Add default set env MC_STORE_MEMCPY is 0 when protocol is tcp

tests/test_mooncake_force_delete.py

@@ -68,6 +68,56 @@ def test_enable_hard_pin_default_off(self):
             assert restored.enable_hard_pin is False
 
 
+class TestMooncakeEnvDefaults:
+    def test_tcp_memcpy_default_is_applied_by_export_env(self):
+        config = MooncakeConfig(protocol="tcp")
+
+        with patch.dict(os.environ, {}, clear=True):
+            config.export_env()
+
+            assert os.environ["MC_STORE_MEMCPY"] == "0"
+
+    def test_tcp_memcpy_default_preserves_user_override(self):
+        config = MooncakeConfig(protocol="tcp")
+
+        with patch.dict(os.environ, {"MC_STORE_MEMCPY": "1"}, clear=True):
+            config.apply_env_defaults()
+
+            assert os.environ["MC_STORE_MEMCPY"] == "1"
+
+    def test_tcp_memcpy_default_not_applied_for_rdma(self):
+        config = MooncakeConfig(protocol="rdma")
+
+        with patch.dict(os.environ, {}, clear=True):
+            config.apply_env_defaults()
+
+            assert "MC_STORE_MEMCPY" not in os.environ
+
+    def test_direct_store_setup_applies_tcp_memcpy_before_mooncake_client_setup(self):
+        config = MooncakeConfig(protocol="tcp", async_put_pool_size=0)
+        mock_raw_store = MagicMock()
+        mock_raw_store.setup.return_value = 0
+
+        class ConcreteStore(MooncakeHiddenStateStore):
+            pass
+
+        def make_raw_store():
+            assert os.environ["MC_STORE_MEMCPY"] == "0"
+            return mock_raw_store
+
+        store = ConcreteStore(config)
+        with (
+            patch.dict(os.environ, {}, clear=True),
+            patch("torchspec.transfer.mooncake.store.MooncakeDistributedStore", make_raw_store),
+            patch.object(ConcreteStore, "_verify_force_delete"),
+            patch.object(ConcreteStore, "_build_replicate_config"),
+            patch("torch.cuda.is_available", return_value=False),
+        ):
+            store.setup()
+
+        mock_raw_store.setup.assert_called_once()
+
+
 # ---------------------------------------------------------------------------
 # Tests 2-3: _verify_force_delete
 # ---------------------------------------------------------------------------

tests/test_placement_group.py

@@ -1,13 +1,12 @@
-from argparse import Namespace
 import importlib.util
 import sys
 import types
+from argparse import Namespace
 from pathlib import Path
 from unittest.mock import MagicMock, patch
 
 import pytest
 
-
 repo_root = Path(__file__).resolve().parents[1]
 torchspec_pkg = sys.modules.get("torchspec")
 if torchspec_pkg is None and importlib.util.find_spec("torch") is None:
@@ -67,8 +66,8 @@ def __init__(self, **kwargs):
 sys.modules["torchspec.ray.train_group"] = train_group_stub
 
 from torchspec.ray.placement_group import (  # noqa: E402
-    _NodeConstraint,
     _build_custom_bundles,
+    _NodeConstraint,
     _sort_probed_bundle_infos,
     create_placement_groups,
 )

torchspec/config/mooncake_config.py

@@ -176,6 +176,7 @@ def export_env(self) -> None:
         os.environ["MOONCAKE_PROTOCOL"] = self.protocol
         os.environ["MOONCAKE_DEVICE_NAME"] = self.device_name
         os.environ["MOONCAKE_ENABLE_GPU_DIRECT"] = "1" if self.enable_gpu_direct else "0"
+        self.apply_env_defaults()
         if self.async_put_pool_size is not None:
             os.environ["MOONCAKE_ASYNC_PUT_POOL_SIZE"] = str(self.async_put_pool_size)
         os.environ["MOONCAKE_STORE_FULL_WAIT_SECONDS"] = str(self.store_full_wait_seconds)
@@ -190,6 +191,14 @@ def export_env(self) -> None:
         os.environ["MOONCAKE_GET_RETRY_MAX_WAIT_SECONDS"] = str(self.get_retry_max_wait_seconds)
         os.environ["MOONCAKE_ENABLE_HARD_PIN"] = "1" if self.enable_hard_pin else "0"
 
+    def apply_env_defaults(self) -> None:
+        """Apply Mooncake process defaults that are needed before client setup."""
+        # Fix: https://github.com/kvcache-ai/Mooncake/issues/1986
+        if self.protocol.lower() == "tcp" and "MC_STORE_MEMCPY" not in os.environ:
+            # Mooncake's TCP-only memcpy fast path can segfault in same-host
+            # multi-process get paths. Preserve an explicit user override.
+            os.environ["MC_STORE_MEMCPY"] = "0"
+
     @classmethod
     def from_env(cls) -> "MooncakeConfig":
         """Create config from environment variables."""

torchspec/ray/placement_group.py

@@ -34,7 +34,6 @@
 from torchspec.ray.train_group import RayTrainGroup
 from torchspec.utils.logging import logger
 
-
 # Ray exposes a tiny "node:<ip>" resource on each node. Requiring a fractional
 # amount pins a bundle to that node without consuming a full logical resource.
 _NODE_RESOURCE_EPSILON = 0.001

torchspec/transfer/mooncake/store.py

@@ -73,6 +73,7 @@ def setup(self, device: torch.device | int | None = None) -> None:
                 "Set mooncake.device_name to a specific RDMA device (e.g. 'mlx5_0')."
             )
 
+        self.config.apply_env_defaults()
         self._store = MooncakeDistributedStore()
         logger.info(
             "Connecting to Mooncake master at %s, metadata server at %s",