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experiment(vegas): vegas_scroll.prefetch_gate runs the prefetch only while the render thread waits on vsync

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The render thread spends most of each refresh in SwapOnVSync with the GIL
released, then needs it back the moment the swap returns. With plugin
rendering on the prefetch thread, it often has to wait for it -- behind
bytecode for up to the switch interval, behind a GIL-holding C call for as
long as that takes -- and hdpi's late frames of 2-5 refreshes went up.

src/common/render_gate.py opens a window around each swap, up to just
before the refresh the swap will return on, and a profile hook on the
prefetch thread parks it outside that window. It is never parked holding a
lock the render thread also takes (the Vegas buffer, cache and state
locks, logging, threading, importlib, the cache), never when no frame has
been swapped for 50ms, and never for more than 50ms at a time.

Off by default and ignored on a binding that keeps the GIL in
SwapOnVSync, where the window would never let the prefetch run.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Chuck
2026-09-24 14:23:09 -04:00
co-authored by Claude Opus 5.5
parent 55a2760892
commit 4f30bc62ca
6 changed files with 656 additions and 1 deletions
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src/common/render_gate.py
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"""Let a background thread run Python only while the render thread waits on vsync.
With plugin rendering moved to Vegas's prefetch thread (DisplayManager.offscreen,
#630) the render thread no longer stops for it, but it still shares the GIL
with it. The render thread spends most of each refresh inside SwapOnVSync,
which releases the GIL, and needs it back the moment the swap returns. If the
prefetch thread is running Python right then, the render thread waits: up to
the switch interval (5ms) behind bytecode, and for as long as a C call that
keeps the GIL takes. On hdpi that showed up as frames 2-5 refreshes late while
a group was being prepared.
The gate turns that around. The display manager opens it just before each swap,
with a deadline shortly ahead of the refresh the swap will return on, and
closes it when the swap returns. A thread inside ``gate.yielding()`` checks it on
every Python and C call through a profile hook, and once the window has closed
it parks -- blocked on a condition, GIL released -- until the next swap opens
it. The render thread then finds the GIL free when its refresh arrives, and the
background work runs in time the render thread was only spending waiting.
Parking a thread is only safe if nothing the render thread needs is stuck
behind it, so it is never parked:
* while it holds a lock registered with ``guard()`` (the Vegas buffers and
caches the render thread also takes);
* inside logging, threading, importlib or the cache, all of which take locks the
render thread can take too;
* when there is no render loop to protect -- no swap for ``STALE_SECONDS``, as
on a static screen or a stalled frame.
And a parked thread is never held more than ``MAX_WAIT_SECONDS`` at a time, so
whatever the gate gets wrong costs a frame, not a freeze.
Only worth enabling on a binding whose SwapOnVSync releases the GIL (see
scripts/build_rgbmatrix_nogil.sh). With one that keeps it, the window never
lets the background thread run and it only makes progress in the timeouts.
"""
from __future__ import annotations
import math
import sys
import threading
import time
from collections import deque
from typing import Any, Callable, Deque, List, Optional
#: Park background threads this long before the refresh a swap will return on,
#: so a short C call already under way has finished by then.
MARGIN_SECONDS = 0.002
#: The longest a background thread is parked in one go.
MAX_WAIT_SECONDS = 0.05
#: No swap for this long means there is no render loop running to protect.
STALE_SECONDS = 0.05
#: Swaps needed before the refresh period is trusted enough to open a window.
MIN_SAMPLES = 8
#: Parking with any of these on the stack could hold a lock the render
#: thread takes: logging handler locks, Condition and Event internals, the
#: module import locks, and the disk and memory cache locks.
_UNSAFE_PATHS = (
"logging",
"threading.py",
"importlib",
"cache",
)
def _unsafe(frame: Any, base: Any) -> bool:
"""True if a frame above ``base`` comes from somewhere parking could deadlock.
``base`` is the frame that entered ``yielding()``; what lies below it (the
thread's own bootstrap in threading.py) holds nothing.
"""
while frame is not None and frame is not base:
filename = frame.f_code.co_filename
for part in _UNSAFE_PATHS:
if part in filename:
return True
frame = frame.f_back
return False
def swap_releases_gil() -> Optional[bool]:
"""Whether the loaded rgbmatrix binding releases the GIL, or None if none is loaded.
The rebuilt binding links PyEval_SaveThread and the stock one never does.
The same test as src.common.frame_timing.binding_releases_gil (#629); one
of the two goes once both have landed.
"""
module = sys.modules.get("rgbmatrix.core")
path = getattr(module, "__file__", None)
if not path:
return None
try:
with open(path, "rb") as handle:
return b"PyEval_SaveThread" in handle.read()
except OSError:
return None
def _held(lock: Any) -> bool:
"""Is ``lock`` held? RLocks report this thread's ownership; plain locks, anyone's."""
is_owned = getattr(lock, "_is_owned", None)
if is_owned is not None:
return is_owned()
return lock.locked()
class RenderGate:
"""Opened by the render thread around each swap; honoured by background threads."""
def __init__(self, clock: Callable[[], float] = time.monotonic):
self.clock = clock
self._cond = threading.Condition()
self._generation = 0
self._open_until = 0.0
self._last_return: Optional[float] = None
self._periods: Deque[float] = deque(maxlen=64)
self._period: Optional[float] = None
self._guarded: List[Any] = []
self._local = threading.local()
#: How often, and for how long in all, background threads were parked.
self.parks = 0
self.parked_seconds = 0.0
def guard(self, *locks: Any) -> None:
"""Never park a thread while it holds (or, for a plain Lock, anyone holds) these."""
self._guarded.extend(lock for lock in locks if lock is not None)
# -- render thread -----------------------------------------------------
def refresh_period(self) -> Optional[float]:
"""The panel's refresh period from recent swaps, or None until known.
The 10th percentile of the gaps between swap returns, each divided by
the hold: a late frame only ever lengthens a gap, so the low end is
the panel's own period.
"""
return self._period
def before_swap(self, hold: int) -> None:
"""The render thread is about to block in SwapOnVSync: open the window."""
hold = max(1, int(hold))
period = self._period
now = self.clock()
last = self._last_return
if period and last is not None and now - last < STALE_SECONDS:
# The swap returns on the first refresh boundary after both the
# current frame's hold is up and this frame has been handed over;
# boundaries fall a whole period apart from the last return.
refreshes = max(hold, math.ceil((now - last) / period))
open_until = last + refreshes * period - MARGIN_SECONDS
else:
open_until = 0.0 # no rhythm to predict from: leave threads be
with self._cond:
self._open_until = open_until
self._generation += 1
self._cond.notify_all()
def after_swap(self, hold: int) -> None:
"""The swap returned and the render thread needs the GIL: close the window."""
now = self.clock()
self._open_until = 0.0
last = self._last_return
if last is not None and now - last < STALE_SECONDS:
self._periods.append((now - last) / max(1, int(hold)))
if len(self._periods) >= MIN_SAMPLES:
ordered = sorted(self._periods)
self._period = ordered[len(ordered) // 10]
self._last_return = now
# -- background threads ------------------------------------------------
def _should_park(self, frame: Any, now: float) -> bool:
if now < self._open_until:
return False # inside the window
last = self._last_return
if last is None or now - last > STALE_SECONDS or self._period is None:
return False # no render loop to protect
for lock in self._guarded:
if _held(lock):
return False
return not _unsafe(frame, getattr(self._local, "base", None))
def _hook(self, frame: Any, _event: str, _arg: Any) -> None:
now = self.clock()
if not self._should_park(frame, now):
return
generation = self._generation
with self._cond:
self._cond.wait_for(lambda: self._generation != generation,
timeout=MAX_WAIT_SECONDS)
self.parks += 1
self.parked_seconds += self.clock() - now
def yielding(self) -> "_Yielding":
"""``with gate.yielding():`` runs the block giving way to the render thread."""
return _Yielding(self)
class _Yielding:
"""Installs a gate's profile hook on the thread for the length of a block."""
def __init__(self, gate: RenderGate):
self.gate = gate
self._previous: Any = None
self._previous_base: Any = None
def __enter__(self) -> RenderGate:
local = self.gate._local # pylint: disable=protected-access
self._previous_base = getattr(local, "base", None)
local.base = sys._getframe(1) # pylint: disable=protected-access
self._previous = sys.getprofile()
sys.setprofile(self.gate._hook) # pylint: disable=protected-access
return self.gate
def __exit__(self, *_exc: Any) -> None:
sys.setprofile(self._previous)
self.gate._local.base = self._previous_base # pylint: disable=protected-access