[Kernel] optimize performance of gptq marlin kernel when n is small

csrc/quantization/gptq_marlin/gptq_marlin.cu

@@ -538,6 +538,7 @@ __global__ void Marlin(
     int prob_n,           // output dimension n
     int prob_k,           // reduction dimension k
     int* locks,           // extra global storage for barrier synchronization
+    bool use_atomic_add,  // whether to use atomic add to reduce
     bool use_fp32_reduce  // whether to use fp32 global reduce
 ) {
   // Each threadblock processes one "stripe" of the B matrix with (roughly) the
@@ -1542,7 +1543,17 @@ __global__ void Marlin(
          i < div_ceil(16 * thread_m_blocks, threads / (2 * thread_n_blocks));
          i++) {
       if (c_gl_wr < c_gl_wr_end) {
-        C[c_gl_wr] = sh_red[c_sh_rd];
+        if (use_atomic_add && slice_count > 1) {
+          scalar_t2* C_half2 = reinterpret_cast<scalar_t2*>(&C[c_gl_wr]);
+          scalar_t2* sh_red_half2 =
+              reinterpret_cast<scalar_t2*>(&sh_red[c_sh_rd]);
+  #pragma unroll
+          for (int a = 0; a < 4; a++) {
+            atomicAdd(&C_half2[a], sh_red_half2[a]);
+          }
+        } else {
+          C[c_gl_wr] = sh_red[c_sh_rd];
+        }
         c_gl_wr += c_gl_wr_delta;
         c_sh_rd += c_sh_rd_delta;
       }
@@ -1644,7 +1655,7 @@ __global__ void Marlin(
           }
           cp_async_fence();
         } else {
-          if (last) {
+          if (last || use_atomic_add) {
             if (s_sh_wr_pred) {
               cp_async4(&sh_s[s_sh_wr], &scales_ptr[s_gl_rd]);
             }
@@ -1664,7 +1675,7 @@ __global__ void Marlin(
           }
 
         } else {
-          if (last) {
+          if (last || use_atomic_add) {
             cp_async_wait<0>();
             __syncthreads();
             if (threadIdx.x / 32 < thread_n_blocks / 4) {
@@ -1703,8 +1714,8 @@ __global__ void Marlin(
         }
       }
 
-      if (slice_count > 1) {  // only globally reduce if there is more than one
-                              // block in a slice
+      if (slice_count > 1 && !use_atomic_add) {
+        // only globally reduce if there is more than one block in a slice
         barrier_acquire(&locks[slice_col], slice_idx);
         if (use_fp32_reduce) {
           global_reduce_fp32(slice_idx == 0, last);
@@ -1713,7 +1724,8 @@ __global__ void Marlin(
         }
         barrier_release(&locks[slice_col], last);
       }
-      if (last)  // only the last block in a slice actually writes the result
+      if (last || use_atomic_add)
+        // only the last block in a slice actuallywrites the result
         write_result();
       slice_row = 0;
       slice_col_par++;
@@ -1768,7 +1780,8 @@ __global__ void Marlin(
                HAS_ZP, GROUP_BLOCKS, IS_ZP_FLOAT>                              \
             <<<blocks, NUM_THREADS, max_shared_mem, stream>>>(                 \
                 A_ptr, B_ptr, C_ptr, C_tmp_ptr, s_ptr, zp_ptr, g_idx_ptr,      \
-                num_groups, prob_m, prob_n, prob_k, locks, use_fp32_reduce);   \
+                num_groups, prob_m, prob_n, prob_k, locks, use_atomic_add,     \
+                use_fp32_reduce);                                              \
       }                                                                        \
     }
 
@@ -2062,7 +2075,8 @@ void marlin_mm(const void* A, const void* B, void* C, void* C_tmp, void* s,
                vllm::ScalarType const& q_type, bool has_act_order,
                bool is_k_full, bool has_zp, int num_groups, int group_size,
                int dev, cudaStream_t stream, int thread_k, int thread_n,
-               int sms, int max_par, bool use_fp32_reduce, bool is_zp_float) {
+               int sms, int max_par, bool use_atomic_add, bool use_fp32_reduce,
+               bool is_zp_float) {
   if (has_zp) {
     TORCH_CHECK(
         q_type == vllm::kU4 || q_type == vllm::kU8,
@@ -2243,7 +2257,7 @@ torch::Tensor gptq_marlin_gemm(torch::Tensor& a, torch::Tensor& b_q_weight,
                                torch::Tensor& workspace,
                                vllm::ScalarTypeId const& b_q_type_id,
                                int64_t size_m, int64_t size_n, int64_t size_k,
-                               bool is_k_full, bool has_zp,
+                               bool is_k_full, bool has_zp, bool use_atomic_add,
                                bool use_fp32_reduce, bool is_zp_float) {
   vllm::ScalarType const b_q_type = vllm::ScalarType::from_id(b_q_type_id);
   if (has_zp) {
@@ -2306,19 +2320,34 @@ torch::Tensor gptq_marlin_gemm(torch::Tensor& a, torch::Tensor& b_q_weight,
   // Alloc buffers
   const at::cuda::OptionalCUDAGuard device_guard(device_of(a));
   auto options = torch::TensorOptions().dtype(a.dtype()).device(a.device());
-  torch::Tensor c = torch::empty({size_m, size_n}, options);
-  torch::Tensor a_tmp = torch::empty({size_m, size_k}, options);
+  torch::Tensor c;
+  if (use_atomic_add) {
+    c = torch::zeros({size_m, size_n}, options);
+  } else {
+    c = torch::empty({size_m, size_n}, options);
+  }
+
+  torch::Tensor a_tmp;
+  bool has_act_order = g_idx.size(0) != 0;
+  if (has_act_order) {
+    a_tmp = torch::empty({size_m, size_k}, options);
+  } else {
+    a_tmp = torch::empty({0}, options);
+  }
 
   // Alloc C tmp buffer that is going to be used for the global reduce
+  torch::Tensor c_tmp;
   int reduce_max_m = marlin::determine_reduce_max_m(size_m, marlin::max_par);
   int reduce_n = size_n;
   auto options_fp32 =
       torch::TensorOptions().dtype(at::kFloat).device(a.device());
-  if (!use_fp32_reduce) {
+  if (use_fp32_reduce) {
+    c_tmp = torch::empty({reduce_max_m, reduce_n}, options_fp32);
+  } else {
     reduce_max_m = 0;
     reduce_n = 0;
+    c_tmp = torch::empty({0}, options_fp32);
   }
-  torch::Tensor c_tmp = torch::empty({reduce_max_m, reduce_n}, options_fp32);
 
   // thread_k: `k` size of a thread_tile in `weights` (can usually be left as
   // auto -1)
@@ -2339,7 +2368,6 @@ torch::Tensor gptq_marlin_gemm(torch::Tensor& a, torch::Tensor& b_q_weight,
   // Detect groupsize and act_order
   int num_groups = -1;
   int group_size = -1;
-  bool has_act_order = g_idx.size(0) != 0;
 
   int rank = b_scales.sizes().size();
   TORCH_CHECK(rank == 2, "b_scales rank = ", rank, " is not 2");
@@ -2407,7 +2435,8 @@ torch::Tensor gptq_marlin_gemm(torch::Tensor& a, torch::Tensor& b_q_weight,
         a_tmp.data_ptr<at::Half>(), size_m, size_n, size_k,
         workspace.data_ptr(), b_q_type, has_act_order, is_k_full, has_zp,
         num_groups, group_size, dev, at::cuda::getCurrentCUDAStream(dev),
-        thread_k, thread_n, sms, marlin::max_par, use_fp32_reduce, is_zp_float);
+        thread_k, thread_n, sms, marlin::max_par, use_atomic_add,
+        use_fp32_reduce, is_zp_float);
   } else if (a.scalar_type() == at::ScalarType::BFloat16) {
     marlin::marlin_mm<nv_bfloat16>(
         a.data_ptr<at::BFloat16>(), b_q_weight.data_ptr(),
@@ -2416,7 +2445,8 @@ torch::Tensor gptq_marlin_gemm(torch::Tensor& a, torch::Tensor& b_q_weight,
         perm.data_ptr(), a_tmp.data_ptr<at::BFloat16>(), size_m, size_n, size_k,
         workspace.data_ptr(), b_q_type, has_act_order, is_k_full, has_zp,
         num_groups, group_size, dev, at::cuda::getCurrentCUDAStream(dev),
-        thread_k, thread_n, sms, marlin::max_par, use_fp32_reduce, is_zp_float);
+        thread_k, thread_n, sms, marlin::max_par, use_atomic_add,
+        use_fp32_reduce, is_zp_float);
   } else {
     TORCH_CHECK(false, "gpt_marlin_gemm only supports bfloat16 and float16");
   }

csrc/torch_bindings.cpp

@@ -272,7 +272,8 @@ TORCH_LIBRARY_EXPAND(TORCH_EXTENSION_NAME, ops) {
       "Tensor b_zeros, Tensor g_idx, Tensor perm, Tensor workspace, "
       "int b_q_type, "
       "SymInt size_m, SymInt size_n, SymInt size_k, bool is_k_full, "
-      "bool has_zp, bool use_fp32_reduce, bool is_zp_float) -> Tensor",
+      "bool has_zp, bool use_atomic_add, bool use_fp32_reduce, "
+      "bool is_zp_float) -> Tensor",
       {stride_tag});
   // conditionally compiled so impl registration is in source file
 

tests/kernels/test_marlin_gemm.py

@@ -34,6 +34,7 @@
 
 ACT_ORDER_OPTS = [False, True]
 K_FULL_OPTS = [False, True]
+USE_ATOMIC_ADD_OPTS = [False, True]
 USE_FP32_REDUCE_OPTS = [False, True]
 
 MARLIN_K_CHUNKS = [128]
@@ -194,6 +195,7 @@ def test_awq_marlin_repack(k_chunk, n_chunk, quant_type, group_size,
 @pytest.mark.parametrize("mnk_factors", MNK_FACTORS)
 @pytest.mark.parametrize("act_order", ACT_ORDER_OPTS)
 @pytest.mark.parametrize("is_k_full", K_FULL_OPTS)
+@pytest.mark.parametrize("use_atomic_add", USE_ATOMIC_ADD_OPTS)
 @pytest.mark.parametrize("use_fp32_reduce", USE_FP32_REDUCE_OPTS)
 def test_gptq_marlin_gemm(
     k_chunk,
@@ -203,6 +205,7 @@ def test_gptq_marlin_gemm(
     mnk_factors,
     act_order,
     is_k_full,
+    use_atomic_add,
     use_fp32_reduce,
 ):
     m_factor, n_factor, k_factor = mnk_factors
@@ -228,12 +231,12 @@ def test_gptq_marlin_gemm(
     workspace = MarlinWorkspace(size_n, GPTQ_MARLIN_MIN_THREAD_N,
                                 GPTQ_MARLIN_MAX_PARALLEL)
 
-    opcheck(
-        torch.ops._C.gptq_marlin_gemm,
-        (a_input, marlin_q_w, marlin_s, marlin_zp, g_idx, sort_indices,
-         workspace.scratch, quant_type.id, a_input.shape[0], b_weight.shape[1],
-         a_input.shape[1], is_k_full, False, use_fp32_reduce, False),
-        test_utils=DEFAULT_OPCHECK_TEST_UTILS)
+    opcheck(torch.ops._C.gptq_marlin_gemm,
+            (a_input, marlin_q_w, marlin_s, marlin_zp, g_idx, sort_indices,
+             workspace.scratch, quant_type.id, a_input.shape[0],
+             b_weight.shape[1], a_input.shape[1], is_k_full, False,
+             use_atomic_add, use_fp32_reduce, False),
+            test_utils=DEFAULT_OPCHECK_TEST_UTILS)
 
     output = ops.gptq_marlin_gemm(
         a_input,
@@ -249,6 +252,7 @@ def test_gptq_marlin_gemm(
         a_input.shape[1],
         is_k_full=is_k_full,
         has_zp=False,
+        use_atomic_add=use_atomic_add,
         use_fp32_reduce=use_fp32_reduce,
         is_zp_float=False,
     )

vllm/_custom_ops.py

@@ -301,6 +301,7 @@ def _gptq_marlin_gemm_fake(a: torch.Tensor,
                                size_k: torch.SymInt,
                                is_k_full: bool,
                                has_zp: bool = False,
+                               use_atomic_add: bool = False,
                                use_fp32_reduce: bool = False,
                                is_zp_float: bool = False) -> torch.Tensor:
         return torch.empty((size_m, size_n), device=a.device, dtype=a.dtype)
@@ -713,12 +714,14 @@ def gptq_marlin_gemm(a: torch.Tensor,
                      size_k: int,
                      is_k_full: bool,
                      has_zp: bool = False,
+                     use_atomic_add: bool = False,
                      use_fp32_reduce: bool = False,
                      is_zp_float: bool = False) -> torch.Tensor:
     return torch.ops._C.gptq_marlin_gemm(a, b_q_weight, b_scales, b_zeros,
                                          g_idx, perm, workspace, b_q_type.id,
                                          size_m, size_n, size_k, is_k_full,
-                                         has_zp, use_fp32_reduce, is_zp_float)
+                                         has_zp, use_atomic_add,
+                                         use_fp32_reduce, is_zp_float)
 
 
 # fp8 marlin

vllm/envs.py

@@ -95,6 +95,7 @@
     VLLM_DP_SIZE: int = 1
     VLLM_DP_MASTER_IP: str = ""
     VLLM_DP_MASTER_PORT: int = 0
+    VLLM_MARLIN_USE_ATOMIC_ADD: bool = False
 
 
 def get_default_cache_root():
@@ -630,6 +631,10 @@ def maybe_convert_int(value: Optional[str]) -> Optional[int]:
     # Whether to use S3 path for model loading in CI via RunAI Streamer
     "VLLM_CI_USE_S3":
     lambda: os.environ.get("VLLM_CI_USE_S3", "0") == "1",
+
+    # Whether to use atomicAdd reduce in gptq/awq marlin kernel.
+    "VLLM_MARLIN_USE_ATOMIC_ADD":
+    lambda: os.environ.get("VLLM_MARLIN_USE_ATOMIC_ADD", "0") == "1",
 }
 
 # end-env-vars-definition

vllm/model_executor/layers/quantization/utils/marlin_utils.py

@@ -5,6 +5,7 @@
 import numpy
 import torch
 
+import vllm.envs as envs
 from vllm import _custom_ops as ops
 from vllm.model_executor.layers.linear import LinearBase
 from vllm.platforms import current_platform
@@ -290,6 +291,23 @@ def moe_awq_to_marlin_zero_points(q_zp_packed: torch.Tensor, size_k: int,
     return output
 
 
+def should_use_atomic_add_reduce(m: int, n: int, k: int, device: torch.device,
+                                 dtype: torch.dtype) -> bool:
+    # disable atomicAdd reduce by default,
+    # one can enable it with VLLM_MARLIN_USE_ATOMIC_ADD=1
+    if not envs.VLLM_MARLIN_USE_ATOMIC_ADD or device.type != "cuda":
+        return False
+
+    # sm8x doesn't support atomicAdd + bfloat16 natively
+    device_capability = torch.cuda.get_device_capability(device)
+    if device_capability[0] < 9 and dtype == torch.bfloat16:
+        return False
+
+    # the performance of atomicAdd is better than global reduce
+    # only when m*n is small and k is large
+    return max(m, 64) * n < 64 * 2048 and k >= 2048
+
+
 def apply_gptq_marlin_linear(
         input: torch.Tensor,
         weight: torch.Tensor,
@@ -307,6 +325,12 @@ def apply_gptq_marlin_linear(
     reshaped_x = input.reshape(-1, input.shape[-1])
     out_shape = input.shape[:-1] + (output_size_per_partition, )
 
+    use_atomic_add = should_use_atomic_add_reduce(m=reshaped_x.size(0),
+                                                  n=output_size_per_partition,
+                                                  k=reshaped_x.size(1),
+                                                  device=input.device,
+                                                  dtype=input.dtype)
+
     output = ops.gptq_marlin_gemm(reshaped_x,
                                   weight,
                                   weight_scale,
@@ -320,6 +344,7 @@ def apply_gptq_marlin_linear(
                                   size_k=input_size_per_partition,
                                   is_k_full=is_k_full,
                                   has_zp=False,
+                                  use_atomic_add=use_atomic_add,
                                   use_fp32_reduce=use_fp32_reduce,
                                   is_zp_float=False)
 
@@ -345,6 +370,12 @@ def apply_awq_marlin_linear(
     reshaped_x = input.reshape(-1, input.shape[-1])
     out_shape = input.shape[:-1] + (output_size_per_partition, )
 
+    use_atomic_add = should_use_atomic_add_reduce(m=reshaped_x.size(0),
+                                                  n=output_size_per_partition,
+                                                  k=reshaped_x.size(1),
+                                                  device=input.device,
+                                                  dtype=input.dtype)
+
     output = ops.gptq_marlin_gemm(reshaped_x,
                                   weight,
                                   weight_scale,
@@ -358,6 +389,7 @@ def apply_awq_marlin_linear(
                                   size_k=input_size_per_partition,
                                   is_k_full=True,
                                   has_zp=True,
+                                  use_atomic_add=use_atomic_add,
                                   use_fp32_reduce=use_fp32_reduce,
                                   is_zp_float=False)