[float8nocompile] Add alternate Triton kernels for FP8 conversion which use atomic_max-based algo instead of reduction-based algo

torchao/prototype/float8nocompile/kernels/fp8_dynamic_tensorwise.py

@@ -7,9 +7,9 @@
 """
 Triton kernels for scaling high precision tensors to float8.
 """
+from enum import Enum
 
 import torch
-
 import triton
 import triton.language as tl
 
@@ -31,8 +31,86 @@
 }
 
 
+class KernelAlgorithm(Enum):
+    """Enum for FP8 conversion strategy."""
+
+    # use atomic max to compute global amax between blocks
+    ATOMIC_MAX = "atomic_max"
+
+    # reduce shared buffer containing local block amaxes to find global amax
+    REDUCTION = "reduction"
+
+
+kernel_configs = [
+    triton.Config({"BLOCK_SIZE": 128}, num_warps=1),
+    triton.Config({"BLOCK_SIZE": 256}, num_warps=2),
+    triton.Config({"BLOCK_SIZE": 512}, num_warps=4),
+]
+
+
+# --- atomic max version of kernel ---
+@triton.autotune(configs=kernel_configs, key=["input_size"])
+@triton.jit
+def _block_amax_atomic(
+    input_ptr,
+    amax_ptr,
+    num_elements,
+    input_dtype: tl.constexpr,
+    BLOCK_SIZE: tl.constexpr,
+    EPS: tl.constexpr,
+):
+    # compute local amax for each block
+    block_id = tl.program_id(axis=0)
+    block_start = block_id * BLOCK_SIZE
+    block_offs = block_start + tl.arange(0, BLOCK_SIZE)
+    block_mask = block_offs < num_elements
+    vals = tl.load(input_ptr + block_offs, mask=block_mask).to(input_dtype)
+    block_amax = tl.max(tl.abs(vals), axis=0)
+    tl.atomic_max(amax_ptr, block_amax)
+
+
+@triton.autotune(configs=kernel_configs, key=["input_size"])
+@triton.jit
+def _to_fp8_atomic(
+    input_ptr,
+    scale_out_ptr,
+    amax_ptr,
+    out_ptr,
+    num_elements,
+    fp8_dtype_min,
+    fp8_dtype_max,
+    input_dtype: tl.constexpr,
+    output_dtype: tl.constexpr,
+    BLOCK_SIZE: tl.constexpr,
+    EPS: tl.constexpr,
+):
+    # compute scale, must be fp32
+    global_amax = tl.load(amax_ptr)
+    scale = (fp8_dtype_max / tl.clamp(global_amax, min=EPS, max=float("inf"))).to(
+        tl.float32
+    )
+
+    # only one program needs to store the scale
+    block_id = tl.program_id(axis=0)
+    if block_id == 0:
+        scale_offs = tl.arange(0, 1)
+        tl.store(scale_out_ptr + scale_offs, scale)
+
+    # load block of input tensor
+    block_start = block_id * BLOCK_SIZE
+    block_offs = block_start + tl.arange(0, BLOCK_SIZE)
+    mask = block_offs < num_elements
+    vals = tl.load(input_ptr + block_offs, mask=mask).to(input_dtype)
+
+    # perform conversion
+    vals = vals * scale
+    fp8_vals = tl.clamp(vals, min=fp8_dtype_min, max=fp8_dtype_max).to(output_dtype)
+    tl.store(out_ptr + block_offs, fp8_vals, mask=mask)
+
+
+# --- reduction version of kernel ---
 @triton.jit
-def _block_amax(
+def _block_amax_reduction(
     input_ptr,
     block_amaxes_ptr,
     num_elements,
@@ -51,7 +129,7 @@ def _block_amax(
 
 
 @triton.jit
-def _fp8_scale(
+def _fp8_scale_reduction(
     block_amaxes_ptr,
     scale_out_ptr,
     num_elements,
@@ -75,7 +153,7 @@ def _fp8_scale(
 
 
 @triton.jit
-def _to_fp8(
+def _to_fp8_reduction(
     input_ptr,
     scale_ptr,
     out_ptr,
@@ -108,12 +186,10 @@ def triton_hp_tensor_to_float8_dynamic(
     fp8_dtype: torch.dtype,
     linear_mm_config: LinearMMConfig,
     gemm_input_role: GemmInputRole = GemmInputRole.INPUT,
+    algo: KernelAlgorithm = KernelAlgorithm.ATOMIC_MAX,
 ) -> Float8Tensor:
-
     assert hp_tensor.is_contiguous(), "tensor must be contiguous"
 
-    BLOCK_SIZE = 8  # TODO(danielvegamyhre): tune this for perf
-
     num_elements = hp_tensor.numel()
     orig_shape = hp_tensor.shape
     flattened_input = hp_tensor.flatten()
@@ -126,47 +202,80 @@ def triton_hp_tensor_to_float8_dynamic(
 
     # allocate memory for computed scale, local block maxes, and output fp8 tensor
     scale_out = torch.empty((1,), dtype=torch.float32, device=hp_tensor.device)
-    block_amaxes = torch.zeros(
-        (num_elements // BLOCK_SIZE,), dtype=torch.float32, device=hp_tensor.device
-    )
+
     fp8_output = torch.empty_like(
         flattened_input, dtype=fp8_dtype, device=hp_tensor.device
     )
 
-    # compute local amax for each block
     grid = lambda meta: (triton.cdiv(num_elements, meta["BLOCK_SIZE"]),)
-    _block_amax[grid](
-        flattened_input,
-        block_amaxes,
-        num_elements,
-        input_dtype=tl_input_dtype,
-        BLOCK_SIZE=BLOCK_SIZE,
-        EPS=EPS,
-    )
 
-    # calculate global amax across all blocks and use it to compute scale
-    _fp8_scale[(1, 1, 1)](
-        block_amaxes,
-        scale_out,
-        num_elements,
-        fp8_dtype_max,
-        BLOCK_SIZE=BLOCK_SIZE,
-        EPS=EPS,
-    )
+    if algo == KernelAlgorithm.ATOMIC_MAX:
+        global_amax = torch.zeros((1,), dtype=torch.float32, device=hp_tensor.device)
+        # compute global amax to be used for scaling
+        _block_amax_atomic[grid](
+            flattened_input,
+            global_amax,
+            num_elements,
+            input_dtype=tl_input_dtype,
+            EPS=EPS,
+        )
 
-    # perform conversion
-    _to_fp8[grid](
-        flattened_input,
-        scale_out,
-        fp8_output,
-        num_elements,
-        fp8_dtype_min,
-        fp8_dtype_max,
-        input_dtype=tl_input_dtype,
-        output_dtype=tl_output_dtype,
-        BLOCK_SIZE=BLOCK_SIZE,
-        EPS=EPS,
-    )
+        torch.cuda.synchronize()
+
+        # perform conversion and store scale for use in Float8Tensor
+        _to_fp8_atomic[grid](
+            flattened_input,
+            scale_out,
+            global_amax,
+            fp8_output,
+            num_elements,
+            fp8_dtype_min,
+            fp8_dtype_max,
+            input_dtype=tl_input_dtype,
+            output_dtype=tl_output_dtype,
+            EPS=EPS,
+        )
+    elif algo == KernelAlgorithm.REDUCTION:
+        max_block_size = 512
+        BLOCK_SIZE = min(max_block_size, num_elements)
+        block_amaxes = torch.zeros(
+            (num_elements // BLOCK_SIZE,), dtype=torch.float32, device=hp_tensor.device
+        )
+        # compute local amax for each block
+        _block_amax_reduction[grid](
+            flattened_input,
+            block_amaxes,
+            num_elements,
+            input_dtype=tl_input_dtype,
+            BLOCK_SIZE=BLOCK_SIZE,
+            EPS=EPS,
+        )
+
+        # calculate global amax across all blocks and use it to compute scale
+        _fp8_scale_reduction[(1, 1, 1)](
+            block_amaxes,
+            scale_out,
+            num_elements,
+            fp8_dtype_max,
+            BLOCK_SIZE=BLOCK_SIZE,
+            EPS=EPS,
+        )
+
+        # perform conversion
+        _to_fp8_reduction[grid](
+            flattened_input,
+            scale_out,
+            fp8_output,
+            num_elements,
+            fp8_dtype_min,
+            fp8_dtype_max,
+            input_dtype=tl_input_dtype,
+            output_dtype=tl_output_dtype,
+            BLOCK_SIZE=BLOCK_SIZE,
+            EPS=EPS,
+        )
+    else:
+        raise ValueError(f"Unsupported kernel algorithm: {algo}")
 
     return Float8Tensor(
         fp8_output.reshape(orig_shape),

torchao/prototype/float8nocompile/kernels/fp8_dynamic_tensorwise_test.py

@@ -3,14 +3,24 @@
 from torchao.float8.float8_scaling_utils import hp_tensor_to_float8_dynamic
 from torchao.float8.float8_tensor import LinearMMConfig
 from torchao.prototype.float8nocompile.kernels.fp8_dynamic_tensorwise import (
+    KernelAlgorithm,
     triton_hp_tensor_to_float8_dynamic,
 )
 
 
-def test_fp8_triton_hp_tensor_to_float8_dynamic():
+@pytest.mark.parametrize(
+    "algo", [KernelAlgorithm.ATOMIC_MAX, KernelAlgorithm.REDUCTION]
+)
+@pytest.mark.parametrize(
+    "input_shape",
+    [(32, 32), (512, 512), (4096, 4096)],
+)
+def test_fp8_triton_hp_tensor_to_float8_dynamic(
+    algo: KernelAlgorithm, input_shape: tuple[int, int]
+):
     assert torch.cuda.is_available()
     device = "cuda"
-    input_bf16 = torch.randn((4, 4), dtype=torch.bfloat16, device=device)
+    input_bf16 = torch.randn(input_shape, dtype=torch.bfloat16, device=device)
     x_bf16 = input_bf16.clone().detach().to(device)
     y_bf16 = input_bf16.clone().detach().to(device)
 
@@ -26,6 +36,7 @@ def test_fp8_triton_hp_tensor_to_float8_dynamic():
         y_bf16,
         torch.float8_e4m3fn,
         LinearMMConfig(),
+        algo=algo,
     )
 
     def allclose_fp8(tensor1, tensor2, atol=1e-3, rtol=1e-3):