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perf(display): skip preview work nobody reads (lazy checksum, 1 Hz snapshot writer) (#717)

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Mid-scroll, update_display() no longer checksums every frame: it asks is_currently_scrolling() once per frame and reuses the answer, hashes only when dirty tracking can skip a static frame, and the preview snapshot asks its policy first and hashes only when a write or touch could follow (decide() is monotone, pinned by a property test). With the preview open the snapshot is written at most once a second (VIEWER_INTERVAL 1.0 s, was 0.2 s; the SSE stream re-read it once a second, so four encodes in five went unread); the stream now polls its mtime every 0.25 s (VIEWER_POLL_INTERVAL), so the preview stays about as fresh. The PNG is written at compress_level=1. --preview soaks are not comparable across this change.

Merged with #716: _scan_segments takes the frame's one scrolling answer and #716's static-handover pass-through, as soaked on ledpi (A B B A, 20 min each: main 0.118% / 0.113% late, with #716 and this 0.107% / 0.104%).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Chuck
2026-10-01 20:27:20 -04:00
committed by GitHub
co-authored by Claude Opus 5.5
parent 1ecf3aba03
commit f72d69c2b0
13 changed files with 804 additions and 47 deletions
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src/common/snapshot_policy.py
+21 -2
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@@ -26,14 +26,33 @@ Policy:
- Unchanged frames are never re-encoded; the mtime is touched every
TOUCH_INTERVAL so the health check (60s threshold) never degrades.
The writer and the SSE reader (web_interface/app.py) have two periods, not
one shared value. The reader sends each write it sees, so the preview shows
at most one frame per VIEWER_INTERVAL. (That was 0.2 s while the reader
slept 1 s between reads, so four encodes in five were overwritten unread.)
The reader checks the file's mtime every VIEWER_POLL_INTERVAL, which is only
a stat, and so sends each write within that long of it landing. Equal
periods would alias: two unsynchronised 1 s clocks leave the preview up to a
second stale, and now and then 2 s between frames.
decide() is monotone in frame_changed: SKIP for a changed frame means SKIP
for an unchanged one. DisplayManager relies on that to skip hashing the
frame when even a changed one would be skipped; test_snapshot_policy.py
checks it.
If any constant here changes, re-check the health threshold in
api_v3/misc.py (get_hardware_status) — TOUCH_INTERVAL must stay well under it.
"""
from enum import Enum
# Snapshot cadence with a browser preview open (seconds).
VIEWER_INTERVAL = 0.2
# Snapshot cadence with a browser preview open (seconds): the shortest gap
# between two preview frames.
VIEWER_INTERVAL = 1.0
# How often the web SSE reader checks the snapshot's mtime (seconds). Must
# stay well under VIEWER_INTERVAL -- half of it at most -- or the two clocks
# alias (see above).
VIEWER_POLL_INTERVAL = 0.25
# Snapshot cadence with no viewers — cheap freshness for page-open (seconds).
IDLE_INTERVAL = 30.0
# Max age of the last write/touch before bumping mtime for the health