KV cache for the decoder turns greedy generation from O(T^2) into O(T)
total work. Per-token decode drops modestly on short transcripts
(7.1 -> 6.0 ms/token at 49 tokens) and compounds on longer ones.
New components in src/moonshine.rs:
RotaryCache::apply_at(x, position, t)
Apply RoPE for a window starting at `position`. Replaces
`apply()` for cached step (which always called positions 0..T).
DecoderSelfAttention::forward_step(xs, cache_k, cache_v, rope, position)
Single-token cached self-attn. Appends new K/V to per-layer cache,
attends across full accumulated history. No causal mask needed
(cache only contains positions <= current).
CrossAttention::precompute_kv(enc) -> (K, V)
One-shot encoder K/V projection for cross-attn. Reused every step.
CrossAttention::forward_step(xs, k, v)
Cached cross-attn. Q computed from new token; K/V from precompute.
DecoderCache { self_k: Vec<Option<Tensor>>, self_v, cross_k, cross_v, position }
Decoder::precompute_cross_kv(enc) -> DecoderCache
Decoder::step(token_id, &mut cache) -> logits (1, vocab)
Decoder::generate_cached(enc, cfg, max_tokens) -> Vec<u32>
Greedy loop using the cached step.
Profile (5 steady-state runs on /tmp/asr_test.flac, 10.42 s LibriSpeech):
warm-up: 344 ms
steady-state mean: 307 ms (p50 305, range 298-319)
realtime factor: 0.0294x
Comparison across all STT in rtx-csm:
Backend RTF Notes
Kyutai STT 1B 1.01x hardware-bound, 3 GB
Whisper-tiny 0.020x breaks CSM (in-process ggml conflict)
Moonshine-tiny 0.0294x pure candle, NO runtime conflict
Moonshine is the only fast STT path that integrates cleanly. ~34x
faster than realtime, ~17x faster than Kyutai 1B, no protobuf or
ggml linkage issues.
New `examples/moonshine_profile` mirrors `stt_profile` and
`whisper_profile` so all three STT backends report comparable numbers.
Phase 8.10 (next): wire as a third AsrEngine variant in converse_server
for English-only deploys. Replace the energy-VAD-gated Kyutai path
when --moonshine flag is set.
Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>
