metal: update rms_norm kernel

This commit double the speed of rms_norm operations by using 512 threads
per threadgroup, combining with SIMD primitives to minimize the need for
thread group barriers.

Author: lshzh-ww

ggml-metal.m

@@ -792,15 +792,15 @@ void ggml_metal_graph_compute(
 
                             const float eps = 1e-6f;
 
-                            const int nth = 256;
+                            const int nth = 512;
 
                             [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:nth*sizeof(float) atIndex:0];
+                            [encoder setThreadgroupMemoryLength:nth/32*sizeof(float) atIndex:0];
 
                             const int64_t nrows = ggml_nrows(src0);
 

ggml-metal.metal

@@ -339,26 +339,33 @@ kernel void kernel_rms_norm(
         threadgroup float  * sum [[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 float * x = (device const float *) ((device const char *) src0 + tgpig*nb01);
+    device const float4 * x = (device const float4 *) ((device const char *) src0 + tgpig*nb01);
+    device const float * x_scalar = (device const float *) x;
+    float4 sumf=0;
+    float all_sum=0;
 
     // parallel sum
-    sum[tpitg] = 0.0f;
-    for (int i00 = tpitg; i00 < ne00; i00 += ntg) {
-        sum[tpitg] += x[i00] * x[i00];
+    for (int i00 = tpitg; i00 < ne00/4; i00 += ntg) {
+        sumf += x[i00] * x[i00];
+    }
+    all_sum = sumf[0] + sumf[1] + sumf[2] + sumf[3];
+    all_sum = simd_sum(all_sum);
+    if (tiisg == 0) {
+        sum[sgitg] = all_sum;
     }
 
-    // reduce
     threadgroup_barrier(mem_flags::mem_threadgroup);
-    for (uint i = ntg/2; i > 0; i /= 2) {
-        if (tpitg < i) {
-            sum[tpitg] += sum[tpitg + i];
-        }
-        threadgroup_barrier(mem_flags::mem_threadgroup);
+    // broadcast, simd group number is ntg / 32
+    for (int i = ntg / 32 / 2; i > 0; i /= 2) {
+       if (tpitg < i) {
+           sum[tpitg] += sum[tpitg + i];
+       }
     }
-
-    // broadcast
     if (tpitg == 0) {
+        for (int i = 4 * (ne00 / 4); i < ne00; i++) {sum[0] += x_scalar[i];}
         sum[0] /= ne00;
     }
 
@@ -367,10 +374,14 @@ kernel void kernel_rms_norm(
     const float mean  = sum[0];
     const float scale = 1.0f/sqrt(mean + eps);
 
-    device float * y = dst + tgpig*ne00;
-    for (int i00 = tpitg; i00 < ne00; i00 += ntg) {
+    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;}
+    }
 }
 
 // putting them in the kernel cause a significant performance penalty