The named next step after WP3's first increment. That increment deliberately
REFUSED to arbitrate HEVC-with-sub-frame -- the fleet default, and the reported
field case -- because engaging split there gives up sub-frame readback, whose
whole value is that the send overlaps the encode. An encoder measuring only
encode time would see split as ~2x faster, take it, and make end-to-end latency
worse while reporting a win. This supplies the missing number.
The real comparison is encode_1eng + send_of_last_slice against
encode_2eng + send_of_whole_AU, so the challenger owes roughly
spread x (slices-1)/slices. Split across the two sides that can each see half:
- Host: new `Encoder::set_send_spread_us` (defaulted, forwarded by
TrackedEncoder -- same trap class as set_wire_chunking, and unforwarded it
would fail SILENTLY IN THE SAFE DIRECTION, which is the hardest kind to
notice). The send thread is the only place a paced send is observed and the
encode loop the only place the encoder can be touched, so it goes over an
AtomicU32 like encoder_ceiling_kbps, EWMA-smoothed 3:1 per completed AU: one
content spike must not flip a verdict that then gets cached.
- Encoder: turns the raw spread into the handicap, because only it knows
`slices`. SplitArbiter::with_handicap charges it to the challenger before the
comparison. A unit test runs identical encode numbers with a cheap and an
expensive send and asserts the verdict REVERSES -- with an expensive send the
arm that looks twice as fast is a loss end to end, and the incumbent must
hold. That is precisely the regression an encode-only arbiter ships.
Gate now opens for HEVC+sub-frame only when a spread has actually been reported
(and slices >= 2); with no hint it still refuses, so behaviour is unchanged until
the host feeds it.
Two mechanics this needed:
- apply_split_mode became a PAIR flip (split + sub-frame), routed through
resolve_split_subframe and restoring from `subframe_opened_with` so a session
that never had sub-frame can never gain it. It also recomputes
`subframe_chunks`, which reconfigure_bitrate does NOT -- spike S1c's finding;
leave it stale and supports_chunked_poll keeps saying yes while numSlices never
advances, so poll_chunk busy-polls its whole budget every AU.
- The arbiter is now fed from BOTH completion points. A sub-frame session
finishes through poll_chunk, so the incumbent arm of an HEVC experiment would
otherwise never deliver a sample -- only the challenger, with sub-frame
dropped, comes through poll.
Verified .21: clippy -D warnings clean for pf-encode AND punktfunk-host with
nvenc, 63 unit tests (1 new), 23/23 NVENC on-hardware green. Verified .133:
Windows clippy -D warnings clean, zero dead_code. fmt clean.
enricobuehler
2366c4fe31