[Core] avoid too many cuda context by caching p2p test (#4021)

vllm/distributed/utils.py

@@ -2,9 +2,18 @@
 # Adapted from
 # https://github.com/NVIDIA/Megatron-LM/blob/main/megatron/core/tensor_parallel/utils.py
 # Copyright (c) 2022, NVIDIA CORPORATION. All rights reserved.
-from typing import Sequence
+import json
+import os
+from typing import Dict, Optional, Sequence
 
 import torch
+import torch.distributed as dist
+
+from vllm.logger import init_logger
+
+from .parallel_state import get_cpu_world_group, get_local_rank
+
+logger = init_logger(__name__)
 
 
 def ensure_divisibility(numerator, denominator):
@@ -46,3 +55,79 @@ def split_tensor_along_last_dim(
         return tuple(chunk.contiguous() for chunk in tensor_list)
 
     return tensor_list
+
+
+# code partly borrowed from
+# https://github.com/turboderp/exllamav2/blob/1c67f97f3d2a968605a9c31ab791a05c85bb7879/exllamav2/compat.py#L10
+# License: MIT
+def _can_actually_p2p(idx_a, idx_b):
+    dev_i = f"cuda:{idx_a}"
+    dev_j = f"cuda:{idx_b}"
+    a = torch.randn(5, device=dev_i) + 123.0
+    b = a.to(dev_j)
+    c = b.to(dev_i)
+    return torch.all(a == c).cpu().item()
+
+
+# why do we need this cache?
+# 1. we can have runtime checks for P2P access, where every process checks
+#  P2P access to all other GPUs. Unfortunately, the test might cost many
+#  (world_size * world_size) cuda context, and reduce the memory available
+#  for the model. see https://github.com/vllm-project/vllm/issues/3821
+# 2. alternatively, we can have a p2p map that is generated by the master
+#  process and broadcasted to all other processes. This still requires
+#  #world_size of cuda context, belonging to the master process, on each GPU.
+# 3. we can have a cache file, that records the p2p access status. The first
+#  time the master process checks the p2p access, it will generate the cache
+#  file, at the cost of #world_size of cuda context. Later on, all processes
+#  can read the cache file to check the p2p access status without any cost of
+#  additional cuda context.
+# Note that the cache file is suffixed by the CUDA_VISIBLE_DEVICES, so that we
+#  can have different cache files for different CUDA_VISIBLE_DEVICES settings,
+#  e.g. used by different vllm engines. The device id in the cache file is a
+#  **local** device id, i.e. from 0 to num_dev-1, where num_dev is the number
+#  of visible devices in the vllm engine.
+_gpu_p2p_access_cache: Optional[Dict[str, bool]] = None
+
+
+def gpu_p2p_access_check(i: int, j: int) -> bool:
+    """Check if GPU i can access GPU j."""
+
+    # if the cache variable is already calculated,
+    # read from the cache instead of checking it again
+    global _gpu_p2p_access_cache
+    if _gpu_p2p_access_cache is not None:
+        return _gpu_p2p_access_cache[f"{i}->{j}"]
+
+    is_distributed = dist.is_initialized()
+
+    num_dev = torch.cuda.device_count()
+    cuda_visible_devices = os.environ.get("CUDA_VISIBLE_DEVICES", None)
+    if cuda_visible_devices is None:
+        cuda_visible_devices = ",".join(str(i) for i in range(num_dev))
+    path = os.path.expanduser(
+        f"~/.config/vllm/gpu_p2p_access_cache_for_{cuda_visible_devices}.json")
+    os.makedirs(os.path.dirname(path), exist_ok=True)
+    if (not is_distributed or get_local_rank() == 0) \
+        and (not os.path.exists(path)):
+        # only the local master process (with local_rank == 0) can
+        #  enter this block to calculate the cache
+        logger.info(f"generating GPU P2P access cache for in {path}")
+        cache = {}
+        for _i in range(num_dev):
+            for _j in range(num_dev):
+                # on some platforms, P2P support might be buggy and we need
+                # additional checks. See also:
+                # https://github.com/vllm-project/vllm/issues/2728
+                cache[f"{_i}->{_j}"] = torch.cuda.can_device_access_peer(
+                    _i, _j) and _can_actually_p2p(_i, _j)
+        with open(path, "w") as f:
+            json.dump(cache, f, indent=4)
+    if is_distributed:
+        cpu_world_group = get_cpu_world_group()
+        dist.barrier(cpu_world_group)
+    logger.info(f"reading GPU P2P access cache from {path}")
+    with open(path, "r") as f:
+        cache = json.load(f)
+    _gpu_p2p_access_cache = cache
+    return _gpu_p2p_access_cache[f"{i}->{j}"]