Send at the same time to improve exo bench concurrency accuracy

bench/exo_bench.py

@@ -22,6 +22,7 @@ import contextlib
 import itertools
 import json
 import sys
+import threading
 import time
 from collections.abc import Callable
 from concurrent.futures import ThreadPoolExecutor, as_completed
@@ -443,19 +444,37 @@ def main() -> int:
                             all_rows.append(row)
                         else:
                             # Concurrent: fire N requests in parallel
-                            # Each thread gets its own ExoClient (separate HTTP connection)
+                            # Pre-build prompt once, barrier ensures simultaneous dispatch
+                            content, actual_pp = prompt_sizer.build(pp)
+                            pre_built_payload: dict[str, Any] = {
+                                "model": full_model_id,
+                                "messages": [{"role": "user", "content": content}],
+                                "stream": False,
+                                "max_tokens": tg,
+                            }
+                            barrier = threading.Barrier(concurrency)
                             batch_results: list[tuple[dict[str, Any], int]] = []
                             batch_errors = 0
 
                             def _run_concurrent(
-                                idx: int, *, _pp: int = pp, _tg: int = tg
+                                idx: int,
                             ) -> tuple[dict[str, Any], int]:
                                 c = ExoClient(
                                     args.host, args.port, timeout_s=args.timeout
                                 )
-                                return run_one_completion(
-                                    c, full_model_id, _pp, _tg, prompt_sizer
-                                )
+                                barrier.wait()
+                                t0 = time.perf_counter()
+                                out = c.post_bench_chat_completions(pre_built_payload)
+                                elapsed = time.perf_counter() - t0
+                                stats = out.get("generation_stats")
+                                choices = out.get("choices") or [{}]
+                                message = choices[0].get("message", {}) if choices else {}
+                                text = message.get("content") or ""
+                                return {
+                                    "elapsed_s": elapsed,
+                                    "output_text_preview": text[:200],
+                                    "stats": stats,
+                                }, actual_pp
 
                             with ThreadPoolExecutor(max_workers=concurrency) as pool:
                                 futures = {