cuda : use 512 threads for soft_max instead of 32

Author: ggerganov

ggml-cuda.cu

@@ -443,6 +443,7 @@ static_assert(sizeof(block_q6_K) == sizeof(ggml_fp16_t) + 13*QK_K/16, "wrong q6_
 #define CUDA_SCALE_BLOCK_SIZE 256
 #define CUDA_CLAMP_BLOCK_SIZE 256
 #define CUDA_ROPE_BLOCK_SIZE 256
+#define CUDA_SOFT_MAX_BLOCK_SIZE 512
 #define CUDA_ALIBI_BLOCK_SIZE 32
 #define CUDA_DIAG_MASK_INF_BLOCK_SIZE 32
 #define CUDA_QUANTIZE_BLOCK_SIZE 256
@@ -4717,45 +4718,59 @@ static __global__ void diag_mask_inf_f32(const float * x, float * dst, const int
     dst[i] = x[i] - (col > n_past + row % rows_per_channel) * INT_MAX; // equivalent within rounding error but slightly faster on GPU
 }
 
-// the CUDA soft max implementation differs from the CPU implementation
-// instead of doubles floats are used
+// TODO: maybe can be improved with some warp-based primitives
 static __global__ void soft_max_f32(const float * x, const float * y, float * dst, const int ncols, const int nrows_y, const float scale) {
-    const int rowx = blockDim.x*blockIdx.x + threadIdx.x;
+    const int tid  = threadIdx.x;
+    const int rowx = blockIdx.x;
     const int rowy = rowx % nrows_y; // broadcast the mask (y) in the row dimension
-    const int block_size = blockDim.y;
-    const int tid = threadIdx.y;
 
-    float max_val = -INFINITY;
+    const int block_size = blockDim.x;
+
+    __shared__ float buf[CUDA_SOFT_MAX_BLOCK_SIZE];
+
+    buf[tid] = -INFINITY;
 
     for (int col = tid; col < ncols; col += block_size) {
         const int ix = rowx*ncols + col;
         const int iy = rowy*ncols + col;
-        max_val = max(max_val, x[ix]*scale + (y ? y[iy] : 0.0f));
+        buf[tid] = max(buf[tid], x[ix]*scale + (y ? y[iy] : 0.0f));
     }
 
+    __syncthreads();
+
     // find the max value in the block
-#pragma unroll
-    for (int mask = 16; mask > 0; mask >>= 1) {
-        max_val = max(max_val, __shfl_xor_sync(0xffffffff, max_val, mask, 32));
+    for (int i = block_size/2; i > 0; i >>= 1) {
+        if (tid < i) {
+            buf[tid] = max(buf[tid], buf[tid + i]);
+        }
+        __syncthreads();
     }
 
     float tmp = 0.f;
 
     for (int col = tid; col < ncols; col += block_size) {
         const int ix = rowx*ncols + col;
         const int iy = rowy*ncols + col;
-        const float val = expf((x[ix]*scale + (y ? y[iy] : 0.0f)) - max_val);
+        const float val = expf((x[ix]*scale + (y ? y[iy] : 0.0f)) - buf[0]);
         tmp += val;
         dst[ix] = val;
     }
 
+    __syncthreads();
+
+    buf[tid] = tmp;
+
+    __syncthreads();
+
     // sum up partial sums
-#pragma unroll
-    for (int mask = 16; mask > 0; mask >>= 1) {
-        tmp += __shfl_xor_sync(0xffffffff, tmp, mask, 32);
+    for (int i = block_size/2; i > 0; i >>= 1) {
+        if (tid < i) {
+            buf[tid] += buf[tid + i];
+        }
+        __syncthreads();
     }
 
-    const float inv_tmp = 1.f / tmp;
+    const float inv_tmp = 1.f / buf[0];
 
     for (int col = tid; col < ncols; col += block_size) {
         const int i = rowx*ncols + col;
@@ -5796,7 +5811,9 @@ static void diag_mask_inf_f32_cuda(const float * x, float * dst, const int ncols
 }
 
 static void soft_max_f32_cuda(const float * x, const float * y, float * dst, const int ncols_x, const int nrows_x, const int nrows_y, const float scale, cudaStream_t stream) {
-    const dim3 block_dims(1, WARP_SIZE, 1);
+    int nth = WARP_SIZE;
+    while (nth < ncols_x && nth < CUDA_SOFT_MAX_BLOCK_SIZE) nth *= 2;
+    const dim3 block_dims(nth,     1, 1);
     const dim3 block_nums(nrows_x, 1, 1);
     soft_max_f32<<<block_nums, block_dims, 0, stream>>>(x, y, dst, ncols_x, nrows_y, scale);
 }
@@ -6853,7 +6870,7 @@ inline void ggml_cuda_op_soft_max(
 
     const int64_t ne00 = src0->ne[0];
     const int64_t nrows_x = ggml_nrows(src0);
-    const int64_t nrows_y = src1 ? ggml_nrows(src1) : 0;
+    const int64_t nrows_y = src1 ? ggml_nrows(src1) : 1;
 
     float scale = 1.0f;
     memcpy(&scale, dst->op_params, sizeof(float));