Update num_splits heuristic

example/ck_tile/01_fmha/fmha_fwd.cpp

@@ -177,53 +177,35 @@ auto get_elimit<FmhaFwdFp8>(std::string init_method)
     }
 }
 
-int num_splits_heuristic(int batch_nhead_mblocks, int num_SMs, int num_n_blocks, int max_splits)
+int num_splits_heuristic(int batch_nhead_mblocks, int num_SMs, int max_splits)
 {
     // If we have enough to almost fill the SMs, then just use 1 split
     if(batch_nhead_mblocks >= 0.8f * num_SMs)
     {
         return 1;
     }
-    max_splits           = std::min({max_splits, num_SMs, num_n_blocks});
+
+    max_splits = std::min({max_splits, num_SMs});
+
     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_nhead_mblocks * num_splits) / num_SMs;
-            float eff     = n_waves / ceil(n_waves);
-            // printf("num_splits = %d, eff = %f\n", num_splits, eff);
+        float n_blocks = float(batch_nhead_mblocks * num_splits) / num_SMs;
+        float eff      = n_blocks / std::ceil(n_blocks);
+
         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);
             return num_splits;
         }
     }
@@ -267,15 +249,15 @@ int override_num_splits_if_necessary(
 
         return 64; // meet unsupported hdim_q/hdim_v
     }();
-    const int kN1 = hdim_v;
+    // const int kN1 = hdim_v;
 
     const int num_m_blocks = ck_tile::integer_divide_ceil(max_seqlen_q, kM0);
-    const int num_n_blocks = ck_tile::integer_divide_ceil(hdim_v, kN1);
+    // const int num_n_blocks = ck_tile::integer_divide_ceil(hdim_v, kN1); // always 1
 
     if(num_splits < 1 && p_drop == 0.0f)
     {
         return num_splits_heuristic(
-            batch * nhead * num_m_blocks, props.multiProcessorCount * 2, num_n_blocks, 128);
+            batch * nhead * num_m_blocks, props.multiProcessorCount * 2, 32);
     }
 
     return num_splits;