[Core] Add xxHash as a high-performance hash option for accelerating prefix caching (#29163)

Signed-off-by: LuminolT <lumischen01@gmail.com>
Signed-off-by: Lumis Chen <lumischen01@gmail.com>
Co-authored-by: Russell Bryant <rbryant@redhat.com>

benchmarks/benchmark_hash.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+"""
+Micro benchmark comparing built-in hash(), SHA-256, and xxHash.
+
+This focuses on a single test payload shaped like the prefix-cache hash input:
+    (32-byte bytes object, 32-int tuple)
+
+Usage:
+    python benchmarks/hash_micro_benchmark.py --iterations 20000
+"""
+
+from __future__ import annotations
+
+import argparse
+import random
+import statistics
+import time
+from collections.abc import Callable, Iterable
+
+from vllm.utils.hashing import sha256, xxhash
+
+
+def _generate_test_data(seed: int) -> tuple[bytes, tuple[int, ...]]:
+    """Generate a deterministic test payload."""
+    random.seed(seed)
+    bytes_data = bytes(random.getrandbits(8) for _ in range(32))
+    int_tuple = tuple(random.randint(1, 1_000_000) for _ in range(32))
+    return (bytes_data, int_tuple)
+
+
+def _benchmark_func(func: Callable[[tuple], object], data: tuple, iterations: int):
+    """Return (avg_seconds, std_seconds) for hashing `data` `iterations` times."""
+    times: list[float] = []
+
+    # Warm-up to avoid first-run noise.
+    for _ in range(200):
+        func(data)
+
+    for _ in range(iterations):
+        start = time.perf_counter()
+        func(data)
+        end = time.perf_counter()
+        times.append(end - start)
+
+    avg = statistics.mean(times)
+    std = statistics.stdev(times) if len(times) > 1 else 0.0
+    return avg, std
+
+
+def _run_benchmarks(
+    benchmarks: Iterable[tuple[str, Callable[[tuple], object]]],
+    data: tuple,
+    iterations: int,
+):
+    """Yield (name, avg, std) for each benchmark, skipping unavailable ones."""
+    for name, func in benchmarks:
+        try:
+            avg, std = _benchmark_func(func, data, iterations)
+        except ModuleNotFoundError as exc:
+            print(f"Skipping {name}: {exc}")
+            continue
+        yield name, avg, std
+
+
+def builtin_hash(data: tuple) -> int:
+    """Wrapper for Python's built-in hash()."""
+    return hash(data)
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "--iterations",
+        type=int,
+        default=10_000,
+        help="Number of measured iterations per hash function.",
+    )
+    parser.add_argument(
+        "--seed", type=int, default=42, help="Random seed for test payload."
+    )
+    args = parser.parse_args()
+
+    data = _generate_test_data(args.seed)
+    benchmarks = (
+        ("SHA256 (pickle)", sha256),
+        ("xxHash (pickle)", xxhash),
+        ("built-in hash()", builtin_hash),
+    )
+
+    print("=" * 60)
+    print("HASH FUNCTION MICRO BENCHMARK")
+    print("=" * 60)
+    print("Test data: (32-byte bytes object, 32-int tuple)")
+    print(f"Iterations: {args.iterations:,}")
+    print("=" * 60)
+
+    results = list(_run_benchmarks(benchmarks, data, args.iterations))
+    builtin_entry = next((r for r in results if r[0] == "built-in hash()"), None)
+
+    print("\nResults:")
+    for name, avg, std in results:
+        print(f"  {name:16s}: {avg * 1e6:8.2f} ± {std * 1e6:6.2f} μs")
+
+    if builtin_entry:
+        _, builtin_avg, _ = builtin_entry
+        print("\n" + "=" * 60)
+        print("SUMMARY (relative to built-in hash())")
+        print("=" * 60)
+        for name, avg, _ in results:
+            if name == "built-in hash()":
+                continue
+            speed_ratio = avg / builtin_avg
+            print(f"• {name} is {speed_ratio:.1f}x slower than built-in hash()")
+    else:
+        print("\nBuilt-in hash() result missing; cannot compute speed ratios.")
+
+
+if __name__ == "__main__":
+    main()

benchmarks/benchmark_prefix_block_hash.py

+# SPDX-License-Identifier: Apache-2.0
+# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+
+"""
+Simple benchmark to compare prefix-cache block hashing algorithms.
+
+Example:
+    python benchmark_prefix_block_hash.py --num-blocks 20000 --block-size 32
+"""
+
+from __future__ import annotations
+
+import argparse
+import random
+import statistics
+import sys
+import time
+from collections.abc import Callable, Iterable, Sequence
+
+from vllm.utils.hashing import get_hash_fn_by_name
+from vllm.v1.core.kv_cache_utils import BlockHash, hash_block_tokens, init_none_hash
+
+SUPPORTED_ALGOS = ("sha256", "sha256_cbor", "xxhash", "xxhash_cbor")
+
+
+def _generate_blocks(
+    num_blocks: int, block_size: int, vocab_size: int, seed: int
+) -> list[list[int]]:
+    rng = random.Random(seed)
+    return [
+        [rng.randrange(vocab_size) for _ in range(block_size)]
+        for _ in range(num_blocks)
+    ]
+
+
+def _hash_all_blocks(
+    hash_fn: Callable[[object], bytes],
+    blocks: Iterable[Sequence[int]],
+) -> float:
+    parent_hash: BlockHash | None = None
+    start = time.perf_counter()
+    for block in blocks:
+        parent_hash = hash_block_tokens(hash_fn, parent_hash, block, extra_keys=None)
+    end = time.perf_counter()
+    return end - start
+
+
+def _benchmark(
+    hash_algo: str,
+    blocks: list[list[int]],
+    trials: int,
+) -> tuple[float, float, float] | None:
+    try:
+        hash_fn = get_hash_fn_by_name(hash_algo)
+        init_none_hash(hash_fn)
+        timings = [_hash_all_blocks(hash_fn, blocks) for _ in range(trials)]
+    except ModuleNotFoundError as exc:
+        print(f"Skipping {hash_algo}: {exc}", file=sys.stderr)
+        return None
+
+    avg = statistics.mean(timings)
+    best = min(timings)
+    # throughput: tokens / second
+    tokens_hashed = len(blocks) * len(blocks[0])
+    throughput = tokens_hashed / best
+    return avg, best, throughput
+
+
+def main() -> None:
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--num-blocks", type=int, default=10000, help="Block count.")
+    parser.add_argument("--block-size", type=int, default=32, help="Tokens per block.")
+    parser.add_argument(
+        "--vocab-size", type=int, default=32000, help="Token id range [0, vocab_size)."
+    )
+    parser.add_argument("--seed", type=int, default=0, help="Random seed.")
+    parser.add_argument(
+        "--trials", type=int, default=5, help="Number of timed trials per algorithm."
+    )
+    parser.add_argument(
+        "--algorithms",
+        nargs="+",
+        default=SUPPORTED_ALGOS,
+        choices=SUPPORTED_ALGOS,
+        help="Hash algorithms to benchmark.",
+    )
+    args = parser.parse_args()
+
+    blocks = _generate_blocks(
+        args.num_blocks, args.block_size, args.vocab_size, args.seed
+    )
+    print(
+        f"Benchmarking {len(args.algorithms)} algorithms on "
+        f"{args.num_blocks} blocks (block size={args.block_size})."
+    )
+
+    for algo in args.algorithms:
+        result = _benchmark(algo, blocks, args.trials)
+        if result is None:
+            continue
+
+        avg, best, throughput = result
+        print(
+            f"{algo:14s} avg: {avg:.6f}s  best: {best:.6f}s  "
+            f"throughput: {throughput / 1e6:.2f}M tokens/s"
+        )
+
+
+if __name__ == "__main__":
+    main()

docs/benchmarking/cli.md

@@ -670,6 +670,35 @@ vllm bench serve \
 
 </details>
 
+### 🧪 Hashing Benchmarks
+
+<details class="admonition abstract" markdown="1">
+<summary>Show more</summary>
+
+Two helper scripts live in `benchmarks/` to compare hashing options used by prefix caching and related utilities. They are standalone (no server required) and help choose a hash algorithm before enabling prefix caching in production.
+
+- `benchmarks/benchmark_hash.py`: Micro-benchmark that measures per-call latency of three implementations on a representative `(bytes, tuple[int])` payload.
+
+```bash
+python benchmarks/benchmark_hash.py --iterations 20000 --seed 42
+```
+
+- `benchmarks/benchmark_prefix_block_hash.py`: End-to-end block hashing benchmark that runs the full prefix-cache hash pipeline (`hash_block_tokens`) across many fake blocks and reports throughput.
+
+```bash
+python benchmarks/benchmark_prefix_block_hash.py --num-blocks 20000 --block-size 32 --trials 5
+```
+
+Supported algorithms: `sha256`, `sha256_cbor`, `xxhash`, `xxhash_cbor`. Install optional deps to exercise all variants:
+
+```bash
+uv pip install xxhash cbor2
+```
+
+If an algorithm’s dependency is missing, the script will skip it and continue.
+
+</details>
+
 ### ⚡ Request Prioritization Benchmark
 
 <details class="admonition abstract" markdown="1">

tests/v1/engine/test_engine_args.py

@@ -9,6 +9,7 @@ from vllm.config import VllmConfig
 from vllm.engine.arg_utils import EngineArgs
 from vllm.usage.usage_lib import UsageContext
 from vllm.utils.argparse_utils import FlexibleArgumentParser
+from vllm.utils.hashing import _xxhash
 
 
 def test_prefix_caching_from_cli():
@@ -48,6 +49,21 @@ def test_prefix_caching_from_cli():
         args = parser.parse_args(["--prefix-caching-hash-algo", "invalid"])
 
 
+@pytest.mark.skipif(_xxhash is None, reason="xxhash not installed")
+def test_prefix_caching_xxhash_from_cli():
+    parser = EngineArgs.add_cli_args(FlexibleArgumentParser())
+
+    # set hash algorithm to xxhash (pickle)
+    args = parser.parse_args(["--prefix-caching-hash-algo", "xxhash"])
+    vllm_config = EngineArgs.from_cli_args(args=args).create_engine_config()
+    assert vllm_config.cache_config.prefix_caching_hash_algo == "xxhash"
+
+    # set hash algorithm to xxhash_cbor
+    args = parser.parse_args(["--prefix-caching-hash-algo", "xxhash_cbor"])
+    vllm_config = EngineArgs.from_cli_args(args=args).create_engine_config()
+    assert vllm_config.cache_config.prefix_caching_hash_algo == "xxhash_cbor"
+
+
 def test_defaults_with_usage_context():
     engine_args = EngineArgs(model="facebook/opt-125m")
     vllm_config: VllmConfig = engine_args.create_engine_config(UsageContext.LLM_CLASS)

vllm/config/cache.py

@@ -30,7 +30,7 @@ CacheDType = Literal[
     "fp8_ds_mla",
 ]
 MambaDType = Literal["auto", "float32"]
-PrefixCachingHashAlgo = Literal["sha256", "sha256_cbor"]
+PrefixCachingHashAlgo = Literal["sha256", "sha256_cbor", "xxhash", "xxhash_cbor"]
 KVOffloadingBackend = Literal["native", "lmcache"]
 
 
@@ -77,9 +77,21 @@ class CacheConfig:
     """Whether to enable prefix caching."""
     prefix_caching_hash_algo: PrefixCachingHashAlgo = "sha256"
     """Set the hash algorithm for prefix caching:\n
-    - "sha256" uses Pickle for object serialization before hashing.\n
+    - "sha256" uses Pickle for object serialization before hashing. This is the
+    current default, as SHA256 is the most secure choice to avoid potential
+    hash collisions.\n
     - "sha256_cbor" provides a reproducible, cross-language compatible hash. It
-    serializes objects using canonical CBOR and hashes them with SHA-256."""
+    serializes objects using canonical CBOR and hashes them with SHA-256.\n
+    - "xxhash" uses Pickle serialization with xxHash (128-bit) for faster,
+    non-cryptographic hashing. Requires the optional ``xxhash`` package.
+    IMPORTANT: Use of a hashing algorithm that is not considered 
+    cryptographically secure theoretically increases the risk of hash collisions,
+    which can cause undefined behavior or even leak private information in
+    multi-tenant environments. Even if collisions are still very unlikely, it is
+    important to consider your security risk tolerance against the performance
+    benefits before turning this on.\n
+    - "xxhash_cbor" combines canonical CBOR serialization with xxHash for
+    reproducible hashing. Requires the optional ``xxhash`` package."""
     cpu_offload_gb: float = Field(default=0, ge=0)
     """The space in GiB to offload to CPU, per GPU. Default is 0, which means
     no offloading. Intuitively, this argument can be seen as a virtual way to

vllm/utils/hashing.py

@@ -11,6 +11,17 @@ from typing import Any
 
 import cbor2
 
+try:
+    # It is important that this remains an optional dependency.
+    # It would not be allowed in environments with strict security controls,
+    # so it's best not to have it installed when not in use.
+    import xxhash as _xxhash
+
+    if not hasattr(_xxhash, "xxh3_128_digest"):
+        _xxhash = None
+except ImportError:  # pragma: no cover
+    _xxhash = None
+
 
 def sha256(input: Any) -> bytes:
     """Hash any picklable Python object using SHA-256.
@@ -47,6 +58,27 @@ def sha256_cbor(input: Any) -> bytes:
     return hashlib.sha256(input_bytes).digest()
 
 
+def _xxhash_digest(input_bytes: bytes) -> bytes:
+    if _xxhash is None:
+        raise ModuleNotFoundError(
+            "xxhash is required for the 'xxhash' prefix caching hash algorithms. "
+            "Install it via `pip install xxhash`."
+        )
+    return _xxhash.xxh3_128_digest(input_bytes)
+
+
+def xxhash(input: Any) -> bytes:
+    """Hash picklable objects using xxHash."""
+    input_bytes = pickle.dumps(input, protocol=pickle.HIGHEST_PROTOCOL)
+    return _xxhash_digest(input_bytes)
+
+
+def xxhash_cbor(input: Any) -> bytes:
+    """Hash objects serialized with CBOR using xxHash."""
+    input_bytes = cbor2.dumps(input, canonical=True)
+    return _xxhash_digest(input_bytes)
+
+
 def get_hash_fn_by_name(hash_fn_name: str) -> Callable[[Any], bytes]:
     """Get a hash function by name, or raise an error if the function is not found.
 
@@ -60,6 +92,10 @@ def get_hash_fn_by_name(hash_fn_name: str) -> Callable[[Any], bytes]:
         return sha256
     if hash_fn_name == "sha256_cbor":
         return sha256_cbor
+    if hash_fn_name == "xxhash":
+        return xxhash
+    if hash_fn_name == "xxhash_cbor":
+        return xxhash_cbor
 
     raise ValueError(f"Unsupported hash function: {hash_fn_name}")
 

vllm/v1/core/kv_cache_utils.py

@@ -12,7 +12,7 @@ from typing import Any, NewType, TypeAlias, overload
 from vllm import envs
 from vllm.config import VllmConfig
 from vllm.logger import init_logger
-from vllm.utils.hashing import sha256_cbor
+from vllm.utils.hashing import sha256_cbor, xxhash_cbor
 from vllm.utils.math_utils import cdiv
 from vllm.utils.mem_constants import GiB_bytes
 from vllm.v1.kv_cache_interface import (
@@ -83,18 +83,19 @@ logger = init_logger(__name__)
 #
 # The function `init_none_hash` initializes this variable globally.
 NONE_HASH: BlockHash
+_CBOR_HASH_FUNCTIONS = frozenset({sha256_cbor, xxhash_cbor})
 
 
 def init_none_hash(hash_fn: Callable[[Any], bytes]):
     global NONE_HASH
 
     hash_seed = os.getenv("PYTHONHASHSEED")
-    if hash_seed is None and hash_fn is sha256_cbor:
+    if hash_seed is None and hash_fn in _CBOR_HASH_FUNCTIONS:
         logger.warning(
             "PYTHONHASHSEED is not set. This will lead to non-reproducible "
-            "block-hashes when using sha256_cbor as the hash function."
-            "Consider setting PYTHONHASHSEED to a fixed value for "
-            "reproducibility."
+            "block-hashes when using CBOR-based hash functions such as "
+            "sha256_cbor or xxhash_cbor. Consider setting PYTHONHASHSEED to a "
+            "fixed value for reproducibility."
         )
 
     if hash_seed is None: