triton/_sources/getting-started/tutorials/03-matrix-multiplication.rst.txt

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.. only:: html

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        Click :ref:`here <sphx_glr_download_getting-started_tutorials_03-matrix-multiplication.py>`
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.. _sphx_glr_getting-started_tutorials_03-matrix-multiplication.py:


Matrix Multiplication
======================
In this tutorial, you will write a 25-lines high-performance FP16 matrix multiplication kernel that achieves performance on par with cuBLAS.
You will specifically learn about:

- Block-level matrix multiplications
- Multi-dimensional pointer arithmetic
- Program re-ordering for improved L2 cache hit rate 
- Automatic performance tuning

.. GENERATED FROM PYTHON SOURCE LINES 14-37

Motivations
-------------
Matrix multiplications are a key building block of most modern high-performance computing systems.
They are notoriously hard to optimize, hence their implementation is generally done by hardware vendors themselves as part of so-called "kernel libraries" (e.g., cuBLAS).
Unfortunately, these libraries are often proprietary and cannot be easily customized to accomodate the needs of modern deep learning workloads (e.g., fused activation functions).
In this tutorial, you will learn how to implement efficient matrix multiplications by yourself with Triton, in a way that is easy to customize and extend.

Roughly speaking, the kernel that we will write will implement the following blocked algorithm:

 .. code-block:: python

   # do in parallel
   for m in range(0, M, BLOCK_M):
     # do in parallel
     for n in range(0, N, BLOCK_N):
       acc = zeros((BLOCK_M, BLOCK_N), dtype=float32)
       for k in range(0, K, BLOCK_K):
         a = A[m : m+BLOCK_M, k : k+BLOCK_K]
         b = B[k : k+BLOCK_K, n : n+BLOCK_N]
         acc += dot(a, b)
       C[m : m+BLOCK_M, n : n+BLOCK_N] = acc;

where each iteration of the doubly-nested for-loop corresponds to a Triton program instance.

.. GENERATED FROM PYTHON SOURCE LINES 39-110

Compute Kernel
----------------

The above algorithm is, actually, fairly straightforward to implement in Triton.
The main difficulty comes from the computation of the memory locations at which blocks of :code:`A` and :code:`B` must be read in the inner loop. For that, we need multi-dimensional pointer arithmetics.

Pointer Arithmetics
~~~~~~~~~~~~~~~~~~~~

For a row-major 2D tensor :code:`X`, the memory location of :code:`X[i, j]` is given by :code:`&X[i, j] = X + i*stride_x_0 + j*stride_x_1`.
Therefore, blocks of pointers for :code:`A[m : m+BLOCK_M, k:k+BLOCK_K]` and :code:`B[k : k+BLOCK_K, n : n+BLOCK_N]` can be defined in pseudo-code as:

 .. code-block:: python

   &A[m : m+BLOCK_M, k:k+BLOCK_K] =  A + (m : m+BLOCK_M)[:, None]*A.stride(0) + (k : k+BLOCK_K)[None, :]*A.stride(1);
   &B[k : k+BLOCK_K, n:n+BLOCK_N] =  B + (k : k+BLOCK_K)[:, None]*B.stride(0) + (n : n+BLOCK_N)[None, :]*B.stride(1);

Which means that pointers for blocks of A and B can be initialized (i.e., :code:`k=0`) in Triton as:

 .. code-block:: python

   pid_m = triton.program_id(0)
   pid_n = triton.program_id(1)
   rm = pid_m * BLOCK_M + triton.arange(0, BLOCK_M)
   rn = pid_n * BLOCK_N + triton.arange(0, BLOCK_N)
   rk = triton.arange(0, BLOCK_K)
   // pointer for A operand
   pa = A + (rm[:, None] * stride_a_0 + rk[None, :] * stride_a_1);
   // pointer for B operand
   pb = B + (rk[:, None] * stride_b_0 + rn[None, :] * stride_b_1);

And then updated in the inner loop as follows:

 .. code-block:: python

   pa += BLOCK_K * stride_a_1;
   pb += BLOCK_K * stride_b_0;


L2 Cache Optimizations
~~~~~~~~~~~~~~~~~~~~~~~~

As mentioned above, each program instance computes an :code:`[BLOCK_M, BLOCK_N]` block of :code:`C`.
It is important to remember that the order in which these blocks are computed does matter, since it affects the L2 cache hit rate of our program.
And unfortunately, a simple row-major ordering

 .. code-block:: Python

   pid = triton.program_id(0);
   grid_m = (M + BLOCK_M - 1) // BLOCK_M;
   grid_n = (N + BLOCK_N - 1) // BLOCK_N;
   pid_m = pid / grid_n;
   pid_n = pid % grid_n;

is just not going to cut it.

One possible solution is to launch blocks in an order that promotes data reuse.
This can be done by 'super-grouping' blocks in groups of :code:`GROUP_M` rows before switching to the next column:

 .. code-block:: python

   pid = triton.program_id(0);
   width = GROUP_M * grid_n;
   group_id = pid // width;
   # we need to handle the case where M % (GROUP_M*BLOCK_M) != 0
   group_size = min(grid_m - group_id * GROUP_M, GROUP_M);
   pid_m = group_id * GROUP_M + (pid % group_size);
   pid_n = (pid % width) // (group_size);

In practice, this can improve the performance of our matrix multiplication kernel by >10\% on some hardware architecture (e.g., 220 to 245 TFLOPS on A100).


.. GENERATED FROM PYTHON SOURCE LINES 112-115

Final Result
-------------


.. GENERATED FROM PYTHON SOURCE LINES 115-190

.. code-block:: default


    import torch
    import triton
    import triton.language as tl

    # %
    # :code:`triton.jit`'ed functions can be auto-tuned by using the `triton.autotune` decorator, which consumes:
    #   - A list of :code:`triton.Config` objects that define different configurations of meta-parameters (e.g., BLOCK_M) and compilation options (e.g., num_warps) to try
    #   - A autotuning *key* whose change in values will trigger evaluation of all the provided configs

    @triton.autotune(
        configs=[
            triton.Config({'BLOCK_M': 128, 'BLOCK_N': 256, 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=3, num_warps=8),
            triton.Config({'BLOCK_M': 256, 'BLOCK_N': 128, 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=3, num_warps=8),
            triton.Config({'BLOCK_M': 256, 'BLOCK_N': 64,  'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),
            triton.Config({'BLOCK_M': 64 , 'BLOCK_N': 256, 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),\
            triton.Config({'BLOCK_M': 128, 'BLOCK_N': 128, 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),\
            triton.Config({'BLOCK_M': 128, 'BLOCK_N': 64 , 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),\
            triton.Config({'BLOCK_M': 64 , 'BLOCK_N': 128, 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),
            triton.Config({'BLOCK_M': 128, 'BLOCK_N': 32 , 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=4, num_warps=4),\
            triton.Config({'BLOCK_M': 64 , 'BLOCK_N': 32 , 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=5, num_warps=2),\
            triton.Config({'BLOCK_M': 32 , 'BLOCK_N': 64 , 'BLOCK_K': 32, 'GROUP_M': 8}, num_stages=5, num_warps=2),
            #triton.Config({'BLOCK_M': 64, 'BLOCK_N': 128, 'BLOCK_K': 32, 'GROUP_M': 8}, num_warps=4),
        ],
        key=['M', 'N', 'K'],
    )
    # %
    # We can now define our kernel as normal, using all the techniques presented above
    @triton.jit
    def _matmul(A, B, C, M, N, K, stride_am, stride_ak, stride_bk, stride_bn, stride_cm, stride_cn, **META):
        # extract meta-parameters
        BLOCK_M = META['BLOCK_M']
        BLOCK_N = META['BLOCK_N']
        BLOCK_K = META['BLOCK_K']
        GROUP_M = 8
        # matrix multiplication
        pid = tl.program_id(0)
        grid_m = (M + BLOCK_M - 1) // BLOCK_M
        grid_n = (N + BLOCK_N - 1) // BLOCK_N
        # re-order program ID for better L2 performance
        width = GROUP_M * grid_n
        group_id = pid // width
        group_size = min(grid_m - group_id * GROUP_M, GROUP_M)
        pid_m = group_id * GROUP_M + (pid % group_size)
        pid_n = (pid % width) // (group_size)
        # do matrix multiplication
        rm = pid_m * BLOCK_M + tl.arange(0, BLOCK_M)
        rn = pid_n * BLOCK_N + tl.arange(0, BLOCK_N)
        rk = tl.arange(0, BLOCK_K)
        A = A + (rm[:, None] * stride_am + rk[None, :] * stride_ak)
        B = B + (rk[:, None] * stride_bk + rn[None, :] * stride_bn)
        acc = tl.zeros((BLOCK_M, BLOCK_N), dtype=tl.float32)
        for k in range(K, 0, -BLOCK_K):
            a = tl.load(A)
            b = tl.load(B)
            acc += tl.dot(a, b)
            A += BLOCK_K * stride_ak
            B += BLOCK_K * stride_bk
        # triton can accept arbitrary activation function
        # via metaparameters!
        if META['ACTIVATION']:
            acc = META['ACTIVATION'](acc)
        # rematerialize rm and rn to save registers
        rm = pid_m * BLOCK_M + tl.arange(0, BLOCK_M)
        rn = pid_n * BLOCK_N + tl.arange(0, BLOCK_N)
        C = C + (rm[:, None] * stride_cm + rn[None, :] * stride_cn)
        mask = (rm[:, None] < M) & (rn[None, :] < N)
        tl.store(C, acc, mask=mask)


    # we can fuse `leaky_relu` by providing it as an `ACTIVATION` meta-parameter in `_matmul`
    @triton.jit
    def leaky_relu(x):
        return tl.where(x >= 0, x, 0.01*x)








.. GENERATED FROM PYTHON SOURCE LINES 191-193

We can now create a convenience wrapper function that only takes two input tensors
and (1) checks any shape constraint; (2) allocates the output; (3) launches the above kernel

.. GENERATED FROM PYTHON SOURCE LINES 193-214

.. code-block:: default


    def matmul(a, b, activation=None):
        # checks constraints
        assert a.shape[1] == b.shape[0], "incompatible dimensions"
        assert a.is_contiguous(), "matrix A must be contiguous"
        assert b.is_contiguous(), "matrix B must be contiguous"
        M, K = a.shape
        _, N = b.shape
        # allocates output
        c = torch.empty((M, N), device=a.device, dtype=a.dtype)
        # launch kernel
        grid = lambda META: (triton.cdiv(M, META['BLOCK_M']) * triton.cdiv(N, META['BLOCK_N']), )
        pgm = _matmul[grid](
            a, b, c, M, N, K, \
            a.stride(0), a.stride(1), b.stride(0), b.stride(1), c.stride(0), c.stride(1),\
            ACTIVATION = activation
        )
        # done; return the output tensor
        return c









.. GENERATED FROM PYTHON SOURCE LINES 215-219

Unit Test
-----------

We can test our custom matrix multiplication operation against a native torch implementation (i.e., cuBLAS)

.. GENERATED FROM PYTHON SOURCE LINES 219-229

.. code-block:: default


    torch.manual_seed(0)
    a = torch.randn((512, 512), device='cuda', dtype=torch.float16)
    b = torch.randn((512, 512), device='cuda', dtype=torch.float16)
    c_0 = matmul(a, b, activation=None)
    c_1 = torch.matmul(a, b)
    print(c_0)
    print(c_1)
    print(triton.testing.allclose(c_0, c_1))





.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    tensor([[  1.1045, -36.9688,  31.4688,  ..., -11.3984,  24.4531, -32.3438],
            [  6.3555, -19.6094,  34.0938,  ...,  -5.8945,   5.2891,   6.8867],
            [-32.0625,   5.9492,  15.3984,  ..., -21.3906, -23.9844, -10.1328],
            ...,
            [ -5.7031,   7.4492,   8.2656,  ..., -10.6953, -40.0000,  17.7500],
            [ 25.5000,  24.3281,  -8.4688,  ..., -18.9375,  32.5312, -29.9219],
            [ -5.3477,   4.9844,  11.8906,  ...,   5.5898,   6.4023, -17.3125]],
           device='cuda:0', dtype=torch.float16)
    tensor([[  1.1045, -36.9688,  31.4688,  ..., -11.3906,  24.4531, -32.3438],
            [  6.3516, -19.6094,  34.0938,  ...,  -5.8906,   5.2812,   6.8828],
            [-32.0625,   5.9531,  15.3984,  ..., -21.4062, -23.9844, -10.1328],
            ...,
            [ -5.7070,   7.4492,   8.2656,  ..., -10.6953, -40.0000,  17.7500],
            [ 25.5000,  24.3438,  -8.4609,  ..., -18.9375,  32.5312, -29.9219],
            [ -5.3477,   4.9805,  11.8828,  ...,   5.5859,   6.4023, -17.3125]],
           device='cuda:0', dtype=torch.float16)
    tensor(True, device='cuda:0')




.. GENERATED FROM PYTHON SOURCE LINES 230-236

Benchmark
--------------

Square Matrix Performance
~~~~~~~~~~~~~~~~~~~~~~~~~~
We can now compare the performance of our kernel against that of cuBLAS. Here we focus on square matrices, but feel free to arrange this script as you wish to benchmark any other matrix shape.

.. GENERATED FROM PYTHON SOURCE LINES 236-268

.. code-block:: default



    @triton.testing.perf_report(
        triton.testing.Benchmark(
            x_names=['M', 'N', 'K'],  # argument names to use as an x-axis for the plot
            x_vals=[128 * i for i in range(1, 33)],  # different possible values for `x_name`
            line_arg='provider',  # argument name whose value corresponds to a different line in the plot
            line_vals=['cublas', 'cublas + relu', 'triton', 'triton + relu'],  # possible values for `line_arg``
            line_names=["cuBLAS", "cuBLAS (+ torch.nn.LeakyReLU)", "Triton", "Triton (+ LeakyReLU)"],  # label name for the lines
            styles=[('green', '-'), ('green', '--'), ('blue', '-'), ('blue', '--')],  # line styles
            ylabel="TFLOPS",  # label name for the y-axis
            plot_name="matmul-performance",  # name for the plot. Used also as a file name for saving the plot.
            args={}
        )
    )
    def benchmark(M, N, K, provider):
        a = torch.randn((M, K), device='cuda', dtype=torch.float16)
        b = torch.randn((K, N), device='cuda', dtype=torch.float16)
        if provider == 'cublas':
            ms, min_ms, max_ms = triton.testing.do_bench(lambda: torch.matmul(a, b))
        if provider == 'triton':
            ms, min_ms, max_ms = triton.testing.do_bench(lambda: matmul(a, b))
        if provider == 'cublas + relu':
            torch_relu = torch.nn.ReLU(inplace=True)
            ms, min_ms, max_ms = triton.testing.do_bench(lambda: torch_relu(torch.matmul(a, b)))
        if provider == 'triton + relu':
            ms, min_ms, max_ms = triton.testing.do_bench(lambda: matmul(a, b, activation=leaky_relu))
        perf = lambda ms: 2 * M * N * K * 1e-12 / (ms * 1e-3)
        return perf(ms), perf(max_ms), perf(min_ms)


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



.. image:: /getting-started/tutorials/images/sphx_glr_03-matrix-multiplication_001.png
    :alt: 03 matrix multiplication
    :class: sphx-glr-single-img


.. rst-class:: sphx-glr-script-out

 Out:

 .. code-block:: none

    matmul-performance:
             M     cuBLAS  ...     Triton  Triton (+ LeakyReLU)
    0    128.0   0.455111  ...   0.512000              0.512000
    1    256.0   2.730667  ...   2.978909              2.978909
    2    384.0   7.372800  ...   8.507077              8.507077
    3    512.0  14.563555  ...  16.384000             15.420235
    4    640.0  22.260869  ...  24.380953             24.380953
    5    768.0  32.768000  ...  34.028308             34.028308
    6    896.0  39.025776  ...  39.025776             39.025776
    7   1024.0  49.932191  ...  52.428801             52.428801
    8   1152.0  44.566925  ...  46.656000             46.656000
    9   1280.0  51.200001  ...  56.109587             56.109587
    10  1408.0  64.138541  ...  65.684049             64.902096
    11  1536.0  80.430545  ...  76.106321             76.106321
    12  1664.0  63.372618  ...  61.636381             61.636381
    13  1792.0  72.983276  ...  69.379162             68.953520
    14  1920.0  68.435645  ...  69.818184             68.776119
    15  2048.0  73.584279  ...  75.915006             75.234154
    16  2176.0  83.500614  ...  78.608000             79.540109
    17  2304.0  68.251065  ...  73.051599             72.607513
    18  2432.0  71.125224  ...  81.669953             80.269900
    19  2560.0  77.833728  ...  75.328737             74.812787
    20  2688.0  81.576466  ...  81.227100             82.823267
    21  2816.0  83.392363  ...  78.868366             77.330158
    22  2944.0  81.698415  ...  80.122235             79.356738
    23  3072.0  80.890151  ...  81.589488             83.391907
    24  3200.0  84.656085  ...  85.447263             88.033012
    25  3328.0  82.939284  ...  86.632127             82.558825
    26  3456.0  81.518272  ...  84.954233             84.420490
    27  3584.0  83.254996  ...  94.747514             95.654673
    28  3712.0  86.192706  ...  88.955779             88.797643
    29  3840.0  84.809814  ...  84.874902             84.356981
    30  3968.0  89.593789  ...  84.738843             84.212518
    31  4096.0  93.596744  ...  83.833686             90.933423

    [32 rows x 5 columns]





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   **Total running time of the script:** ( 2 minutes  16.432 seconds)


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