92 lines
4.3 KiB
C++
92 lines
4.3 KiB
C++
// Copyright (c) 2022, Tri Dao.
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#pragma once
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#include <vector>
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#include <cuda_fp16.h>
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#include <cuda_bf16.h>
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#include "static_switch.h"
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#include "fmha.h"
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#include "fmha_fprop_kernel_1xN.h"
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// Find the number of splits that maximizes the occupancy. For example, if we have
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// batch * n_heads = 48 and we have 108 SMs, having 2 splits (efficiency = 0.89) is
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// better than having 3 splits (efficiency = 0.67). However, we also don't want too many
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// splits as that would incur more HBM reads/writes.
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// So we find the best efficiency, then find the smallest number of splits that gets 95%
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// of the best efficiency.
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// [2022-11-25] TD: Mark this as "inline" otherwise we get "multiple definition" error.
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inline int num_splits_heuristic_fwd(int batch_nheads, int num_SMs, int ctas_per_sm, int max_splits) {
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float max_efficiency = 0.f;
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std::vector<float> efficiency;
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efficiency.reserve(max_splits);
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for (int num_splits = 1; num_splits <= max_splits; num_splits++) {
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float n_waves = float(batch_nheads * num_splits) / (num_SMs * ctas_per_sm);
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float eff = n_waves / ceil(n_waves);
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// printf("num_splits = %d, eff = %f\n", num_splits, eff);
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if (eff > max_efficiency) { max_efficiency = eff; }
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efficiency.push_back(eff);
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}
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for (int num_splits = 1; num_splits <= max_splits; num_splits++) {
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if (efficiency[num_splits - 1] > 0.95 * max_efficiency) {
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// printf("num_splits chosen = %d\n", num_splits);
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return num_splits;
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}
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}
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return 1;
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}
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template<typename Kernel_traits, bool Is_dropout, bool Is_causal, bool Return_softmax>
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__global__ void fmha_fwd_loop_kernel(FMHA_fprop_params params) {
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fmha::device_1xN_loop<Kernel_traits, Is_dropout, Is_causal, Return_softmax>(params);
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}
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template<typename Kernel_traits>
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void run_fmha_fwd_loop(Launch_params<FMHA_fprop_params> &launch_params) {
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constexpr int blocksize_c = Kernel_traits::Cta_tile_p::N;
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const int loop_steps = (launch_params.params.seqlen_k + blocksize_c - 1) / blocksize_c;
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constexpr int smem_size_softmax_lse = Kernel_traits::Smem_dp_sum::BYTES_PER_TILE;
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// Don't need smem_size_softmax_lse if we're not looping
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const int smem_size = fmha::get_dynamic_smem_size<Kernel_traits>()
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+ (loop_steps > 1 ? smem_size_softmax_lse : 0);
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// Work-around for gcc 7. It doesn't like nested BOOL_SWITCH.
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// https://github.com/kokkos/kokkos-kernels/issues/349
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// https://github.com/HazyResearch/flash-attention/issues/21
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BOOL_SWITCH(launch_params.is_dropout, IsDropoutConst, ({
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auto kernel = launch_params.params.is_causal
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? (launch_params.return_softmax
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? &fmha_fwd_loop_kernel<Kernel_traits, IsDropoutConst, true, true>
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: &fmha_fwd_loop_kernel<Kernel_traits, IsDropoutConst, true, false>)
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: (launch_params.return_softmax
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? &fmha_fwd_loop_kernel<Kernel_traits, IsDropoutConst, false, true>
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: &fmha_fwd_loop_kernel<Kernel_traits, IsDropoutConst, false, false>);
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if( smem_size >= 48 * 1024 ) {
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FMHA_CHECK_CUDA(cudaFuncSetAttribute(
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kernel, cudaFuncAttributeMaxDynamicSharedMemorySize, smem_size));
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}
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// Automatically set num_splits to maximize occupancy
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if (launch_params.params.num_splits <= 0) {
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int ctas_per_sm;
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cudaError status_ = cudaOccupancyMaxActiveBlocksPerMultiprocessor(
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&ctas_per_sm, kernel, Kernel_traits::THREADS, smem_size);
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auto dprops = at::cuda::getCurrentDeviceProperties();
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// printf("CTAS_PER_SM = %d, nSMs = %d\n", ctas_per_sm, dprops->multiProcessorCount);
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constexpr int M = Kernel_traits::Cta_tile_p::M;
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launch_params.params.num_splits = num_splits_heuristic_fwd(
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launch_params.params.b * launch_params.params.h, dprops->multiProcessorCount,
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ctas_per_sm,
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/*max_splits=*/std::min(30, (launch_params.params.seqlen_q + M - 1 / M))
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);
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}
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// printf("smem_size = %d\n", smem_size);
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dim3 grid(launch_params.params.b, launch_params.params.h, launch_params.params.num_splits);
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kernel<<<grid, Kernel_traits::THREADS, smem_size, launch_params.stream>>>(
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launch_params.params);
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FMHA_CHECK_CUDA(cudaPeekAtLastError());
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}));
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}
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