Add UCCL backend integration for NIXL

meson_options.txt

@@ -19,6 +19,7 @@ option('etcd_inc_path', type: 'string', value: '', description: 'Path to ETCD He
 option('etcd_lib_path', type: 'string', value: '', description: 'Path to ETCD Libraries')
 option('disable_gds_backend', type : 'boolean', value : false, description : 'disable gds backend')
 option('disable_mooncake_backend', type : 'boolean', value : false, description : 'disable mooncake backend')
+option('disable_uccl_backend', type : 'boolean', value : false, description : 'disable uccl backend')
 option('install_headers', type : 'boolean', value : true, description : 'install headers')
 option('gds_path', type: 'string', value: '/usr/local/cuda/', description: 'Path to GDS CuFile install')
 option('cudapath_inc', type: 'string', value: '', description: 'Include path for CUDA')

src/api/python/_api.py

@@ -232,6 +232,8 @@ def __init__(
                         init["num_threads"] = str(nixl_conf.num_threads)
                     elif bknd == "GDS_MT":
                         init["thread_count"] = str(nixl_conf.num_threads)
+                    elif bknd == "Uccl":
+                        init["num_cpus"] = str(nixl_conf.num_threads)
                 self.create_backend(bknd, init)
 
         self.nixl_mems = {

src/plugins/meson.build

@@ -39,6 +39,11 @@ if libtransfer_engine.found() and not get_option('disable_mooncake_backend')
     subdir('mooncake')
 endif
 
+libuccl_engine = cc.find_library('uccl_engine', required: false)
+if libuccl_engine.found() and not get_option('disable_uccl_backend')
+    subdir('uccl')
+endif
+
 hf3fs_lib_path = '/usr/lib/'
 hf3fs_lib_file = 'hf3fs_api_shared'
 hf3fs_lib_found = cc.find_library(hf3fs_lib_file, dirs: [hf3fs_lib_path], required: false)

src/plugins/uccl/README.md

@@ -0,0 +1,53 @@
+## UCCL Backend Plugin [Preview]
+
+[UCCL](https://github.com/uccl-project/uccl) is an efficient communication library to perform GPU memory transfers, with a focus on flexibility (evolving ML workloads) and portability (heteregenous GPUs). UCCL provides a software transport stack which runs on the CPUs and are easily extensible to support different techniques like congestion control, multipathing, efficient loss recovery, etc.
+UCCL supports collectives, p2p communication and gpu-driven communication for expert parallelism.
+
+## Capabilities
+
+Currently, the UCCL backend supports internode communication over RDMA. Intranode communication will be added soon.
+
+## Installation Guide
+
+1. Install UCCL's p2p engine manually. You can refer to the [installation guide here](https://https://github.com/uccl-project/uccl).
+
+    ```cpp
+    git clone https://github.com/uccl-project/uccl.git
+    cd uccl/p2p
+    make -j
+    sudo make install
+    ```
+
+2. Build NIXL using regular method as in [README](https://github.com/ai-dynamo/nixl/blob/main/README.md) ensuring `disable_uccl_backend` is set to `false`.
+
+## Usage Guide
+
+Example Usage to create a NIXL agent with uccl engine:
+
+    ```python
+    config = nixl_agent_config(backends=["UCCL"])
+    agent = nixl_agent("agent-name", config)
+    ```
+UCCL engine would auto discover the right NIC to be used for the GPU based on the PCIe distance during memory registration based on the data locality.
+
+### Environment Variables
+
+Refer to [README](https://github.com/uccl-project/uccl/tree/main/collective/rdma#environment-variables-in-uccl) for the complete list of environment variables that can be set to customize UCCL.
+
+**Important**: For `NIXL_READ` operations set `UCCL_RCMODE=1`. By default, UCCL uses RDMA UC (Unreliable Connection). However, `READ` operations need to operate on RDMA RC (Reliable Connection).
+
+### Usage References
+
+1) [NIXL Benchmark](https://github.com/uccl-project/uccl/blob/main/p2p/benchmarks/benchmark_nixl.py) in UCCL: Refer to  this [README](https://github.com/uccl-project/uccl/tree/main/p2p) on how to run the script.
+
+2) [NIXL connector](https://github.com/praveingk/vllm/commit/fa67cd7edff076fee4914cc316a9833c2311a65d) in vLLM. vLLM's NIXL connector uses `NIXL_READ` operations, hence set env `UCCL_RCMODE` to 1.
+
+### Road Map
+
+- [ ] Add Intra-node communication support
+
+- [ ] Add Progress Thread support
+
+- [ ] Add asynchronous posting of reads over multiple workers to mitigate latency increase upon fragmentation
+
+- [ ] Add support for other transport (TCP, TCP-X, etc.)

src/plugins/uccl/meson.build

@@ -0,0 +1,48 @@
+# SPDX-FileCopyrightText: Copyright (c) 2025 NVIDIA CORPORATION & AFFILIATES. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+cpp = meson.get_compiler('cpp')
+uccl_lib = cpp.find_library('uccl_engine')
+
+compile_flags = []
+
+
+if 'UCCL' in static_plugins
+    uccl_backend_lib = static_library('UCCL',
+               'uccl_backend.cpp', 'uccl_backend.h', 'uccl_plugin.cpp',
+               dependencies: [nixl_infra, serdes_interface, cuda_dep, uccl_lib, nixl_common_dep],
+               include_directories: nixl_inc_dirs,
+               install: false,
+               cpp_args : compile_flags,
+               name_prefix: 'libplugin_')  # Custom prefix for plugin libraries
+else
+    uccl_backend_lib = shared_library('UCCL',
+               'uccl_backend.cpp', 'uccl_backend.h', 'uccl_plugin.cpp',
+               dependencies: [nixl_infra, serdes_interface, cuda_dep, uccl_lib, nixl_common_dep],
+               include_directories: [nixl_inc_dirs, utils_inc_dirs],
+               install: true,
+               cpp_args : compile_flags + ['-fPIC'],
+               name_prefix: 'libplugin_',  # Custom prefix for plugin libraries
+               install_dir: plugin_install_dir)
+
+    if get_option('buildtype') == 'debug'
+        run_command('sh', '-c',
+                    'echo "UCCL=' + uccl_backend_lib.full_path() + '" >> ' + plugin_build_dir + '/pluginlist',
+                    check: true
+                )
+    endif
+endif
+
+uccl_backend_interface = declare_dependency(link_with: uccl_backend_lib)