[Bugfix] Fix failure to launch in Tensor Parallel TP mode on macOS. (#14948)

Signed-off-by: Kebe <mail@kebe7jun.com>
Signed-off-by: youkaichao <youkaichao@gmail.com>
Co-authored-by: youkaichao <youkaichao@gmail.com>

docs/source/design/multiprocessing.md

@@ -24,7 +24,7 @@ This document describes how vLLM deals with these challenges.
 [Python multiprocessing methods](https://docs.python.org/3/library/multiprocessing.html#contexts-and-start-methods) include:
 
 - `spawn` - spawn a new Python process. This will be the default as of Python
-  3.14.
+  3.14. In macOS, this is already the default.
 
 - `fork` - Use `os.fork()` to fork the Python interpreter. This is the default
   in Python versions prior to 3.14.
@@ -34,7 +34,7 @@ This document describes how vLLM deals with these challenges.
 ### Tradeoffs
 
 `fork` is the fastest method, but is incompatible with dependencies that use
-threads.
+threads. If you are under macOS, using `fork` may cause the process to crash.
 
 `spawn` is more compatible with dependencies, but can be problematic when vLLM
 is used as a library. If the consuming code does not use a `__main__` guard (`if

vllm/distributed/device_communicators/shm_broadcast.py

@@ -125,8 +125,13 @@ class ShmRingBuffer:
                        lambda *args, **kwargs: None):
                 try:
                     self.shared_memory = shared_memory.SharedMemory(name=name)
-                    assert (
-                        self.shared_memory.size == self.total_bytes_of_buffer)
+                    # See https://docs.python.org/3/library/multiprocessing.shared_memory.html # noqa
+                    # Some platforms allocate memory based on page size,
+                    # so the shared memory block size may be larger or equal
+                    # to the requested size. The size parameter is ignored
+                    # when attaching to an existing block.
+                    assert (self.shared_memory.size
+                            >= self.total_bytes_of_buffer)
                 except FileNotFoundError:
                     # we might deserialize the object in a different node
                     # in this case, this object is not used,

vllm/platforms/cpu.py

 # SPDX-License-Identifier: Apache-2.0
 
 import os
+import sys
 from typing import TYPE_CHECKING, Optional
 
 import psutil
@@ -148,6 +149,13 @@ class CpuPlatform(Platform):
         # To hint IPEX uses shared memory based AllReduce
         os.environ["LOCAL_WORLD_SIZE"] = str(
             vllm_config.parallel_config.tensor_parallel_size)
+        if sys.platform == "darwin" and \
+                envs.VLLM_WORKER_MULTIPROC_METHOD == "fork":
+            if os.environ.get('VLLM_WORKER_MULTIPROC_METHOD', None) is None:
+                logger.warning(
+                    "Default to spawn method on MacOS. If this is not desired,"
+                    " set VLLM_WORKER_MULTIPROC_METHOD to fork explicitly.")
+                os.environ['VLLM_WORKER_MULTIPROC_METHOD'] = 'spawn'
 
     @classmethod
     def is_pin_memory_available(cls) -> bool: