Backpressure cannot be implemented in this repository. Every node output in KPN
uses the dropping push() (pool_node.hpp:404 and :710, plus branch, fanout and
interrupt_node). A lossless push_blocking() already exists on both Channel and
OutputPort — "wait for the consumer to drain instead of dropping; the producer
just runs slower" — and nothing calls it. The fix is a per-channel policy or a
network default in KPN, and this pipeline should select lossless: a dropped
frame here does not degrade a result, it silently changes one.
Measured rather than inferred. One 77s clip at 5 fps should yield ~385 sampled
frames. On CPU it produced 49, ending at 51s, with 285 dropped at camera_pos
and 51 at face_aligner. Rebuilt with CUDA the same clip ran in 29s and reached
EOF correctly, and still dropped 320 at camera_pos, yielding 65. Faster
hardware moves where the queue backs up; it does not change what happens when
it does — which is why this is a correctness requirement rather than a
throughput one.
Raising channel capacity is therefore a stopgap: it lowers the probability of
overflow without changing the behaviour on overflow, and the failure it hides
is silent corruption of the output.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
TRACES: AR-003, AR-004, VR-001 | SR-002
dtourolleandClaude Opus 5
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