metal : warp-based reduce for rms_norm

Author: ggerganov

ggml-metal.m

@@ -1358,15 +1358,19 @@ void ggml_metal_graph_compute(
                             float eps;
                             memcpy(&eps, dst->op_params, sizeof(float));
 
-                            const int nth = MIN(512, ne00);
+                            int nth = 32; // SIMD width
+
+                            while (nth < ne00/4 && nth < 1024) {
+                                nth *= 2;
+                            }
 
                             [encoder setComputePipelineState:ctx->pipeline_rms_norm];
-                            [encoder setBuffer:id_src0 offset:offs_src0 atIndex:0];
-                            [encoder setBuffer:id_dst  offset:offs_dst  atIndex:1];
-                            [encoder setBytes:&ne00 length:sizeof( int64_t) atIndex:2];
-                            [encoder setBytes:&nb01 length:sizeof(uint64_t) atIndex:3];
-                            [encoder setBytes:&eps  length:sizeof(   float) atIndex:4];
-                            [encoder setThreadgroupMemoryLength:GGML_PAD(nth/32*sizeof(float), 16) atIndex:0];
+                            [encoder setBuffer:id_src0 offset:offs_src0        atIndex:0];
+                            [encoder setBuffer:id_dst  offset:offs_dst         atIndex:1];
+                            [encoder setBytes:&ne00    length:sizeof( int64_t) atIndex:2];
+                            [encoder setBytes:&nb01    length:sizeof(uint64_t) atIndex:3];
+                            [encoder setBytes:&eps     length:sizeof(   float) atIndex:4];
+                            [encoder setThreadgroupMemoryLength:32*sizeof(float) atIndex:0];
 
                             const int64_t nrows = ggml_nrows(src0);
 

ggml-metal.metal

@@ -447,14 +447,13 @@ kernel void kernel_rms_norm(
         constant   int64_t & ne00,
         constant  uint64_t & nb01,
         constant     float & eps,
-        threadgroup float  * sum [[threadgroup(0)]],
+        threadgroup float  * buf [[threadgroup(0)]],
         uint tgpig[[threadgroup_position_in_grid]],
         uint tpitg[[thread_position_in_threadgroup]],
         uint sgitg[[simdgroup_index_in_threadgroup]],
         uint tiisg[[thread_index_in_simdgroup]],
         uint   ntg[[threads_per_threadgroup]]) {
-    device const float4 * x        = (device const float4 *) ((device const char *) src0 + tgpig*nb01);
-    device const float  * x_scalar = (device const float  *) x;
+    device const float4 * x = (device const float4 *) ((device const char *) src0 + tgpig*nb01);
 
     float4 sumf = 0;
     float all_sum = 0;
@@ -465,40 +464,30 @@ kernel void kernel_rms_norm(
     }
     all_sum = sumf[0] + sumf[1] + sumf[2] + sumf[3];
     all_sum = simd_sum(all_sum);
-    if (tiisg == 0) {
-        sum[sgitg] = all_sum;
-    }
+    if (ntg > N_SIMDWIDTH) {
+        if (sgitg == 0) {
+            buf[tiisg] = 0.0f;
+        }
 
-    threadgroup_barrier(mem_flags::mem_threadgroup);
+        threadgroup_barrier(mem_flags::mem_threadgroup);
 
-    // broadcast, simd group number is ntg / 32
-    for (uint i = ntg / 32 / 2; i > 0; i /= 2) {
-       if (tpitg < i) {
-           sum[tpitg] += sum[tpitg + i];
-       }
-    }
-    if (tpitg == 0) {
-        for (int i = 4 * (ne00 / 4); i < ne00; i++) {
-            sum[0] += x_scalar[i];
+        if (tiisg == 0) {
+            buf[sgitg] = all_sum;
         }
-        sum[0] /= ne00;
-    }
 
-    threadgroup_barrier(mem_flags::mem_threadgroup);
+        threadgroup_barrier(mem_flags::mem_threadgroup);
 
-    const float mean  = sum[0];
+        all_sum = buf[tiisg];
+        all_sum = simd_sum(all_sum);
+    }
+
+    const float mean  = all_sum/ne00;
     const float scale = 1.0f/sqrt(mean + eps);
 
     device float4 * y = (device float4 *) (dst + tgpig*ne00);
-    device float * y_scalar = (device float *) y;
     for (int i00 = tpitg; i00 < ne00/4; i00 += ntg) {
         y[i00] = x[i00] * scale;
     }
-    if (tpitg == 0) {
-        for (int i00 = 4 * (ne00 / 4); i00 < ne00; i00++) {
-            y_scalar[i00] = x_scalar[i00] * scale;
-        }
-    }
 }
 
 // function for calculate inner product between half a q4_0 block and 16 floats (yl), sumy is SUM(yl[i])