Use more reasonable splitkv heuristic

Author: poyenc

csrc/flash_attn_ck/flash_common.cpp

@@ -18,15 +18,15 @@ int override_num_splits_if_necessary(int batch, int nhead, int max_seqlen_q, int
         return num_splits;
 
     // TODO - tile size should match the TileFmhaShape, hardcode for now
-    const int kM0 = 128;
+    const int kM0 = 64;
     const int kN1 = hdim_v;
 
     const int num_m_blocks = (max_seqlen_q + kM0 - 1) / kM0;
     const int num_n_blocks = (hdim_v + kN1 - 1) / kN1;
 
     if(num_splits < 1 && p_drop == 0.0f)
         return num_splits_heuristic_ck(
-            batch * nhead * num_m_blocks, props.multiProcessorCount * 2, num_n_blocks, 128);
+            batch * nhead * num_m_blocks, props.multiProcessorCount, num_n_blocks, 16);
 
     return num_splits;
 }

csrc/flash_attn_ck/flash_common.hpp

@@ -35,40 +35,15 @@ inline __global__ void ParsePhiloxCudaState(at::PhiloxCudaState arg, uint64_t* r
     }
 }
 
-inline int num_splits_heuristic_ck(int batch_nheads_mblocks, int num_SMs, int num_n_blocks, int max_splits) {
+inline int num_splits_heuristic_ck(int batch_nheads_mblocks, int num_SMs, [[maybe_unused]] int num_n_blocks, int max_splits) {
     // If we have enough to almost fill the SMs, then just use 1 split
-    if (batch_nheads_mblocks >= 0.8f * num_SMs) { return 1; }
-    max_splits = std::min({max_splits, num_SMs, num_n_blocks});
-    float max_efficiency = 0.f;
-    std::vector<float> efficiency;
-    efficiency.reserve(max_splits);
-    auto ceildiv = [](int a, int b) { return (a + b - 1) / b; };
-    // Some splits are not eligible. For example, if we have 64 blocks and choose 11 splits,
-    // we'll have 6 * 10 + 4 blocks. If we choose 12 splits, we'll have 6 * 11 + (-2) blocks
-    // (i.e. it's 11 splits anyway).
-    // So we check if the number of blocks per split is the same as the previous num_splits.
-    auto is_split_eligible = [&ceildiv, &num_n_blocks](int num_splits) {
-        return num_splits == 1 || ceildiv(num_n_blocks, num_splits) != ceildiv(num_n_blocks, num_splits - 1);
-    };
-    for (int num_splits = 1; num_splits <= max_splits; num_splits++) {
-        if (!is_split_eligible(num_splits)) {
-            efficiency.push_back(0.f);
-        } else {
-            float n_waves = float(batch_nheads_mblocks * num_splits) / num_SMs;
-            float eff = n_waves / ceil(n_waves);
-            // printf("num_splits = %d, eff = %f\n", num_splits, eff);
-            if (eff > max_efficiency) { max_efficiency = eff; }
-            efficiency.push_back(eff);
-        }
-    }
-    for (int num_splits = 1; num_splits <= max_splits; num_splits++) {
-        if (!is_split_eligible(num_splits)) { continue; }
-        if (efficiency[num_splits - 1] >= 0.85 * max_efficiency) {
-            // printf("num_splits chosen = %d\n", num_splits);
+    for (int num_splits = 1; num_splits <= max_splits; num_splits *= 2) {
+        if (num_SMs < batch_nheads_mblocks * (num_splits * 2)) {
             return num_splits;
         }
     }
-    return 1;
+
+    return max_splits;
 }
 
 int override_num_splits_if_necessary(int batch, int nhead, int max_seqlen_q, int hdim_v, float p_drop, int num_splits);