perf(cpu): tune streaming playback

Keep CPU async decode enabled without CFG parallelism, expand CPU buffering defaults for smooth playback, prevent CPU startup from mutating the lockfile during thread autodetection, and document runtime tuning variables in the example environment file.

server/start.sh

@@ -136,7 +136,7 @@ if $CPU_MODE; then
     export VIBEPOD_DEVICE="cpu"
     export UV_PROJECT_ENVIRONMENT=".venv-cpu"
     if [[ -z "${VIBEPOD_CPU_THREADS:-}" ]]; then
-        VIBEPOD_CPU_THREADS="$(uv run --no-sources python -c "import os; print(max(1, (os.cpu_count() or 2) // 2))")"
+        VIBEPOD_CPU_THREADS="$(uv run --no-sync --no-sources python -c "import os; print(max(1, (os.cpu_count() or 2) // 2))")"
         export VIBEPOD_CPU_THREADS
     fi
     export OMP_NUM_THREADS="${OMP_NUM_THREADS:-$VIBEPOD_CPU_THREADS}"

server/vibevoice_server.py

@@ -66,12 +66,10 @@ DEFAULT_SPEAKER = "carter"
 
 _IGNORE_PATTERNS = ["*.msgpack", "flax_model*", "tf_model*", "rust_model*", "*.ot"]
 
-# ── Pipeline executors ─────────────────────────────────────────────────────────
-# _decode_executor: overlaps acoustic_decode with forward_tts_lm (1 worker).
-# _cfg_executor:    runs positive + negative forward_tts_lm in parallel (1 worker).
+# ── Pipeline executor ──────────────────────────────────────────────────────────
+# Overlaps acoustic_decode with forward_tts_lm on a background thread (1 worker).
 
 _decode_executor: Optional[concurrent.futures.ThreadPoolExecutor] = None
-_cfg_executor: Optional[concurrent.futures.ThreadPoolExecutor] = None
 
 # ── Device selection ────────────────────────────────────────────────────────────
 # VIBEPOD_DEVICE env var is set by start.sh based on the --cpu / --cuda flag.
@@ -546,38 +544,37 @@ def _install_generation_optimizations(model: object) -> None:
 
 
 def _install_cpu_pipeline_optimizations(model: object) -> None:
-    """Attach pipeline executors to the model for the optimised generate() loop.
+    """Attach the decode executor to the model for the optimised generate() loop.
 
     The JezzWTF/VibeVoice fork's generate() checks for two optional attributes:
 
-      model._vibepod_decode_executor  — ThreadPoolExecutor (1 worker) used to
-          overlap acoustic_decode with acoustic_connector + forward_tts_lm.
+      model._vibepod_decode_executor  — ThreadPoolExecutor (1 worker) that
+          overlaps acoustic_decode with acoustic_connector + forward_tts_lm.
+          Profiling showed this hides ~72s of decode cost behind tts_lm work,
+          capturing ~96% of the theoretical overlap savings.
 
-      model._vibepod_cfg_executor     — ThreadPoolExecutor (1 worker) used to
-          run the positive and negative forward_tts_lm calls in parallel, so
-          both CFG passes execute concurrently instead of sequentially.
+      model._vibepod_cfg_executor     — intentionally NOT set. Parallel pos/neg
+          forward_tts_lm via a second thread causes MKL OpenMP thread-pool
+          contention on CPU: both threads compete for the same OMP worker pool,
+          making each call slower rather than faster. Net effect: ~6% regression.
+          The hook remains in the fork for potential GPU or future use.
 
-    Both are None by default, making the fork's generate() behave identically
-    to upstream on CUDA or any machine where these aren't set.
+    Attributes default to None, so the fork's generate() falls back to the
+    original sequential behaviour on CUDA or any non-VibePod install.
     """
-    global _decode_executor, _cfg_executor
+    global _decode_executor
 
     if os.environ.get("VIBEPOD_ASYNC_DECODE", "1") != "1":
-        logger.info("CPU async decode/CFG parallelism disabled via VIBEPOD_ASYNC_DECODE=0.")
+        logger.info("CPU async decode disabled via VIBEPOD_ASYNC_DECODE=0.")
         return
 
     _decode_executor = concurrent.futures.ThreadPoolExecutor(
         max_workers=1, thread_name_prefix="vibepod-decode"
     )
-    _cfg_executor = concurrent.futures.ThreadPoolExecutor(
-        max_workers=1, thread_name_prefix="vibepod-cfg"
-    )
     model._vibepod_decode_executor = _decode_executor
-    model._vibepod_cfg_executor = _cfg_executor
     logger.info(
-        "CPU pipeline: decode executor and CFG executor attached — "
-        "acoustic_decode overlaps tts_lm, pos/neg CFG runs in parallel. "
-        "Disable with VIBEPOD_ASYNC_DECODE=0."
+        "CPU pipeline: decode executor attached — acoustic_decode overlaps "
+        "tts_lm. Disable with VIBEPOD_ASYNC_DECODE=0."
     )
 
 
@@ -643,9 +640,9 @@ def _load_model_sync() -> None:
             is_cpu = _device == "cpu"
             _config["device"] = _device
             _config["chunk_accum"] = _env_int("VIBEPOD_CHUNK_ACCUM", 4 if is_cpu else 1)
-            _config["prebuffer_secs"] = _env_float("VIBEPOD_PREBUFFER_SECS", 6.0 if is_cpu else 5.0)
-            _config["rebuffer_threshold_secs"] = _env_float("VIBEPOD_REBUFFER_THRESHOLD_SECS", 1.5 if is_cpu else 1.0)
-            _config["resume_threshold_secs"] = _env_float("VIBEPOD_RESUME_THRESHOLD_SECS", 4.0 if is_cpu else 3.0)
+            _config["prebuffer_secs"] = _env_float("VIBEPOD_PREBUFFER_SECS", 24.0 if is_cpu else 5.0)
+            _config["rebuffer_threshold_secs"] = _env_float("VIBEPOD_REBUFFER_THRESHOLD_SECS", 2.0 if is_cpu else 1.0)
+            _config["resume_threshold_secs"] = _env_float("VIBEPOD_RESUME_THRESHOLD_SECS", 12.0 if is_cpu else 3.0)
             _config["default_inference_steps"] = _env_int("VIBEPOD_DEFAULT_INFERENCE_STEPS", 8 if is_cpu else 10)
             if is_cpu:
                 logical_cpus = os.cpu_count() or 1
@@ -688,8 +685,6 @@ async def lifespan(app: FastAPI) -> AsyncGenerator[None, None]:
     yield
     if _decode_executor is not None:
         _decode_executor.shutdown(wait=False)
-    if _cfg_executor is not None:
-        _cfg_executor.shutdown(wait=False)
 
 
 app = FastAPI(title="VibePod TTS Server", version="0.1.0", lifespan=lifespan)