[Misc] Add common random prefix option to structured-output serving benchmark (#41632)

Signed-off-by: Viktor Pus <viktorpus@tenstorrent.com>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

benchmarks/benchmark_serving_structured_output.py

@@ -115,6 +115,39 @@ class SampleRequest:
 def sample_requests(
     tokenizer: PreTrainedTokenizerBase, args: argparse.Namespace
 ) -> list[SampleRequest]:
+    def _apply_random_prefix(
+        tokenizer: PreTrainedTokenizerBase,
+        requests: list[SampleRequest],
+        prefix_len: int,
+        seed: int,
+    ) -> list[SampleRequest]:
+        if prefix_len <= 0:
+            return requests
+        rng = np.random.default_rng(seed)
+        vocab_size = tokenizer.vocab_size
+        prohibited = getattr(tokenizer, "all_special_ids", None) or []
+        allowed = np.array([i for i in range(vocab_size) if i not in prohibited])
+        if len(allowed) == 0:
+            return requests
+        prefix_ids = rng.integers(0, len(allowed), size=prefix_len)
+        prefix_token_ids = allowed[prefix_ids].tolist()
+        out = []
+        for req in requests:
+            prompt_ids = tokenizer(req.prompt, add_special_tokens=False).input_ids
+            full_ids = prefix_token_ids + prompt_ids
+            full_prompt = tokenizer.decode(full_ids, skip_special_tokens=False)
+            out.append(
+                SampleRequest(
+                    prompt=full_prompt,
+                    prompt_len=len(tokenizer(full_prompt).input_ids),
+                    expected_output_len=req.expected_output_len,
+                    schema=req.schema,
+                    structure_type=req.structure_type,
+                    completion=req.completion,
+                )
+            )
+        return out
+
     if args.dataset == "json" or args.dataset == "json-unique":
         if args.json_schema_path is None:
             dir_path = os.path.dirname(os.path.realpath(__file__))
@@ -261,6 +294,9 @@ def sample_requests(
                 )
             )
 
+    requests = _apply_random_prefix(
+        tokenizer, requests, args.random_prefix_len, args.seed
+    )
     return requests
 
 
@@ -945,6 +981,15 @@ def create_argument_parser():
         "results in a more uniform arrival of requests.",
     )
     parser.add_argument("--seed", type=int, default=0)
+    parser.add_argument(
+        "--random-prefix-len",
+        type=int,
+        default=0,
+        help=(
+            "Number of prefix tokens to prepend to every prompt. "
+            "The same prefix is used for all prompts to enable prefix caching."
+        ),
+    )
     parser.add_argument(
         "--trust-remote-code",
         action="store_true",