[caffe2] support serializing float data as bfloat16 (#53735)

Summary:
Pull Request resolved: #53735

Add an option to BlobSerializationOptions to request that float data be
serialized as bfloat16.  This reduces the serialized data size at the expense
of some loss in precision.
ghstack-source-id: 124317910

Test Plan: Included a new unit test.

Reviewed By: mraway

Differential Revision: D26658205

fbshipit-source-id: 74521ed161059066355a3f208488ed01a344dbb5

caffe2/core/blob_serialization.cc

@@ -10,6 +10,9 @@
 #include "caffe2/core/blob.h"
 #include "caffe2/core/common.h"
 #include "caffe2/utils/proto_utils.h"
+#ifdef USE_FBGEMM
+#include "fbgemm/FbgemmConvert.h"
+#endif
 
 C10_DEFINE_int(
     caffe2_tensor_chunk_size,
@@ -388,7 +391,75 @@ void SerializeTensorData(const SerializeParams<at::Half>& params) {
       params.tensor_proto);
 }
 
+#ifdef USE_FBGEMM
+namespace {
+// Unfortunately we can't include folly/lang/Bits.h here,
+// so provide our own byte-swapping code.
+fbgemm::bfloat16 ByteSwap(fbgemm::bfloat16 n) {
+#ifdef _MSC_VER
+  return _byteswap_ushort(n);
+#else
+  return __builtin_bswap16(n);
+#endif
+}
+
+void ByteSwapArray(
+    const fbgemm::bfloat16* src,
+    fbgemm::bfloat16* dest,
+    size_t num_elements) {
+  // Note that we support src and dest pointing to the same location.
+  // We currently only use this function on big-endian machines, so it isn't
+  // worth trying to build a fancier SIMD version.
+  for (size_t n = 0; n < num_elements; ++n) {
+    dest[n] = ByteSwap(src[n]);
+  }
+}
+} // namespace
+#endif // USE_FBGEMM
+
 void SerializeTensorData(const SerializeParams<float>& params) {
+  // The FLOAT_BFLOAT16 option requests doing a conversion to bfloat16.  This
+  // reduces the serialized data size at the cost of some lost precision.
+  // We currently only support doing this when compiled with fbgemm.
+#ifdef USE_FBGEMM
+  if (params.options.float_format() ==
+      BlobSerializationOptions_FloatFormat_FLOAT_BFLOAT16) {
+    std::unique_ptr<float[]> tmp_buffer;
+    const float* src;
+    if (params.context.device() == CPU) {
+      src = params.input.data();
+    } else {
+      tmp_buffer.reset(new float[params.input.size()]);
+      params.context.CopyToCPU(
+          params.input.size(), params.input.data(), tmp_buffer.get());
+    }
+
+    params.SetDataFormat(TensorProto_SerializationFormat_FMT_BFLOAT16);
+    // TODO: it would be nice if we could use
+    // folly::resizeWithoutInitialization() here
+    params.tensor_proto.mutable_raw_data()->resize(
+        params.input.size() * sizeof(fbgemm::bfloat16));
+
+    Range<fbgemm::bfloat16*> dest(
+        reinterpret_cast<fbgemm::bfloat16*>(
+            &(*params.tensor_proto.mutable_raw_data())[0]),
+        params.input.size());
+
+    fbgemm::FloatToBfloat16_simd(src, dest.data(), params.input.size());
+
+    // Note: technically a platform can have different integer from floating
+    // point endianness, and we ideally should check floating point endianness
+    // here.  However, the fbgemm code doesn't appear to make this distinction,
+    // and at least in the Bfloat16ToFloat_ref() code it appears to assume that
+    // floating point and integer endianness are the same.
+    if (!kIsLittleEndian) {
+      ByteSwapArray(dest.data(), dest.data(), dest.size());
+    }
+    return;
+  }
+#endif
+
+  params.SetDataFormat(TensorProto_SerializationFormat_FMT_PROTOBUF);
   params.CopyToRepeatedField(params.tensor_proto.mutable_float_data());
 }
 
@@ -792,6 +863,48 @@ DESERIALIZE_IMPL(float, FMT_PROTOBUF) {
   params.CopyFromRepeatedField(params.tensor_proto.float_data());
 }
 
+DESERIALIZE_IMPL(float, FMT_BFLOAT16) {
+#ifdef USE_FBGEMM
+  CAFFE_ENFORCE_EQ(
+      params.dest.size() * sizeof(fbgemm::bfloat16),
+      params.tensor_proto.raw_data().size(),
+      "incorrect data size in serialized bfloat16 data");
+  auto raw_src = reinterpret_cast<const fbgemm::bfloat16*>(
+      params.tensor_proto.raw_data().data());
+
+  // If we are on a big-endian machine, byte-swap the serialized data.
+  const fbgemm::bfloat16* src;
+  std::unique_ptr<fbgemm::bfloat16[]> bswap_buffer;
+  if (kIsLittleEndian) {
+    src = raw_src;
+  } else {
+    bswap_buffer.reset(new fbgemm::bfloat16[params.dest.size()]);
+    ByteSwapArray(raw_src, bswap_buffer.get(), params.dest.size());
+    src = bswap_buffer.get();
+  }
+
+  // If we are on a non-CPU device, we need an intermediate CPU buffer for the
+  // bfloat16 to float conversion.
+  std::unique_ptr<float[]> tmp_buffer;
+  float* dest;
+  if (params.context.device() == CPU) {
+    dest = params.dest.data();
+  } else {
+    tmp_buffer.reset(new float[params.dest.size()]);
+    dest = tmp_buffer.get();
+  }
+
+  fbgemm::Bfloat16ToFloat_simd(src, dest, params.dest.size());
+  if (params.context.device() != CPU) {
+    params.context.CopyFromCPU(params.dest.size(), dest, params.dest.data());
+  }
+#else
+  // We cannot load serialized bfloat16 data without fbgemm.
+  CAFFE_ENFORCE(
+      false, "cannot perform bfloat16 to float conversion without fbgemm");
+#endif
+}
+
 DESERIALIZE_IMPL(double, FMT_PROTOBUF) {
   params.CopyFromRepeatedField(params.tensor_proto.double_data());
 }
@@ -825,6 +938,7 @@ void DeserializeTensorBody(
   DeserializeParams<T> params(dest, tensor_proto, context);
   switch (format) {
     DESERIALIZE_FORMAT_CASE(FMT_PROTOBUF);
+    DESERIALIZE_FORMAT_CASE(FMT_BFLOAT16);
   }
 
   // This can happen if the blob was serialized by a newer version of the code

caffe2/proto/caffe2.proto

@@ -49,6 +49,8 @@ message TensorProto {
     // the protobuf typed fields, although in some cases raw little endian data
     // is stored in the byte_data field instead.
     FMT_PROTOBUF = 0;
+    // bfloat16 data stored in the raw_data field.
+    FMT_BFLOAT16 = 1;
   }
   // data_format is a SerializationFormat enum value.
   // However, we intentionally store it as an integer value so we can
@@ -504,6 +506,19 @@ message BlobSerializationOptions {
   // - a chunk size of -1 means to disable chunking, and serialize the blob in
   //   a single chunk.
   optional int64 chunk_size = 2;
+
+  enum FloatFormat {
+    // Use the current default serialization format, as chosen by the
+    // current version of the code.  (At the time of writing this is PROTOBUF)
+    FLOAT_DEFAULT = 0;
+    // Store the data in the TensorProto's float_data field
+    FLOAT_PROTOBUF = 1;
+    // Serialize float values as bfloat16.  Note that this conversion is lossy.
+    FLOAT_BFLOAT16 = 2;
+  }
+
+  // Settings for how to serialize tensors containing float values
+  optional FloatFormat float_format = 3;
 }
 
 message SerializationOptions {

caffe2/proto/caffe2_pb2.pyi

@@ -80,9 +80,11 @@ class TensorProto(google.protobuf.message.Message):
     class _SerializationFormat(google.protobuf.internal.enum_type_wrapper._EnumTypeWrapper[SerializationFormat.V], builtins.type):
         DESCRIPTOR: google.protobuf.descriptor.EnumDescriptor = ...
         FMT_PROTOBUF = TensorProto.SerializationFormat.V(0)
+        FMT_BFLOAT16 = TensorProto.SerializationFormat.V(1)
     class SerializationFormat(metaclass=_SerializationFormat):
         V = typing.NewType('V', builtins.int)
     FMT_PROTOBUF = TensorProto.SerializationFormat.V(0)
+    FMT_BFLOAT16 = TensorProto.SerializationFormat.V(1)
 
     class Segment(google.protobuf.message.Message):
         DESCRIPTOR: google.protobuf.descriptor.Descriptor = ...
@@ -708,18 +710,32 @@ global___DBReaderProto = DBReaderProto
 
 class BlobSerializationOptions(google.protobuf.message.Message):
     DESCRIPTOR: google.protobuf.descriptor.Descriptor = ...
+    class _FloatFormat(google.protobuf.internal.enum_type_wrapper._EnumTypeWrapper[FloatFormat.V], builtins.type):
+        DESCRIPTOR: google.protobuf.descriptor.EnumDescriptor = ...
+        FLOAT_DEFAULT = BlobSerializationOptions.FloatFormat.V(0)
+        FLOAT_PROTOBUF = BlobSerializationOptions.FloatFormat.V(1)
+        FLOAT_BFLOAT16 = BlobSerializationOptions.FloatFormat.V(2)
+    class FloatFormat(metaclass=_FloatFormat):
+        V = typing.NewType('V', builtins.int)
+    FLOAT_DEFAULT = BlobSerializationOptions.FloatFormat.V(0)
+    FLOAT_PROTOBUF = BlobSerializationOptions.FloatFormat.V(1)
+    FLOAT_BFLOAT16 = BlobSerializationOptions.FloatFormat.V(2)
+
     BLOB_NAME_REGEX_FIELD_NUMBER: builtins.int
     CHUNK_SIZE_FIELD_NUMBER: builtins.int
+    FLOAT_FORMAT_FIELD_NUMBER: builtins.int
     blob_name_regex: typing.Text = ...
     chunk_size: builtins.int = ...
+    float_format: global___BlobSerializationOptions.FloatFormat.V = ...
 
     def __init__(self,
         *,
         blob_name_regex : typing.Optional[typing.Text] = ...,
         chunk_size : typing.Optional[builtins.int] = ...,
+        float_format : typing.Optional[global___BlobSerializationOptions.FloatFormat.V] = ...,
         ) -> None: ...
-    def HasField(self, field_name: typing_extensions.Literal[u"blob_name_regex",b"blob_name_regex",u"chunk_size",b"chunk_size"]) -> builtins.bool: ...
-    def ClearField(self, field_name: typing_extensions.Literal[u"blob_name_regex",b"blob_name_regex",u"chunk_size",b"chunk_size"]) -> None: ...
+    def HasField(self, field_name: typing_extensions.Literal[u"blob_name_regex",b"blob_name_regex",u"chunk_size",b"chunk_size",u"float_format",b"float_format"]) -> builtins.bool: ...
+    def ClearField(self, field_name: typing_extensions.Literal[u"blob_name_regex",b"blob_name_regex",u"chunk_size",b"chunk_size",u"float_format",b"float_format"]) -> None: ...
 global___BlobSerializationOptions = BlobSerializationOptions
 
 class SerializationOptions(google.protobuf.message.Message):

caffe2/python/operator_test/load_save_test.py

@@ -461,9 +461,9 @@ def float_array(dtype: Type[np.floating], size: int) -> np.ndarray:
 
         return blobs
 
-    def load_and_check_blobs(
+    def load_blobs(
         self,
-        blobs: List[Tuple[str, np.ndarray]],
+        blob_names: List[str],
         dbs: List[str],
         db_type: Optional[str] = None
     ) -> None:
@@ -472,13 +472,21 @@ def load_and_check_blobs(
         load_op = core.CreateOperator(
             "Load",
             [],
-            [name for name, data in blobs],
+            blob_names,
             absolute_path=1,
             dbs=dbs,
             db_type=db_type or self._db_type,
         )
         self.assertTrue(workspace.RunOperatorOnce(load_op))
-        self.assertEqual(len(workspace.Blobs()), len(blobs))
+        self.assertEqual(len(workspace.Blobs()), len(blob_names))
+
+    def load_and_check_blobs(
+        self,
+        blobs: List[Tuple[str, np.ndarray]],
+        dbs: List[str],
+        db_type: Optional[str] = None
+    ) -> None:
+        self.load_blobs([name for name, data in blobs], dbs, db_type)
         for name, data in blobs:
             np.testing.assert_array_equal(workspace.FetchBlob(name), data)
 
@@ -636,5 +644,55 @@ def testSaveWithOptions(self) -> None:
         )
 
 
+    def testSaveFloatToBfloat16(self) -> None:
+        tmp_folder = self.make_tempdir()
+        tmp_file = str(tmp_folder / "save.output")
+
+        # Create 2 blobs with the same float data
+        float_data = np.random.random_sample(4000).astype(np.float32)
+        workspace.FeedBlob("float1", float_data)
+        workspace.FeedBlob("float2", float_data)
+        blob_names = ["float1", "float2"]
+
+        # Serialize the data, using bfloat16 serialization for one of the blobs
+        save_op = core.CreateOperator(
+            "Save",
+            blob_names,
+            [],
+            absolute_path=1,
+            db=tmp_file,
+            db_type=self._db_type,
+            options=caffe2_pb2.SerializationOptions(
+                options=[
+                    BlobSerializationOptions(
+                        blob_name_regex="float1",
+                        float_format=BlobSerializationOptions.FLOAT_BFLOAT16,
+                    ),
+                ],
+            ),
+        )
+        self.assertTrue(workspace.RunOperatorOnce(save_op))
+
+        # As long as fbgemm was available for us to perform bfloat16 conversion,
+        # the serialized data for float1 should be almost half the size of float2
+        if workspace.has_fbgemm:
+            blob_chunks = self._read_chunk_info(Path(tmp_file))
+            self.assertEqual(len(blob_chunks["float1"]), 1, blob_chunks["float1"])
+            self.assertEqual(len(blob_chunks["float2"]), 1, blob_chunks["float2"])
+            self.assertLess(
+                blob_chunks["float1"][0].value_size,
+                0.6 * blob_chunks["float2"][0].value_size
+            )
+
+        self.load_blobs(blob_names, [tmp_file])
+
+        # float2 should be exactly the same as the input data
+        np.testing.assert_array_equal(workspace.FetchBlob("float2"), float_data)
+        # float2 should be close-ish to the input data
+        np.testing.assert_array_almost_equal(
+            workspace.FetchBlob("float1"), float_data, decimal=2
+        )
+
+
 if __name__ == '__main__':
     unittest.main()