CUDA: route batch>=4 quantized matmul to MMQ on AMD MFMA hardware (ggml-org#23227)

* CUDA: per-quant MMVQ/MMQ batch threshold on AMD MFMA hardware

The dispatcher uses a single global threshold (MMVQ_MAX_BATCH_SIZE = 8)
to choose between mul_mat_vec_q (per-row GEMV) and mul_mat_q (MFMA-tiled
GEMM) for quantized matmul. On AMD CDNA, the optimal crossover differs
substantially by quant family because the per-row GEMV cost is dominated
by dequantisation, not the dot-product itself: K-quants pay a heavier
super-block decode and so MMQ wins sooner; legacy and IQ quants have
lean decode and stay ahead until the batch fully populates an MFMA tile.

This patch introduces ggml_cuda_should_use_mmvq(type, cc, ne11) -> bool,
mirroring the existing ggml_cuda_should_use_mmq, and gates per-quant
thresholds on amd_mfma_available(cc):

  Q3_K, Q4_K, Q5_K  : MMVQ <= 3   (MMQ wins from batch=4: +5% .. +76%)
  Q2_K, Q6_K        : MMVQ <= 5   (MMQ wins from batch=6: +8% .. +35%)
  others            : MMVQ <= 8   (legacy & IQ regress under MMQ; unchanged)

Non-AMD-MFMA paths (NVIDIA, RDNA, CDNA1 without MFMA) are byte-identical
to master. GGML_CUDA_FORCE_MMVQ=1 restores the original global threshold
for A/B testing.

Measured on MI250X (gfx90a, ROCm 7.2.1) with Llama-3.2-3B-Instruct,
llama-bench pp512 across all 20 supported quants, ubatch 1..8, 10 reps.
Full table in PR description.

  Selected pp512 throughput (tok/s, ub=8):
    Q4_K_S:  559 -> 940  (+68%)
    Q5_K_S:  503 -> 884  (+76%)
    Q3_K_S:  629 -> 879  (+40%)
    Q2_K  :  615 -> 809  (+32%)
    Q6_K  :  582 -> 776  (+33%)

  Selected pp512 throughput (tok/s, ub=4):
    Q4_K_S:  444 -> 480  (+ 8%)
    Q4_0  :  682 -> 685  (+ 0%)   (no regression - retains MMVQ)
    IQ4_XS:  706 -> 698  (- 1%)   (no regression - retains MMVQ)

* CUDA: address review — inline MMVQ batch table, drop env hatch & doc block

* tune kernel selection logic for CDNA1

---------

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

Author: jadenmach2

ggml/src/ggml-cuda/ggml-cuda.cu

@@ -2570,6 +2570,7 @@ static void ggml_cuda_mul_mat(ggml_backend_cuda_context & ctx, const ggml_tensor
             use_mul_mat_q           = use_mul_mat_q             && ggml_cuda_should_use_mmq(src0->type, cc, src1->ne[1], /*n_experts=*/0);
             use_mul_mat_f           = use_mul_mat_f             && ggml_cuda_should_use_mmf(src0->type, cc, warp_size, src0->ne, src0->nb, src1->ne[1], /*mul_mat_id=*/false);
             use_mul_mat_vec_f       = use_mul_mat_vec_f         && ggml_cuda_should_use_mmvf(src0->type, cc, src0->ne, src0->nb, src1->ne[1]);
+            use_mul_mat_vec_q       = use_mul_mat_vec_q         && ggml_cuda_should_use_mmvq(src0->type, cc, src1->ne[1]);
             any_gpus_with_slow_fp16 = any_gpus_with_slow_fp16   || !fast_fp16_hardware_available(cc);
         }
     } else {
@@ -2578,6 +2579,7 @@ static void ggml_cuda_mul_mat(ggml_backend_cuda_context & ctx, const ggml_tensor
         use_mul_mat_q           = use_mul_mat_q             && ggml_cuda_should_use_mmq(src0->type, cc, src1->ne[1], /*n_experts=*/0);
         use_mul_mat_f           = use_mul_mat_f             && ggml_cuda_should_use_mmf(src0->type, cc, warp_size, src0->ne, src0->nb, src1->ne[1], /*mul_mat_id=*/false);
         use_mul_mat_vec_f       = use_mul_mat_vec_f         && ggml_cuda_should_use_mmvf(src0->type, cc, src0->ne, src0->nb, src1->ne[1]);
+        use_mul_mat_vec_q       = use_mul_mat_vec_q         && ggml_cuda_should_use_mmvq(src0->type, cc, src1->ne[1]);
         any_gpus_with_slow_fp16 = any_gpus_with_slow_fp16   || !fast_fp16_hardware_available(cc);
     }
 

ggml/src/ggml-cuda/mmvq.cu

@@ -271,6 +271,53 @@ int get_mmvq_mmid_max_batch(ggml_type type, int cc) {
     return MMVQ_MAX_BATCH_SIZE;
 }
 
+bool ggml_cuda_should_use_mmvq(enum ggml_type type, int cc, int64_t ne11) {
+    if (GGML_CUDA_CC_IS_CDNA(cc)) {
+        if (GGML_CUDA_CC_IS_CDNA1(cc)) {
+            switch (type) {
+                case GGML_TYPE_Q4_0:
+                case GGML_TYPE_Q4_1:
+                    return ne11 <= 7;
+                case GGML_TYPE_Q5_1:
+                    return ne11 <= 7;
+                case GGML_TYPE_Q8_0:
+                    return ne11 <= 6;
+                case GGML_TYPE_Q2_K:
+                    return ne11 <= 4;
+                case GGML_TYPE_Q3_K:
+                    return ne11 <= 3;
+                case GGML_TYPE_Q4_K:
+                    return ne11 <= 2;
+                case GGML_TYPE_Q5_K:
+                    return ne11 <= 3;
+                case GGML_TYPE_Q6_K:
+                    return ne11 <= 4;
+                case GGML_TYPE_IQ1_S:
+                    return ne11 <= 5;
+                case GGML_TYPE_IQ2_XXS:
+                case GGML_TYPE_IQ3_S:
+                case GGML_TYPE_IQ4_XS:
+                    return ne11 <= 6;
+                default:
+                    return ne11 <= MMVQ_MAX_BATCH_SIZE;
+            }
+        }
+        switch (type) { // tuned for CDNA2
+            case GGML_TYPE_Q2_K:
+                return ne11 <= 5;
+            case GGML_TYPE_Q3_K:
+            case GGML_TYPE_Q4_K:
+            case GGML_TYPE_Q5_K:
+                return ne11 <= 3;
+            case GGML_TYPE_Q6_K:
+                return ne11 <= 5;
+            default:
+                return ne11 <= MMVQ_MAX_BATCH_SIZE;
+        }
+    }
+    return ne11 <= MMVQ_MAX_BATCH_SIZE;
+}
+
 // Device constexpr: returns the max batch size for the current arch+type at compile time.
 template <ggml_type type>
 static constexpr __device__ int get_mmvq_mmid_max_batch_for_device() {

ggml/src/ggml-cuda/mmvq.cuh

@@ -2,6 +2,8 @@
 
 #define MMVQ_MAX_BATCH_SIZE 8 // Max. batch size for which to use MMVQ kernels.
 
+bool ggml_cuda_should_use_mmvq(enum ggml_type type, int cc, int64_t ne11);
+
 // Returns the maximum batch size for which MMVQ should be used for MUL_MAT_ID,
 // based on the quantization type and GPU architecture (compute capability).
 int get_mmvq_mmid_max_batch(ggml_type type, int cc);