feat: support ep size < 32 for sgl kernel (#4348)

sgl-kernel/benchmark/bench_moe_align_block_size.py

@@ -196,6 +196,8 @@ def calculate_diff(num_tokens, num_experts=256, block_size=128, topk=8):
         expert_ids_triton,
         num_tokens_post_pad_triton,
     )
+
+    try:
         ops.moe_align_block_size(
             topk_ids,
             num_experts,
@@ -204,6 +206,11 @@ def calculate_diff(num_tokens, num_experts=256, block_size=128, topk=8):
             expert_ids_vllm,
             num_tokens_post_pad_vllm,
         )
+        print(f"✅ VLLM implementation works with {num_experts} experts!")
+        vllm_works = True
+    except RuntimeError as e:
+        print(f"❌ VLLM implementation failed with {num_experts} experts: {e}")
+        vllm_works = False
 
     if torch.allclose(expert_ids_cuda, expert_ids_triton) and torch.allclose(
         num_tokens_post_pad_cuda, num_tokens_post_pad_triton
@@ -216,10 +223,15 @@ def calculate_diff(num_tokens, num_experts=256, block_size=128, topk=8):
         print("SGL num_tokens_post_pad:", num_tokens_post_pad_cuda)
         print("Triton num_tokens_post_pad:", num_tokens_post_pad_triton)
 
-    if torch.allclose(expert_ids_cuda, expert_ids_vllm) and torch.allclose(
-        num_tokens_post_pad_cuda, num_tokens_post_pad_vllm
+    if (
+        vllm_works
+        and torch.allclose(expert_ids_cuda, expert_ids_vllm)
+        and torch.allclose(num_tokens_post_pad_cuda, num_tokens_post_pad_vllm)
     ):
         print("✅ SGL and VLLM implementations match")
+    else:
+        if not vllm_works:
+            print("⚠️ VLLM comparison skipped due to failure")
         else:
             print("❌ SGL and VLLM implementations do not match")
             print("SGL expert_ids:", expert_ids_cuda)
@@ -228,8 +240,9 @@ def calculate_diff(num_tokens, num_experts=256, block_size=128, topk=8):
             print("VLLM num_tokens_post_pad:", num_tokens_post_pad_vllm)
 
 
+# Test range
 num_tokens_range = [16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192]
-num_experts_range = [32, 64, 128, 256]
+num_experts_range = [8, 32, 64, 128, 256]
 topk_range = [2, 4, 8]
 
 configs = list(itertools.product(num_tokens_range, num_experts_range, topk_range))
@@ -316,6 +329,7 @@ def benchmark(num_tokens, num_experts, topk, provider):
             quantiles=quantiles,
         )
     else:  # vllm
+        try:
             ms, min_ms, max_ms = triton.testing.do_bench(
                 lambda: ops.moe_align_block_size(
                     topk_ids,
@@ -327,6 +341,10 @@ def benchmark(num_tokens, num_experts, topk, provider):
                 ),
                 quantiles=quantiles,
             )
+        except RuntimeError as e:
+            print(f"❌ VLLM benchmark failed with {num_experts} experts: {e}")
+            # Return extreme values to indicate failure in the chart
+            return float("inf"), float("inf"), float("inf")
 
     return 1000 * ms, 1000 * max_ms, 1000 * min_ms
 
@@ -343,7 +361,7 @@ if __name__ == "__main__":
         "--num_experts",
         type=int,
         default=256,
-        choices=[8, 64, 128, 256],
+        choices=[8, 16, 32, 64, 128, 256],
         help="Number of experts for benchmark",
     )
     parser.add_argument(
@@ -353,8 +371,15 @@ if __name__ == "__main__":
         choices=[2, 4, 8],
         help="Top-k value for benchmark",
     )
+    parser.add_argument(
+        "--skip_full_benchmark",
+        action="store_true",
+        help="Only run the calculate_diff function, skip full benchmarking",
+    )
     args = parser.parse_args()
 
     calculate_diff(num_tokens=1024, num_experts=args.num_experts, topk=args.topk)
 
+    if not args.skip_full_benchmark:
+        print(f"\n📊 Running performance benchmark for {args.num_experts} experts...")
         benchmark.run(print_data=True)

sgl-kernel/csrc/moe/moe_align_kernel.cu

@@ -47,6 +47,7 @@ __global__ void moe_align_block_size_kernel(
     int32_t* __restrict__ expert_ids,
     int32_t* __restrict__ total_tokens_post_pad,
     int32_t num_experts,
+    int32_t padded_num_experts,
     int32_t experts_per_warp,
     int32_t block_size,
     size_t numel,
@@ -57,7 +58,7 @@ __global__ void moe_align_block_size_kernel(
   const int my_expert_start = warp_id * experts_per_warp;
 
   for (int i = 0; i < experts_per_warp; ++i) {
-    if (my_expert_start + i < num_experts) {
+    if (my_expert_start + i < padded_num_experts) {
       shared_counts[warp_id * experts_per_warp + i] = 0;
     }
   }
@@ -108,23 +109,44 @@ void moe_align_block_size(
     torch::Tensor token_cnts_buffer,
     torch::Tensor cumsum_buffer) {
   const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
-  TORCH_CHECK(num_experts % WARP_SIZE == 0);
-  int experts_per_warp = num_experts / WARP_SIZE;
+
+  int64_t padded_num_experts = ((num_experts + WARP_SIZE - 1) / WARP_SIZE) * WARP_SIZE;
+
+  int experts_per_warp;
+  int threads;
+
+  if (num_experts <= 8) {
+    experts_per_warp = 8;
+    threads = 256;
+  } else if (num_experts <= 16) {
+    experts_per_warp = 16;
+    threads = 512;
+  } else {
+    experts_per_warp = WARP_SIZE;
+    threads = 1024;
+  }
+
+  threads = ((threads + WARP_SIZE - 1) / WARP_SIZE) * WARP_SIZE;
+
   DISPATCH_INTEGRAL_TYPES(topk_ids.scalar_type(), "moe_align_block_size_kernel", [&] {
     auto align_kernel = moe_align_block_size_kernel<scalar_t>;
-    size_t shared_mem_size = 32 * experts_per_warp * sizeof(int32_t);
-    align_kernel<<<1, 1024, shared_mem_size, stream>>>(
+
+    size_t num_warps = CEILDIV(padded_num_experts, experts_per_warp);
+    size_t shared_mem_size = num_warps * experts_per_warp * sizeof(int32_t);
+
+    align_kernel<<<1, threads, shared_mem_size, stream>>>(
         topk_ids.data_ptr<scalar_t>(),
         sorted_token_ids.data_ptr<int32_t>(),
         experts_ids.data_ptr<int32_t>(),
         num_tokens_post_pad.data_ptr<int32_t>(),
         num_experts,
+        padded_num_experts,
         experts_per_warp,
         block_size,
         topk_ids.numel(),
         cumsum_buffer.data_ptr<int32_t>());
 
-    const int block_threads = 256;
+    const int block_threads = std::min(256, (int)threads);
     const int num_blocks = (topk_ids.numel() + block_threads - 1) / block_threads;
     const int max_blocks = 65535;
     const int actual_blocks = std::min(num_blocks, max_blocks);