[Perf] Optimize reshape_and_cache CUDA Kernel (vllm-project#25955)

ZJY0516 · Liu-congo · web-flow · commit eb0fa43868cf · 2025-10-03T01:33:46.000-07:00
Signed-off-by: zjy0516 &lt;riverclouds.zhu@qq.com&gt;
Co-authored-by: Liu-congo &lt;1502632128@qq.com&gt;
diff --git a/benchmarks/kernels/benchmark_reshape_and_cache.py b/benchmarks/kernels/benchmark_reshape_and_cache.py
@@ -0,0 +1,174 @@
+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from __future__ import annotations
+
+import random
+import time
+
+import torch
+from tabulate import tabulate
+
+from vllm import _custom_ops as ops
+from vllm.logger import init_logger
+from vllm.platforms import current_platform
+from vllm.utils import (
+    STR_DTYPE_TO_TORCH_DTYPE,
+    FlexibleArgumentParser,
+    create_kv_caches_with_random,
+)
+
+logger = init_logger(__name__)
+
+
+@torch.inference_mode()
+def run_benchmark(
+    num_tokens: int,
+    num_heads: int,
+    head_size: int,
+    block_size: int,
+    num_blocks: int,
+    dtype: torch.dtype,
+    kv_cache_dtype: str,
+    num_iters: int,
+    benchmark_mode: str,
+    device: str = "cuda",
+) -> float:
+    """Return latency (seconds) for given num_tokens."""
+
+    if kv_cache_dtype == "fp8" and head_size % 16:
+        raise ValueError("fp8 kv-cache requires head_size to be a multiple of 16.")
+
+    current_platform.seed_everything(42)
+    torch.set_default_device(device)
+
+    # create random key / value tensors [T, H, D].
+    key = torch.randn(num_tokens, num_heads, head_size, dtype=dtype, device=device)
+    value = torch.randn_like(key)
+
+    # prepare the slot mapping.
+    # each token is assigned a unique slot in the KV-cache.
+    num_slots = block_size * num_blocks
+    if num_tokens > num_slots:
+        raise ValueError("num_tokens cannot exceed the total number of cache slots")
+    slot_mapping_lst = random.sample(range(num_slots), num_tokens)
+    slot_mapping = torch.tensor(slot_mapping_lst, dtype=torch.long, device=device)
+
+    key_caches, value_caches = create_kv_caches_with_random(
+        num_blocks,
+        block_size,
+        1,  # num_layers
+        num_heads,
+        head_size,
+        kv_cache_dtype,
+        dtype,
+        device=device,
+    )
+    key_cache, value_cache = key_caches[0], value_caches[0]
+    # to free unused memory
+    del key_caches, value_caches
+
+    # compute per-kernel scaling factors for fp8 conversion (if used).
+    k_scale = (key.amax() / 64.0).to(torch.float32)
+    v_scale = (value.amax() / 64.0).to(torch.float32)
+
+    function_under_test = lambda: ops.reshape_and_cache(
+        key,  # noqa: F821
+        value,  # noqa: F821
+        key_cache,  # noqa: F821
+        value_cache,  # noqa: F821
+        slot_mapping,  # noqa: F821
+        kv_cache_dtype,
+        k_scale,
+        v_scale,
+    )
+
+    if benchmark_mode == "cudagraph":
+        g = torch.cuda.CUDAGraph()
+        with torch.cuda.graph(g):
+            function_under_test()
+        torch.cuda.synchronize()
+        function_under_test = lambda: g.replay()
+
+    def run_cuda_benchmark(n_iters: int) -> float:
+        nonlocal key, value, key_cache, value_cache, slot_mapping
+        torch.cuda.synchronize()
+        start = time.perf_counter()
+        for _ in range(n_iters):
+            function_under_test()
+            torch.cuda.synchronize()
+        end = time.perf_counter()
+        return (end - start) / n_iters
+
+    # warm-up
+    run_cuda_benchmark(3)
+
+    lat = run_cuda_benchmark(num_iters)
+
+    # free tensors to mitigate OOM when sweeping
+    del key, value, key_cache, value_cache, slot_mapping
+    torch.cuda.empty_cache()
+
+    return lat
+
+
+def main(args):
+    rows = []
+    for exp in range(1, 17):
+        n_tok = 2**exp
+        lat = run_benchmark(
+            num_tokens=n_tok,
+            num_heads=args.num_heads,
+            head_size=args.head_size,
+            block_size=args.block_size,
+            num_blocks=args.num_blocks,
+            dtype=STR_DTYPE_TO_TORCH_DTYPE[args.dtype],
+            kv_cache_dtype=args.kv_cache_dtype,
+            num_iters=args.iters,
+            benchmark_mode=args.mode,
+            device="cuda",
+        )
+        rows.append([n_tok, lat * 1e6])  # convert to microseconds
+
+    print(f"Benchmark results for implementation cuda (measuring with {args.mode}):")
+    print(tabulate(rows, headers=["num_tokens", "latency (µs)"], floatfmt=".3f"))
+
+
+if __name__ == "__main__":
+    parser = FlexibleArgumentParser()
+
+    parser.add_argument("--num-heads", type=int, default=128)
+    parser.add_argument(
+        "--head-size",
+        type=int,
+        choices=[64, 80, 96, 112, 120, 128, 192, 256],
+        default=128,
+    )
+    parser.add_argument("--block-size", type=int, choices=[16, 32], default=16)
+    parser.add_argument("--num-blocks", type=int, default=128 * 128)
+
+    parser.add_argument(
+        "--dtype",
+        type=str,
+        choices=["half", "bfloat16", "float"],
+        default="bfloat16",
+    )
+
+    parser.add_argument(
+        "--kv-cache-dtype",
+        type=str,
+        choices=["auto", "fp8"],
+        default="auto",
+    )
+
+    parser.add_argument("--iters", type=int, default=200)
+
+    parser.add_argument(
+        "--mode",
+        type=str,
+        choices=["cudagraph", "no_graph"],
+        default="cudagraph",
+    )
+
+    args = parser.parse_args()
+
+    main(args)
diff --git a/csrc/cache_kernels.cu b/csrc/cache_kernels.cu
@@ -16,8 +16,7 @@
 
 #include <algorithm>
 #include <cassert>
-#include <map>
-#include <vector>
+#include <cfloat>  // FLT_MIN
 
 #ifdef USE_ROCM
   #include <hip/hip_bf16.h>
@@ -209,6 +208,20 @@ void copy_blocks_mla(std::vector<torch::Tensor> const& kv_caches,
 
 namespace vllm {
 
+// Used to copy/convert one element
+template <typename OutT, typename InT, Fp8KVCacheDataType kv_dt>
+struct CopyWithScaleOp {
+  float scale;
+
+  __device__ __forceinline__ void operator()(OutT& dst, const InT src) const {
+    if constexpr (kv_dt == Fp8KVCacheDataType::kAuto) {
+      dst = static_cast<OutT>(src);
+    } else {
+      dst = fp8::scaled_convert<OutT, InT, kv_dt>(src, scale);
+    }
+  }
+};
+
 template <typename scalar_t, typename cache_t, Fp8KVCacheDataType kv_dt>
 __global__ void reshape_and_cache_kernel(
     const scalar_t* __restrict__ key,    // [num_tokens, num_heads, head_size]
@@ -224,58 +237,50 @@ __global__ void reshape_and_cache_kernel(
   const int64_t token_idx = blockIdx.x;
   const int64_t slot_idx = slot_mapping[token_idx];
   if (slot_idx < 0) {
-    // Padding token that should be ignored.
     return;
   }
 
   const int64_t block_idx = slot_idx / block_size;
   const int64_t block_offset = slot_idx % block_size;
+  const int h_block_count = head_size / x;  // head_size//x
 
-  const int n = num_heads * head_size;
-  for (int i = threadIdx.x; i < n; i += blockDim.x) {
-    const int64_t src_key_idx = token_idx * key_stride + i;
-    const int64_t src_value_idx = token_idx * value_stride + i;
-
-    const int head_idx = i / head_size;
-    const int head_offset = i % head_size;
-    const int x_idx = head_offset / x;
-    const int x_offset = head_offset % x;
-
-    const int64_t tgt_key_idx =
-        block_idx * num_heads * (head_size / x) * block_size * x +
-        head_idx * (head_size / x) * block_size * x + x_idx * block_size * x +
-        block_offset * x + x_offset;
-    const int64_t tgt_value_idx =
-        block_idx * num_heads * head_size * block_size +
-        head_idx * head_size * block_size + head_offset * block_size +
-        block_offset;
-    scalar_t tgt_key = key[src_key_idx];
-    scalar_t tgt_value = value[src_value_idx];
-    if constexpr (kv_dt == Fp8KVCacheDataType::kAuto) {
-      key_cache[tgt_key_idx] = tgt_key;
-      value_cache[tgt_value_idx] = tgt_value;
-    } else {
-      key_cache[tgt_key_idx] =
-          fp8::scaled_convert<cache_t, scalar_t, kv_dt>(tgt_key, *k_scale);
-      value_cache[tgt_value_idx] =
-          fp8::scaled_convert<cache_t, scalar_t, kv_dt>(tgt_value, *v_scale);
-    }
+  const int h_block_idx = threadIdx.x;
+  if (h_block_idx >= num_heads * h_block_count) {
+    return;
   }
-}
 
-// Used by vectorization_utils to copy/convert one element
-template <typename OutT, typename InT, Fp8KVCacheDataType kv_dt>
-struct CopyWithScaleOp {
-  float scale;
+  const int head_idx = h_block_idx / h_block_count;
+  const int h_block = h_block_idx % h_block_count;
 
-  __device__ __forceinline__ void operator()(OutT& dst, const InT src) const {
-    if constexpr (kv_dt == Fp8KVCacheDataType::kAuto) {
-      dst = static_cast<OutT>(src);
-    } else {
-      dst = fp8::scaled_convert<OutT, InT, kv_dt>(src, scale);
-    }
+  const scalar_t* __restrict__ key_src =
+      key + token_idx * key_stride + head_idx * head_size + h_block * x;
+  const int64_t src_value_start =
+      token_idx * value_stride + head_idx * head_size + h_block * x;
+
+  cache_t* __restrict__ key_dst =
+      key_cache + block_idx * num_heads * h_block_count * block_size * x +
+      head_idx * h_block_count * block_size * x + h_block * block_size * x +
+      block_offset * x;
+  const int64_t tgt_value_start =
+      block_idx * num_heads * h_block_count * x * block_size +
+      head_idx * h_block_count * x * block_size + h_block * x * block_size +
+      block_offset;
+
+  constexpr int VEC_SIZE = (sizeof(scalar_t) == 2) ? 8 : 4;
+  float k_scale_val = (kv_dt == Fp8KVCacheDataType::kAuto) ? 0.f : *k_scale;
+  CopyWithScaleOp<cache_t, scalar_t, kv_dt> k_op{k_scale_val};
+  float v_scale_val = (kv_dt == Fp8KVCacheDataType::kAuto) ? 0.f : *v_scale;
+  CopyWithScaleOp<cache_t, scalar_t, kv_dt> v_op{v_scale_val};
+
+  vectorize_with_alignment<VEC_SIZE>(key_src, key_dst, x, 0, 1, k_op);
+
+  const scalar_t* __restrict__ value_src = value + src_value_start;
+  cache_t* __restrict__ value_dst = value_cache + tgt_value_start;
+#pragma unroll
+  for (int i = 0; i < x; i++) {
+    v_op(value_dst[i * block_size], value_src[i]);
   }
-};
+}
 
 template <typename scalar_t, typename cache_t, Fp8KVCacheDataType kv_dt>
 __global__ void reshape_and_cache_flash_kernel(
@@ -601,9 +606,10 @@ void reshape_and_cache(
 
   int key_stride = key.stride(0);
   int value_stride = value.stride(0);
+  int head_div_x = head_size / x;
 
   dim3 grid(num_tokens);
-  dim3 block(std::min(num_heads * head_size, 512));
+  dim3 block(std::min(num_heads * head_div_x, 512));
   const at::cuda::OptionalCUDAGuard device_guard(device_of(key));
   const cudaStream_t stream = at::cuda::getCurrentCUDAStream();
 
