02-fused-softmax.py

"""
Fused Softmax
=============

In this tutorial, you will write a fused softmax operation that is significantly faster
than PyTorch's native op for a particular class of matrices: those whose rows can fit in
the GPU's SRAM.

In doing so, you will learn about:

* The benefits of kernel fusion for bandwidth-bound operations.

* Reduction operators in Triton.

"""

# %%
# Motivations
# -----------
#
# Custom GPU kernels for elementwise additions are educationally valuable but won't get you very far in practice.
# Let us consider instead the case of a simple (numerically stabilized) softmax operation:

import torch

import triton
import triton.language as tl
from triton.runtime import driver


def naive_softmax(x):
    """Compute row-wise softmax of X using native pytorch

    We subtract the maximum element in order to avoid overflows. Softmax is invariant to
    this shift.
    """
    # read  MN elements ; write M  elements
    x_max = x.max(dim=1)[0]
    # read MN + M elements ; write MN elements
    z = x - x_max[:, None]
    # read  MN elements ; write MN elements
    numerator = torch.exp(z)
    # read  MN elements ; write M  elements
    denominator = numerator.sum(dim=1)
    # read MN + M elements ; write MN elements
    ret = numerator / denominator[:, None]
    # in total: read 5MN + 2M elements ; wrote 3MN + 2M elements
    return ret


# %%
# When implemented naively in PyTorch, computing :code:`y = naive_softmax(x)` for :math:`x \in R^{M \times N}`
# requires reading :math:`5MN + 2M` elements from DRAM and writing back :math:`3MN + 2M` elements.
# This is obviously wasteful; we'd prefer to have a custom "fused" kernel that only reads
# X once and does all the necessary computations on-chip.
# Doing so would require reading and writing back only :math:`MN` bytes, so we could
# expect a theoretical speed-up of ~4x (i.e., :math:`(8MN + 4M) / 2MN`).
# The `torch.jit.script` flags aims to perform this kind of "kernel fusion" automatically
# but, as we will see later, it is still far from ideal.

# %%
# Compute Kernel
# --------------
#
# Our softmax kernel works as follows: each program loads a set of rows of the input matrix X strided by number of programs,
# normalizes it and writes back the result to the output Y.
#
# Note that one important limitation of Triton is that each block must have a
# power-of-two number of elements, so we need to internally "pad" each row and guard the
# memory operations properly if we want to handle any possible input shapes:


@triton.jit
def softmax_kernel(output_ptr, input_ptr, input_row_stride, output_row_stride, n_rows, n_cols, BLOCK_SIZE: tl.constexpr,
                   num_stages: tl.constexpr):
    # starting row of the program
    row_start = tl.program_id(0)
    row_step = tl.num_programs(0)
    for row_idx in tl.range(row_start, n_rows, row_step, num_stages=num_stages):
        # The stride represents how much we need to increase the pointer to advance 1 row
        row_start_ptr = input_ptr + row_idx * input_row_stride
        # The block size is the next power of two greater than n_cols, so we can fit each
        # row in a single block
        col_offsets = tl.arange(0, BLOCK_SIZE)
        input_ptrs = row_start_ptr + col_offsets
        # Load the row into SRAM, using a mask since BLOCK_SIZE may be > than n_cols
        mask = col_offsets < n_cols
        row = tl.load(input_ptrs, mask=mask, other=-float('inf'))
        # Subtract maximum for numerical stability
        row_minus_max = row - tl.max(row, axis=0)
        # Note that exponentiation in Triton is fast but approximate (i.e., think __expf in CUDA)
        numerator = tl.exp(row_minus_max)
        denominator = tl.sum(numerator, axis=0)
        softmax_output = numerator / denominator
        # Write back output to DRAM
        output_row_start_ptr = output_ptr + row_idx * output_row_stride
        output_ptrs = output_row_start_ptr + col_offsets
        tl.store(output_ptrs, softmax_output, mask=mask)


# %%
# We can create a helper function that enqueues the kernel and its (meta-)arguments for any given input tensor.

device = torch.cuda.current_device()
properties = driver.active.utils.get_device_properties(device)
NUM_SM = properties["multiprocessor_count"]
NUM_REGS = properties["max_num_regs"]
SIZE_SMEM = properties["max_shared_mem"]
WARP_SIZE = properties["warpSize"]
target = triton.runtime.driver.active.get_current_target()
kernels = {}


def softmax(x):
    n_rows, n_cols = x.shape

    # The block size of each loop iteration is the smallest power of two greater than the number of columns in `x`
    BLOCK_SIZE = triton.next_power_of_2(n_cols)

    # Another trick we can use is to ask the compiler to use more threads per row by
    # increasing the number of warps (`num_warps`) over which each row is distributed.
    # You will see in the next tutorial how to auto-tune this value in a more natural
    # way so you don't have to come up with manual heuristics yourself.
    num_warps = 8

    # Number of software piepling stages.
    num_stages = 4 if SIZE_SMEM > 200000 else 2

    # Allocate output
    y = torch.empty_like(x)

    # pre-compile kernel to get register usage and compute thread occupancy.
    kernel, num_programs = kernels.get(BLOCK_SIZE, (None, 0))
    if kernel is None:
        kernel = softmax_kernel.warmup(y, x, x.stride(0), y.stride(0), n_rows, n_cols, BLOCK_SIZE=BLOCK_SIZE,
                                       num_stages=num_stages, num_warps=num_warps, grid=(1, ))
        kernel._init_handles()
        n_regs = kernel.n_regs
        size_smem = kernel.metadata.shared
        occupancy = NUM_REGS // (n_regs * WARP_SIZE * num_warps)
        occupancy = min(occupancy, SIZE_SMEM // size_smem)
        num_programs = NUM_SM * occupancy
        kernels[BLOCK_SIZE] = (kernel, num_programs)

    num_programs = min(num_programs, n_rows)

    # Create a number of persistent programs.
    kernel[(num_programs, 1, 1)](
        y,
        x,
        x.stride(0),
        y.stride(0),
        n_rows,
        n_cols,
    )
    return y


# %%
# Unit Test
# ---------

# %%
# We make sure that we test our kernel on a matrix with an irregular number of rows and columns.
# This will allow us to verify that our padding mechanism works.

torch.manual_seed(0)
x = torch.randn(1823, 781, device='cuda')
y_triton = softmax(x)
y_torch = torch.softmax(x, axis=1)
assert torch.allclose(y_triton, y_torch), (y_triton, y_torch)

# %%
# As expected, the results are identical.

# %%
# Benchmark
# ---------
#
# Here we will benchmark our operation as a function of the number of columns in the input matrix -- assuming 4096 rows.
# We will then compare its performance against (1) :code:`torch.softmax` and (2) the :code:`naive_softmax` defined above.


@triton.testing.perf_report(
    triton.testing.Benchmark(
        x_names=['N'],  # argument names to use as an x-axis for the plot
        x_vals=[128 * i for i in range(2, 100)],  # different possible values for `x_name`
        line_arg='provider',  # argument name whose value corresponds to a different line in the plot
        line_vals=['triton', 'torch'],  # possible values for `line_arg``
        line_names=[
            "Triton",
            "Torch",
        ],  # label name for the lines
        styles=[('blue', '-'), ('green', '-')],  # line styles
        ylabel="GB/s",  # label name for the y-axis
        plot_name="softmax-performance",  # name for the plot. Used also as a file name for saving the plot.
        args={'M': 4096},  # values for function arguments not in `x_names` and `y_name`
    ))
def benchmark(M, N, provider):
    x = torch.randn(M, N, device='cuda', dtype=torch.float32)
    stream = torch.cuda.Stream()
    torch.cuda.set_stream(stream)
    if provider == 'torch':
        ms = triton.testing.do_bench(lambda: torch.softmax(x, axis=-1))
    if provider == 'triton':
        ms = triton.testing.do_bench(lambda: softmax(x))
    gbps = lambda ms: 2 * x.nelement() * x.element_size() * 1e-9 / (ms * 1e-3)
    return gbps(ms)


benchmark.run(show_plots=True, print_data=True)

# %%
# In the above plot, we can see that:
#  - Triton is 4x faster than the Torch JIT. This confirms our suspicions that the Torch JIT does not do any fusion here.
#  - Triton is noticeably faster than :code:`torch.softmax` -- in addition to being **easier to read, understand and maintain**.
#    Note however that the PyTorch `softmax` operation is more general and will work on tensors of any shape.