[PD]feat(bench): add --fake-prefill flag for decode-only stress testing (#22973)

python/sglang/bench_serving.py

@@ -44,6 +44,7 @@ from sglang.benchmark.utils import (
     remove_prefix,
     set_ulimit,
 )
+from sglang.srt.disaggregation.utils import FAKE_BOOTSTRAP_HOST
 from sglang.srt.utils.network import NetworkAddress
 
 _ROUTING_KEY_HEADER = "X-SMG-Routing-Key"
@@ -1709,6 +1710,11 @@ def run_benchmark(args_: argparse.Namespace):
     if args.extra_request_body:
         extra_request_body = json.loads(args.extra_request_body)
 
+    # Inject bootstrap fields for fake decode benchmarking
+    if getattr(args, "fake_prefill", False):
+        extra_request_body["bootstrap_host"] = FAKE_BOOTSTRAP_HOST
+        extra_request_body["bootstrap_room"] = 0
+
     if args.tokenize_prompt:
         assert (
             args.backend == "sglang"
@@ -2339,6 +2345,14 @@ if __name__ == "__main__":
         ],
         help="Underlying workload for the mooncake dataset.",
     )
+    parser.add_argument(
+        "--fake-prefill",
+        action="store_true",
+        default=False,
+        help="Enable fake prefill mode for decode-only benchmarking. "
+        "Use with a decode server running --disaggregation-transfer-backend fake "
+        "to benchmark pure decode performance without a real prefill node.",
+    )
     parser.add_argument(
         "--tag", type=str, default=None, help="The tag to be dumped to output."
     )