[wip] store inv_scale on Float8Tensor

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ghstack-source-id: 6ab4322ffb9456f244a1c3987842e10f3ecc2a83
ghstack-comment-id: 2273795212
Pull Request resolved: #628

benchmarks/float8/profile_linear_float8.py

@@ -209,6 +209,7 @@ def main(
     scaling_type_grad_output: str = "dynamic",
     model_type: str = "linear",
     dtype_filter: str = "both",
+    skip_amax_sync: bool = False,
 ):
     assert model_type in ("linear", "ln_linear", "norm_ffn_norm", "norm_ffn_norm_small"), "unsupported"
     assert dtype_filter in ("both", "float8", "bfloat16")
@@ -220,6 +221,9 @@ def main(
         cast_config_input=CastConfig(scaling_type=scaling_type_input),
         cast_config_weight=CastConfig(scaling_type=scaling_type_weight),
         cast_config_grad_output=CastConfig(scaling_type=scaling_type_grad_output),
+        # for now we don't care about amax init for performance profiling
+        enable_amax_init=False,
+        enable_pre_and_post_forward = not skip_amax_sync,
     )
     scaling_repr = "_".join(
         [
@@ -290,7 +294,7 @@ def float8_forw_backward_wrapper(x):
         # inspection of the fw+bw torch.compile without the scale
         # syncing code
         # TODO(future): make this better
-        if linear_requires_sync(config):
+        if linear_requires_sync(config) and not skip_amax_sync:
             with record_function("scale_amax_and_scales"):
                 sync_amax_history(m_float8)
         out = float8_forw(x)

test/float8/test_base.py

@@ -128,6 +128,7 @@ def test_copy_(self):
         fp8_b = Float8Tensor(
             torch.empty(16, dtype=torch.float8_e4m3fn),
             scale_a,
+            scale_a.reciprocal(),
             torch.bfloat16,
             fp8_a._linear_mm_config,
         )
@@ -417,14 +418,14 @@ def test_scaled_mm_vs_emulated(self, base_dtype, use_fast_accum):
 
         out_scaled_mm = addmm_float8_unwrapped(
             a_fp8._data,
-            a_fp8._scale,
+            a_fp8._inv_scale,
             b_fp8._data,
-            b_fp8._scale,
+            b_fp8._inv_scale,
             output_dtype=output_dtype,
             use_fast_accum=use_fast_accum,
         )
         out_emulated = torch.ops.aten.mm_float8_emulated(
-            a_fp8._data, a_fp8._scale, b_fp8._data, b_fp8._scale, output_dtype
+            a_fp8._data, a_fp8._inv_scale, b_fp8._data, b_fp8._inv_scale, output_dtype
         )
 
         if output_dtype != base_dtype:

torchao/float8/float8_aten_api.py

@@ -15,14 +15,14 @@
 
 def mm_float8_emulated(
     m1,  # input 1 data
-    s1,  # input 1 scale
+    inv_s1,  # input 1 inverse scale
     m2,  # input 2 data
-    s2,  # input 2 scale
+    inv_s2,  # input 2 inverse scale
     dtype3,  # output dtype
 ):
     # naive implementation: dq -> op -> q
-    m1_fp32 = m1.float() / s1
-    m2_fp32 = m2.float() / s2
+    m1_fp32 = m1.float() * inv_s1
+    m2_fp32 = m2.float() * inv_s2
     m3_fp32 = torch.mm(m1_fp32, m2_fp32)
 
     return m3_fp32.to(dtype3)
@@ -37,13 +37,13 @@ def mm_float8_emulated(
 lib = Library("aten", "FRAGMENT")
 
 lib.define(
-    "mm_float8_emulated(Tensor m1, Tensor s1, Tensor m2, Tensor s2, ScalarType dtype3) -> Tensor"
+    "mm_float8_emulated(Tensor m1, Tensor inv_s1, Tensor m2, Tensor inv_s2, ScalarType dtype3) -> Tensor"
 )
 lib.impl("mm_float8_emulated", mm_float8_emulated, "CPU")
 lib.impl("mm_float8_emulated", mm_float8_emulated, "CUDA")
 
 
 @torch.library.impl(lib, "mm_float8_emulated", "Meta")
-def _mm_float8_emulated_meta(m1, s1, m2, s2, dtype3):
+def _mm_float8_emulated_meta(m1, inv_s1, m2, inv_s2, dtype3):
     out = torch.mm(m1.float(), m2.float()).to(dtype3)
     return out

torchao/float8/float8_ops.py

@@ -48,6 +48,7 @@ def float8_desugar_op(aten_op, args, kwargs=None):
     return Float8Tensor(
         new_data,
         args[0]._scale,
+        args[0]._inv_scale,
         args[0]._orig_dtype,
         args[0]._linear_mm_config,
         args[0]._gemm_input_role,
@@ -62,6 +63,7 @@ def make_float8(data):
         return Float8Tensor(
             data,
             args[0]._scale,
+            args[0]._inv_scale,
             args[0]._orig_dtype,
             args[0]._linear_mm_config,
             args[0]._gemm_input_role,
@@ -78,6 +80,7 @@ def float8_cat(aten_op, args, kwargs=None):
 
     orig_dtype = chunked_tensors[0]._orig_dtype
     scale = chunked_tensors[0]._scale
+    inv_scale = chunked_tensors[0]._inv_scale
     mm_config = chunked_tensors[0]._linear_mm_config
     fp8_dtype = chunked_tensors[0]._data.dtype
     gemm_input_role = chunked_tensors[0]._gemm_input_role
@@ -105,7 +108,7 @@ def float8_cat(aten_op, args, kwargs=None):
 
     new_data = aten_op(chunk_data, *args[1:], **kwargs)
     new_data = new_data.view(fp8_dtype)
-    return Float8Tensor(new_data, scale, orig_dtype, mm_config, gemm_input_role)
+    return Float8Tensor(new_data, scale, inv_scale, orig_dtype, mm_config, gemm_input_role)
 
 
 @implements([aten.sum.dim_IntList])
@@ -130,7 +133,7 @@ def unwrap(x):
 
 def preprocess_addmm(a: Float8Tensor, b: Float8Tensor):
     a_data = a._data
-    a_scale = a._scale
+    a_inv_scale = a._inv_scale
     b_data = b._data
 
     scaled_mm_config = choose_scaled_mm_config(
@@ -151,8 +154,8 @@ def preprocess_addmm(a: Float8Tensor, b: Float8Tensor):
         a_data = a_data.contiguous()
     if is_row_major(b_data.stride()):
         b_data = b_data.t().contiguous().t()
-    b_scale = b._scale
-    return a_data, a_scale, b_data, b_scale
+    b_inv_scale = b._inv_scale
+    return a_data, a_inv_scale, b_data, b_inv_scale
 
 
 @implements([aten.mm.default, aten.matmul.default])
@@ -165,7 +168,7 @@ def float8_mm(aten_op, args, kwargs=None):
     ), "Expecting  both Float8Tensor for mm inputs but found {} and {}".format(
         type(a), type(b)
     )
-    a_data, a_scale, b_data, b_scale = preprocess_addmm(a, b)
+    a_data, a_inv_scale, b_data, b_inv_scale = preprocess_addmm(a, b)
     output_dtype = a._orig_dtype
     scaled_mm_config = choose_scaled_mm_config(
         a._gemm_input_role,
@@ -175,13 +178,13 @@ def float8_mm(aten_op, args, kwargs=None):
     )
     if scaled_mm_config.emulate:
         return torch.ops.aten.mm_float8_emulated(
-            a._data, a._scale, b._data, b._scale, output_dtype
+            a._data, a._inv_scale, b._data, b._inv_scale, output_dtype
         )
     tensor_out = addmm_float8_unwrapped(
         a_data,
-        a_scale,
+        a_inv_scale,
         b_data,
-        b_scale,
+        b_inv_scale,
         output_dtype,
         output_scale=None,
         bias=None,
@@ -200,7 +203,7 @@ def float8_addmm(aten_op, args, kwargs=None):
     bias = args[0]
     a = args[1]
     b = args[2]
-    a_data, a_scale, b_data, b_scale = preprocess_addmm(a, b)
+    a_data, a_inv_scale, b_data, b_inv_scale = preprocess_addmm(a, b)
     output_dtype = a._orig_dtype
     assert bias.dtype == output_dtype, "bias dtype must match output dtype"
     scaled_mm_config = choose_scaled_mm_config(
@@ -210,15 +213,16 @@ def float8_addmm(aten_op, args, kwargs=None):
         b._linear_mm_config,
     )
     if scaled_mm_config.emulate:
+        # TODO inv scale here
         out = torch.ops.aten.mm_float8_emulated(
             a._data, a._scale, b._data, b._scale, output_dtype
         )
         return out + bias
     tensor_out = addmm_float8_unwrapped(
         a_data,
-        a_scale,
+        a_inv_scale,
         b_data,
-        b_scale,
+        b_inv_scale,
         output_dtype,
         output_scale=None,
         bias=bias,
@@ -249,6 +253,7 @@ def autocast_to_copy(aten_op, args, kwargs=None):
     return Float8Tensor(
         args[0]._data,
         args[0]._scale,
+        args[0]._inv_scale,
         kwargs["dtype"],
         args[0]._linear_mm_config,
         args[0]._gemm_input_role,
@@ -276,6 +281,7 @@ def allgather_fp8(aten_op, args, kwargs=None):
     return Float8Tensor(
         fp8_out,
         fp8_input._scale,
+        fp8_input._inv_scale,
         fp8_input._orig_dtype,
         fp8_input._linear_mm_config,
         fp8_input._gemm_input_role,
@@ -292,6 +298,7 @@ def wait_tensor_fp8(aten_op, args, kwargs=None):
     return Float8Tensor(
         fp8_out,
         fp8_input._scale,
+        fp8_input._inv_scale,
         fp8_input._orig_dtype,
         fp8_input._linear_mm_config,
         fp8_input._gemm_input_role,
@@ -314,6 +321,7 @@ def index_put_fp8(aten_op, args, kwargs=None):
     return Float8Tensor(
         fp8_out,
         fp8_self._scale,
+        fp8_self._inv_scale,
         fp8_self._orig_dtype,
         fp8_self._linear_mm_config,
         fp8_self._gemm_input_role,
@@ -355,6 +363,7 @@ def copy_fp8(aten_op, args, kwargs=None):
         return Float8Tensor(
             fp8_out,
             self._scale,
+            self._inv_scale,
             self._orig_dtype,
             self._linear_mm_config,
             self._gemm_input_role,

torchao/float8/float8_python_api.py

@@ -23,9 +23,9 @@
 # For output going from fp32 -> fp8 we multiply by the scale
 def addmm_float8_unwrapped(
     a_data: torch.Tensor,
-    a_scale: torch.Tensor,
+    a_inv_scale: torch.Tensor,
     b_data: torch.Tensor,
-    b_scale: torch.tensor,
+    b_inv_scale: torch.tensor,
     output_dtype: torch.dtype,
     output_scale: Optional[torch.Tensor] = None,
     bias: Optional[torch.Tensor] = None,
@@ -36,8 +36,10 @@ def addmm_float8_unwrapped(
     as inputs. This is used to standardize the logic between subclassed and non subclassed
     versions of the linear module.
     """
-    a_inverse_scale = a_scale.reciprocal()
-    b_inverse_scale = b_scale.reciprocal()
+    # a_inverse_scale = a_scale.reciprocal()
+    # b_inverse_scale = b_scale.reciprocal()
+    a_inverse_scale = a_inv_scale
+    b_inverse_scale = b_inv_scale
     if output_dtype == torch.float32 and bias is not None:
         # Bias is not supported by _scaled_mm when output is fp32
         output = torch._scaled_mm(

torchao/float8/float8_tensor.py

@@ -157,6 +157,7 @@ def forward(
         """
         tensor_scaled = tensor * scale
         bits_fp8 = to_fp8_saturated(tensor_scaled, float8_dtype)
+        inv_scale = scale.reciprocal()
 
         if isinstance(bits_fp8, DTensor):
             assert isinstance(
@@ -166,9 +167,11 @@ def forward(
             bits_placements = bits_fp8.placements
             local_bits = bits_fp8.to_local()
             local_scale = scale.to_local()
+            local_inv_scale = inv_scale.to_local()
             inner_float8_tensor = Float8Tensor(
                 local_bits,
                 local_scale,
+                local_inv_scale,
                 tensor.dtype,
                 linear_mm_config=linear_mm_config,
                 gemm_input_role=gemm_input_role,
@@ -185,6 +188,7 @@ def forward(
         return Float8Tensor(
             bits_fp8,
             scale,
+            inv_scale,
             tensor.dtype,
             linear_mm_config=linear_mm_config,
             gemm_input_role=gemm_input_role,
@@ -251,6 +255,11 @@ class Float8Tensor(torch.Tensor):
     * `_scale`: the scale used to scale the original fp32 tensor. We multiply
       by scale to go from fp32 range to fp8 range, and divide by scale to go
       from fp8 range to fp32 range.
+    * `_inv_scale`: the inverse of `_scale`. We need this because the 
+      `torch._scaled_mm` function requires inverse scales, and torch.compile
+      does not reliably fuse this into preceding ops, which can lead to extra
+      GPU kernel launches. If we calculate the inverse scale colocated with 
+      creating the `Float8Tensor` instance, we don't see the extra GPU kernels.
     * `_orig_dtype`: the original dtype of the tensor used to create this
       tensor.
     * `_emulate`: if true using fp32 emulation for the matmuls, helpful
@@ -275,6 +284,7 @@ def __new__(
         cls,
         data: torch.Tensor,
         scale: torch.Tensor,
+        inv_scale: torch.Tensor,
         orig_dtype: torch.dtype,
         linear_mm_config: Optional[LinearMMConfig],
         gemm_input_role: Optional[GemmInputRole] = GemmInputRole.INPUT,
@@ -297,6 +307,7 @@ def __new__(
         )
         self._data = data
         self._scale = scale
+        self._inv_scale = inv_scale
         self._orig_dtype = orig_dtype
         self._linear_mm_config = (
             linear_mm_config if linear_mm_config is not None else LinearMMConfig()
@@ -314,14 +325,15 @@ def __tensor_flatten__(self):
             "_linear_mm_config": self._linear_mm_config,
             "_gemm_input_role": self._gemm_input_role,
         }
-        return ["_data", "_scale"], ctx
+        return ["_data", "_scale", "_inv_scale"], ctx
 
     @staticmethod
     def __tensor_unflatten__(inner_tensors: Dict, metadata, outer_size, outer_stride):
-        assert len(inner_tensors) == 2
+        assert len(inner_tensors) == 3
         return Float8Tensor(
             inner_tensors["_data"],
             inner_tensors["_scale"],
+            inner_tensors["_inv_scale"],
             metadata["_orig_dtype"],
             metadata["_linear_mm_config"],
             metadata["_gemm_input_role"],

torchao/float8/fsdp_utils.py

@@ -217,7 +217,7 @@ def fsdp_pre_all_gather(self, mesh):
                 reduce_amax=True,
                 gemm_input_role=GemmInputRole.WEIGHT,
             )
-        return (float8_tensor._data,), (float8_tensor._scale,)
+        return (float8_tensor._data,), (float8_tensor._scale, float8_tensor._inv_scale)
 
     def fsdp_post_all_gather(
         self,
@@ -228,7 +228,7 @@ def fsdp_post_all_gather(
         out: Optional[torch.Tensor] = None,
     ):
         (data,) = all_gather_outputs
-        (scale,) = metadata
+        (scale, inv_scale) = metadata
         if out is not None:
             from torch.distributed._tensor import DTensor
             if isinstance(out, Float8Tensor):
@@ -245,6 +245,7 @@ def fsdp_post_all_gather(
         return Float8Tensor(
             data,
             scale,
+            inv_scale,
             param_dtype,
             self._linear_mm_config,
             gemm_input_role=GemmInputRole.WEIGHT,
@@ -407,7 +408,7 @@ def fsdp_pre_all_gather(self, mesh):
             self._linear_mm_config,
             GemmInputRole.WEIGHT,
         )
-        return (float8_tensor._data,), (float8_tensor._scale,)
+        return (float8_tensor._data,), (float8_tensor._scale, float8_tensor._inv_scale)
 
     def fsdp_post_all_gather(
         self,
@@ -418,14 +419,16 @@ def fsdp_post_all_gather(
         out: Optional[torch.Tensor] = None,
     ):
         (data,) = all_gather_outputs
-        (scale,) = metadata
+        (scale, inv_scale) = metadata
         if out is not None:
             assert isinstance(out, Float8Tensor), f"{type(out)}"
             out._scale = scale
+            out._inv_scale = inv_scale
             return
         return Float8Tensor(
             data,
             scale,
+            inv_scale,
             param_dtype,
             self._linear_mm_config,
             gemm_input_role=GemmInputRole.WEIGHT,