metal : improve decoding speed for batches of 2-16

Author: ggerganov

ggml-metal.m

@@ -993,14 +993,34 @@ void ggml_metal_graph_compute(
                             uint gqa = ne12/ne02;
                             GGML_ASSERT(ne03 == ne13);
 
+                            // find the break-even point where the matrix-matrix kernel becomes more efficient compared
+                            // to the matrix-vector kernel. the numbers below are measure on M2 Ultra
+                            // not sure if this translates across all chips
+                            int ne11_mm_min = 1;
+
+                            switch (src0t) {
+                                case GGML_TYPE_F16:  ne11_mm_min = 2;  break;
+                                case GGML_TYPE_Q8_0: ne11_mm_min = 7;  break;
+                                case GGML_TYPE_Q2_K: ne11_mm_min = 15; break;
+                                case GGML_TYPE_Q3_K: ne11_mm_min = 7;  break;
+                                case GGML_TYPE_Q4_0:
+                                case GGML_TYPE_Q4_1: ne11_mm_min = 15; break;
+                                case GGML_TYPE_Q4_K: ne11_mm_min = 11; break;
+                                case GGML_TYPE_Q5_0:                          // not tested yet
+                                case GGML_TYPE_Q5_1: ne11_mm_min = 13; break; // not tested yet
+                                case GGML_TYPE_Q5_K: ne11_mm_min = 7;  break;
+                                case GGML_TYPE_Q6_K: ne11_mm_min = 7;  break;
+                                default:             ne11_mm_min = 1;  break;
+                            }
+
                             // for now the matrix-matrix multiplication kernel only works on A14+/M1+ SoCs
                             // AMD GPU and older A-chips will reuse matrix-vector multiplication kernel
                             if (!ggml_is_transposed(src0) &&
                                 !ggml_is_transposed(src1) &&
                                 src1t == GGML_TYPE_F32 &&
                                 [ctx->device supportsFamily:MTLGPUFamilyApple7] &&
                                 ne00%32 == 0 &&
-                                ne11 > 2) {
+                                ne11 > ne11_mm_min) {
                                 switch (src0->type) {
                                     case GGML_TYPE_F32:  [encoder setComputePipelineState:ctx->pipeline_mul_mm_f32_f32];  break;
                                     case GGML_TYPE_F16:  [encoder setComputePipelineState:ctx->pipeline_mul_mm_f16_f32];  break;