator(communicator **comm,
+  int myrank, int numranks, int mylocal, int numlocal, int mynode, int numnodes,
+  std::function<void(void**, void*, size_t, ExtComm)> ext_alloc_copy_allgather,
+  std::function<void(ExtComm)> ext_barrier,
+  std::function<void(void*)> ext_free);
+
+int create_communicator_grouped2_mpi(communicator **comm,
+  int pipegpus, int pipenodes, int tensorgpus, int tensornodes);
+
+int create_communicator_grouped_mpi(communicator **comm, int pipegpus, int pipenodes);
+
+int create_communicator_mpi(communicator **comm);
+
+void destroy_communicator(communicator *comm);
 
-int create_communicator_grouped(communicator **comm, int pipegpus, int pipenodes);
-int create_communicator_grouped2(communicator **comm, int pipegpus, int pipenodes, int tensorgpus,
-                                 int tensornodes);
-/*  creates communicator with
-    allreduce1 to happen in datagpus x datanodes groups,
-    allreduce2 to happen in tensorgpus x tensor nodes,
-        where num_nodes = pipenodes x tensornodes x datanodes
-            nvlink_size = pipegpus x tensorgpus x datagpus
- */
+void destroy_communicator_mpi(communicator *comm);
 
 // int check_user_buffer_registration(void* gpubuff, int bytes, communicator* comm, size_t* offset);
 /*
@@ -167,8 +242,7 @@ int pipe_rank(communicator *comm,
                           // data-parallel and tensor-parallel position within data and tensor
                           // groups would be preserved
 
-int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *comm,
-                                    bool alloc = false);
+int register_user_buffer_collective(void **gpubuff, size_t bytes, communicator *comm, bool alloc);
 /*  returns handler and registers buffers. assumed to be collective i.e. you use same groups and
    dont mix buffers for different operations returns -1 if cant register (too many preregistered
    regions already) if alloc==true will allocate memory and fill the pointers (required for NVL

transformer_engine/pytorch/module/__init__.py

@@ -8,3 +8,4 @@ from .linear import Linear
 from .layernorm_mlp import LayerNormMLP
 from .layernorm import LayerNorm
 from .rmsnorm import RMSNorm
+from .base import initialize_ub, destroy_ub

transformer_engine/pytorch/module/base.py

@@ -36,6 +36,8 @@ from ..cpp_extensions import (
 from ..constants import dist_group_type
 from ..float8_tensor import Float8Tensor
 
+__all__ = ["initialize_ub", "destroy_ub"]
+
 _2X_ACC_FPROP = False
 _2X_ACC_DGRAD = True
 _2X_ACC_WGRAD = True
@@ -64,7 +66,7 @@ def get_workspace() -> torch.Tensor:
 
 def initialize_ub(
     shape: list,
-    tp_size: int,
+    tp_group: dist_group_type,
     use_fp8: bool = False,
     dtype: torch.dtype = torch.bfloat16,
     ub_cfgs: Optional[dict] = None
@@ -74,6 +76,9 @@ def initialize_ub(
     assert _ub_communicators is None, "UB communicators are already initialized."
     _ub_communicators = {}
     rank_id = torch.distributed.get_rank()
+    world_size = torch.distributed.get_world_size()
+    tp_id = torch.distributed.get_rank(tp_group)
+    tp_size = torch.distributed.get_world_size(tp_group)
 
     # Increase the workspace by the number of maximum concurrent streams
     global _cublas_workspace
@@ -158,6 +163,8 @@ def initialize_ub(
             ub_obj = tex.UbufP2PCommOverlap(
                     sample_buffer,          # Sample userbuffer
                     rank_id,                # Rank id
+                    world_size,             # World size
+                    tp_id,                  # TP id
                     tp_size,                # TP size
                     num_sm,                 # Number of communication SMs
                     cga_size,               # CGA cluster size
@@ -172,6 +179,8 @@ def initialize_ub(
             ub_obj = tex.UbufCommOverlap(
                     sample_buffer,          # Sample userbuffer
                     rank_id,                # Rank id
+                    world_size,             # World size
+                    tp_id,                  # TP id
                     tp_size,                # TP size
                     num_sm,                 # Number of communication SMs
                     cga_size,               # CGA cluster size
@@ -183,6 +192,26 @@ def initialize_ub(
                 )
         _ub_communicators[name] = ub_obj
 
+    def alloc_copy_allgather_callback(local_data: torch.Tensor, group: str) -> torch.Tensor:
+        pg = None if group == "world" else tp_group
+        global_size = local_data.numel() * torch.distributed.get_world_size(pg)
+        global_data = torch.zeros(global_size, dtype=local_data.dtype, device='cuda')
+        torch.distributed.all_gather_into_tensor(global_data, local_data.cuda(), group=pg)
+        return global_data.cpu()
+
+    def barrier_callback(group: str) -> None:
+        pg = None if group == "world" else tp_group
+        torch.distributed.barrier(group=pg)
+
+    def free_callback(data: torch.Tensor) -> None:
+        data.data = torch.Tensor()
+
+    tex.set_ubuf_bootstrap_callbacks(
+        alloc_copy_allgather_callback,
+        barrier_callback,
+        free_callback
+    )
+
     if ub_cfgs is not None:
         for name in dgrad_reduce_scatter_overlap:
             if name in ub_cfgs and 'method' in ub_cfgs[name] and ub_cfgs[name]['method'] != 'bulk':
@@ -235,6 +264,13 @@ def get_ub(name: str):
     assert name in _ub_communicators, f"UB for {name} is not registered."
     return _ub_communicators[name]
 
+def destroy_ub():
+    """Destroy all allocated userbuffer communicators."""
+    global _ub_communicators
+    _ub_communicators = None
+    global layers_atomic_ring_exchange
+    layers_atomic_ring_exchange = []
+
 
 class TransformerEngineBaseModule(torch.nn.Module, ABC):
     """Base TE module."""

transformer_engine/pytorch/module/layernorm_linear.py

@@ -865,11 +865,6 @@ class LayerNormLinear(TransformerEngineBaseModule):
             assert ub_name is not None, "Userbuffer name [string] is not set."
         self.ub_name = ub_name
 
-        if any([ub_bulk_wgrad, ub_bulk_dgrad, ub_overlap_ag]):
-            assert (
-                tex.userbuf_comm_available()
-            ), "Userbuffer communication backend not available."
-
         if tp_group is None:
             self.tp_size = tp_size
             if tp_size == 1:

transformer_engine/pytorch/module/layernorm_mlp.py

@@ -1290,11 +1290,6 @@ class LayerNormMLP(TransformerEngineBaseModule):
             (bool(int(os.getenv("NVTE_GEMM_GELU_FUSION", "0"))) and
             self.activation == 'gelu' and not get_ub("fc1_fprop").is_atomic_gemm())
 
-        if any([ub_bulk_wgrad, ub_bulk_dgrad, ub_overlap_rs, ub_overlap_ag, ub_overlap_rs_dgrad]):
-            assert (
-                tex.userbuf_comm_available()
-            ), "Userbuffer communication backend not available."
-
         if tp_group is None:
             self.tp_size = tp_size
             if tp_size == 1: