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perf(ggml-cuda): skip shared reduction and barrier for single-wave mul_mat_vec_q #521

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Merged
davide221 merged 1 commit into from
Jul 16, 2026
+38 −25
Merged

perf(ggml-cuda): skip shared reduction and barrier for single-wave mul_mat_vec_q #521

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63 changes: 38 additions & 25 deletions server/deps/llama.cpp/ggml/src/ggml-cuda/mmvq.cu
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Original file line number Diff line number Diff line change
Expand Up @@ -608,32 +608,54 @@ static __global__ void mul_mat_vec_q(
}
}

__shared__ float tmp_shared[nwarps-1 > 0 ? nwarps-1 : 1][ncols_dst][rows_per_cuda_block][warp_size];
__shared__ float tmp_shared_gate[(has_fusion && (nwarps-1 > 0)) ? nwarps-1 : 1][ncols_dst][rows_per_cuda_block][warp_size];
if constexpr (!has_fusion) {
(void) tmp_shared_gate;
} else if (!use_gate) {
(void) tmp_shared_gate;
}
// A one-wave specialization has no cross-wave partials. Avoid reserving
// shared memory and issuing a block barrier in that common decode path.
// This does not change the per-lane accumulation or warp reduction order.
if constexpr (nwarps > 1) {
__shared__ float tmp_shared[nwarps-1][ncols_dst][rows_per_cuda_block][warp_size];
__shared__ float tmp_shared_gate[has_fusion ? nwarps-1 : 1][ncols_dst][rows_per_cuda_block][warp_size];
if constexpr (!has_fusion) {
(void) tmp_shared_gate;
} else if (!use_gate) {
(void) tmp_shared_gate;
}

if (threadIdx.y > 0) {
#pragma unroll
for (int j = 0; j < ncols_dst; ++j) {
#pragma unroll
for (int i = 0; i < rows_per_cuda_block; ++i) {
tmp_shared[threadIdx.y-1][j][i][threadIdx.x] = tmp[j][i];
if constexpr (has_fusion) {
if (use_gate) {
tmp_shared_gate[threadIdx.y-1][j][i][threadIdx.x] = tmp_gate[j][i];
}
}
}
}
}

__syncthreads();
if (threadIdx.y > 0) {
return;
}

if (threadIdx.y > 0) {
#pragma unroll
for (int j = 0; j < ncols_dst; ++j) {
#pragma unroll
for (int i = 0; i < rows_per_cuda_block; ++i) {
tmp_shared[threadIdx.y-1][j][i][threadIdx.x] = tmp[j][i];
if constexpr (has_fusion) {
if (use_gate) {
tmp_shared_gate[threadIdx.y-1][j][i][threadIdx.x] = tmp_gate[j][i];
#pragma unroll
for (int l = 0; l < nwarps-1; ++l) {
tmp[j][i] += tmp_shared[l][j][i][threadIdx.x];
if constexpr (has_fusion) {
if (use_gate) {
tmp_gate[j][i] += tmp_shared_gate[l][j][i][threadIdx.x];
}
}
}
}
}
}
__syncthreads();
if (threadIdx.y > 0) {
return;
}

dst += sample_dst*stride_sample_dst + channel_dst*stride_channel_dst + row0;

Expand All @@ -642,15 +664,6 @@ static __global__ void mul_mat_vec_q(
for (int j = 0; j < ncols_dst; ++j) {
#pragma unroll
for (int i = 0; i < rows_per_cuda_block; ++i) {
#pragma unroll
for (int l = 0; l < nwarps-1; ++l) {
tmp[j][i] += tmp_shared[l][j][i][threadIdx.x];
if constexpr (has_fusion) {
if (use_gate) {
tmp_gate[j][i] += tmp_shared_gate[l][j][i][threadIdx.x];
}
}
}
tmp[j][i] = warp_reduce_sum<warp_size>(tmp[j][i]);
if constexpr (has_fusion) {
if (use_gate) {
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