[BugFix] Handle non-contiguous tensors properly when serializing (#16492)

Signed-off-by: Nick Hill <nhill@redhat.com>

tests/v1/test_serial_utils.py

@@ -22,6 +22,10 @@ class MyType:
     list_of_tensors: list[torch.Tensor]
     numpy_array: np.ndarray
     unrecognized: UnrecognizedType
+    small_f_contig_tensor: torch.Tensor
+    large_f_contig_tensor: torch.Tensor
+    small_non_contig_tensor: torch.Tensor
+    large_non_contig_tensor: torch.Tensor
 
 
 def test_encode_decode():
@@ -40,6 +44,10 @@ def test_encode_decode():
         ],
         numpy_array=np.arange(512),
         unrecognized=UnrecognizedType(33),
+        small_f_contig_tensor=torch.rand(5, 4).t(),
+        large_f_contig_tensor=torch.rand(1024, 4).t(),
+        small_non_contig_tensor=torch.rand(2, 4)[:, 1:3],
+        large_non_contig_tensor=torch.rand(1024, 512)[:, 10:20],
     )
 
     encoder = MsgpackEncoder()
@@ -47,10 +55,10 @@ def test_encode_decode():
 
     encoded = encoder.encode(obj)
 
-    # There should be the main buffer + 2 large tensor buffers
-    # + 1 large numpy array. "large" is <= 256 bytes.
+    # There should be the main buffer + 4 large tensor buffers
+    # + 1 large numpy array. "large" is <= 512 bytes.
     # The two small tensors are encoded inline.
-    assert len(encoded) == 4
+    assert len(encoded) == 6
 
     decoded: MyType = decoder.decode(encoded)
 
@@ -62,7 +70,7 @@ def test_encode_decode():
 
     encoded2 = encoder.encode_into(obj, preallocated)
 
-    assert len(encoded2) == 4
+    assert len(encoded2) == 6
     assert encoded2[0] is preallocated
 
     decoded2: MyType = decoder.decode(encoded2)
@@ -78,3 +86,9 @@ def assert_equal(obj1: MyType, obj2: MyType):
         for a, b in zip(obj1.list_of_tensors, obj2.list_of_tensors))
     assert np.array_equal(obj1.numpy_array, obj2.numpy_array)
     assert obj1.unrecognized.an_int == obj2.unrecognized.an_int
+    assert torch.equal(obj1.small_f_contig_tensor, obj2.small_f_contig_tensor)
+    assert torch.equal(obj1.large_f_contig_tensor, obj2.large_f_contig_tensor)
+    assert torch.equal(obj1.small_non_contig_tensor,
+                       obj2.small_non_contig_tensor)
+    assert torch.equal(obj1.large_non_contig_tensor,
+                       obj2.large_non_contig_tensor)

vllm/v1/serial_utils.py

@@ -14,9 +14,10 @@ from msgspec import msgpack
 
 CUSTOM_TYPE_PICKLE = 1
 CUSTOM_TYPE_CLOUDPICKLE = 2
+CUSTOM_TYPE_RAW_VIEW = 3
 
 # TODO calibrate this size
-INLINE_BUF_SIZE_THRESHOLD = 256
+MIN_NOCOPY_BUF_SIZE = 512
 
 bytestr = Union[bytes, bytearray, memoryview, zmq.Frame]
 
@@ -76,14 +77,16 @@ class MsgpackEncoder:
         self, obj: np.ndarray
     ) -> tuple[str, tuple[int, ...], Union[int, memoryview]]:
         assert self.aux_buffers is not None
-        if not obj.shape or obj.nbytes < INLINE_BUF_SIZE_THRESHOLD:
-            # Encode small arrays and scalars inline.
-            data = obj.data
+        arr_data = obj.data if obj.data.c_contiguous else obj.tobytes()
+        if not obj.shape or obj.nbytes < MIN_NOCOPY_BUF_SIZE:
+            # Encode small arrays and scalars inline. Using this extension type
+            # ensures we can avoid copying when decoding.
+            data = msgpack.Ext(CUSTOM_TYPE_RAW_VIEW, arr_data)
         else:
-            # Otherwise encode index of backing buffer.
-            obj = np.ascontiguousarray(obj)
+            # Otherwise encode index of backing buffer to avoid copy.
             data = len(self.aux_buffers)
-            self.aux_buffers.append(obj.data)
+            self.aux_buffers.append(arr_data)
+
         # We serialize the ndarray as a tuple of native types.
         # The data is either inlined if small, or an index into a list of
         # backing buffers that we've stashed in `aux_buffers`.
@@ -131,6 +134,8 @@ class MsgpackDecoder:
         return np.ndarray(buffer=buffer, dtype=np.dtype(dtype), shape=shape)
 
     def ext_hook(self, code: int, data: memoryview) -> Any:
+        if code == CUSTOM_TYPE_RAW_VIEW:
+            return data
         if code == CUSTOM_TYPE_PICKLE:
             return pickle.loads(data)
         if code == CUSTOM_TYPE_CLOUDPICKLE: