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CUDA/HIP: Flash Attention tuning (gfx1201) (#28102)

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* HIP: enable mma FA for head size 256 on RDNA4, tune configs

Assisted-by: Claude
Assisted-by: Codex

* HIP: prefer whole-tile FA grids over stream-k on AMD WMMA

Assisted-by: Claude
Assisted-by: Codex

* revise stream_k logic

* revise kernel selection logic

---------

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>
This commit is contained in:
Piotr Wilkin (ilintar)
2026-09-11 09:58:20 +02:00
committed by GitHub
parent 451b89bae0
commit 16378d93f9
4 changed files with 54 additions and 10 deletions
Download Patch File Download Diff File Expand all files Collapse all files
ggml/src/ggml-cuda/fattn-common.cuh
+14 -5
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@@ -1133,12 +1133,21 @@ void launch_fattn(
dim3 blocks_num;
if (stream_k) {
// For short contexts it can be faster to have the SMs work on whole tiles because this lets us skip the fixup.
const int max_blocks = max_blocks_per_sm*nsm;
const int tiles_nwaves = (ntiles_dst + max_blocks - 1) / max_blocks;
const int tiles_efficiency_percent = 100 * ntiles_dst / (max_blocks*tiles_nwaves);
auto should_use_stream_k = [](const int cc, const int ntiles_dst, const int max_blocks, const int DKQ) {
const int tiles_nwaves = (ntiles_dst + max_blocks - 1) / max_blocks;
const int tiles_efficiency_percent = 100 * ntiles_dst / (max_blocks*tiles_nwaves);
const bool use_stream_k = cc >= GGML_CUDA_CC_ADA_LOVELACE || amd_wmma_available(cc) || tiles_efficiency_percent < 75;
if (GGML_CUDA_CC_IS_NVIDIA(cc) && cc >= GGML_CUDA_CC_ADA_LOVELACE) {
return true;
}
if (amd_wmma_available(cc) && DKQ == 64) {
return true; // TODO better configuration
}
return tiles_efficiency_percent < 75;
};
const int max_blocks = max_blocks_per_sm*nsm;
const bool use_stream_k = should_use_stream_k(cc, ntiles_dst, max_blocks, Q->ne[0]);
blocks_num.x = ntiles_dst;
blocks_num.y = 1;
ggml/src/ggml-cuda/fattn-mma-f16.cuh
+3 -3
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@@ -158,8 +158,8 @@ static constexpr __host__ __device__ fattn_mma_config ggml_cuda_fattn_mma_get_co
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 8, 64, 2, 32, 128, 128, 128, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 16, 64, 2, 32, 128, 128, 128, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 32, 128, 2, 64, 128, 128, 64, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 64, 128, 2, 64, 128, 128, 64, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 32, 256, 2, 64, 128, 128, 64, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(256, 256, 64, 256, 2, 64, 128, 128, 64, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(320, 256, 32, 128, 2, 32, 160, 128, 128, 1, true);
GGML_CUDA_FATTN_MMA_CONFIG_CASE(320, 256, 64, 128, 2, 32, 160, 128, 128, 1, true);
@@ -1826,7 +1826,7 @@ static __global__ void flash_attn_ext_f16(
#endif // __CUDA_ARCH__ == GGML_CUDA_CC_TURING
#if defined(AMD_WMMA_AVAILABLE)
if (ncols1*ncols2 < 16 || ncols2 == 1 || DKQ > 128) {
if (ncols1*ncols2 < 16 || ncols2 == 1 || DKQ > 256) {
NO_DEVICE_CODE;
return;
}
ggml/src/ggml-cuda/fattn.cu
+21 -2
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@@ -221,6 +221,24 @@ static void ggml_cuda_flash_attn_ext_mma_f16_switch_ncols2(ggml_backend_cuda_con
}
}
// On RDNA it is preferable to minimize wasted compute vs. duplicate I/O for the mask.
if (amd_wmma_available(cc)) {
if (use_gqa_opt && gqa_ratio % 8 == 0) {
ggml_cuda_flash_attn_ext_mma_f16_switch_ncols1<DKQ, DV, 8>(ctx, dst);
return;
}
if (use_gqa_opt && gqa_ratio % 4 == 0) {
ggml_cuda_flash_attn_ext_mma_f16_switch_ncols1<DKQ, DV, 4>(ctx, dst);
return;
}
if (use_gqa_opt && gqa_ratio % 2 == 0) {
ggml_cuda_flash_attn_ext_mma_f16_switch_ncols1<DKQ, DV, 2>(ctx, dst);
return;
}
}
if (use_gqa_opt && gqa_ratio > 4) {
ggml_cuda_flash_attn_ext_mma_f16_switch_ncols1<DKQ, DV, 8>(ctx, dst);
return;
@@ -646,8 +664,9 @@ static best_fattn_kernel ggml_cuda_get_best_fattn_kernel(const int device, const
}
}
// AMD WMMA is always faster than the tile kernel if the full tile width of 16 can be utilized.
if ((amd_wmma_available(cc) && gqa_opt_applies && Q->ne[0] <= 128) && Q->ne[0] != 40 && Q->ne[0] != 72 && Q->ne[1] * gqa_ratio_eff > 8) {
// AMD WMMA is faster than the tile kernel if the wide tiles with high arithmetic intensity can be utilized.
if ((amd_wmma_available(cc) && gqa_opt_applies && Q->ne[0] <= 256) && Q->ne[0] != 40 && Q->ne[0] != 72 &&
Q->ne[1] * gqa_ratio_eff > (Q->ne[0] <= 128 ? 8 : 16)) {
return BEST_FATTN_KERNEL_MMA_F16;
}
tests/test-backend-ops.cpp
+16
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@@ -10574,6 +10574,13 @@ static std::vector<std::unique_ptr<test_case>> make_test_cases_eval() {
GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
}
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 512, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 4096, 64, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 4096, 16, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {2, 1}, 4096, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {4, 1}, 4096, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 2, {12, 1}, 4096, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
// dense-allocated (non-view) quant K/V at batch >= 64, in cache and native layouts
test_cases.emplace_back(new test_flash_attn_ext(64, 64, 4, {1, 1}, 512, 75, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_Q8_0, GGML_TYPE_Q8_0, {0, 2, 1, 3}, false));
test_cases.emplace_back(new test_flash_attn_ext(64, 64, 4, {4, 1}, 512, 75, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_Q8_0, GGML_TYPE_Q8_0, {0, 2, 1, 3}, false));
@@ -11031,6 +11038,15 @@ static std::vector<std::unique_ptr<test_case>> make_test_cases_perf() {
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 2, {16, 1}, 10000, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 2, {16, 1}, 20000, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 4096, 1, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 4096, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 16384, 1, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 16384, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 65536, 1, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 65536, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 131072, 1, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
test_cases.emplace_back(new test_flash_attn_ext(256, 256, 4, {6, 1}, 131072, 512, true, false, 0, 0, GGML_PREC_F32, GGML_TYPE_F16, GGML_TYPE_F16));
for (int kv : { 4096, 8192, 16384,32768, 65536, }) {
for (int hs : { 64, 128, 256, 576, }) {
const int hsv = hs == 576 ? 512 : hs;