A prefill chunk that shares a step with decoding sequences holds the device for
the chunk's whole time: measured on this shape, decode-only steps 15.5 ms
against chunk steps 166 ms, and 4.7 % of the steps carry 34 % of the engine's
step time. Harness-visible decode falls 166 -> 13 tok/s (per stream 6.7 = one
token per shared step) once concurrent prefill passes 5k tok/s, and TTFT p90 for
an arriving request goes 2.4 -> 18.0 s.
BUSY_PREFILL_CHUNK stays `auto`: it is a prefill-speed dial, and the source
refuses to halve the chunk for decode rows alone -- which is why
engine_steps_held_total read zero for the whole 34.7 h boot. PREFILL_INTERLEAVE
is the knob that acts on the measured mechanism: N decode-only steps follow any
chunk step taken beside decoders. N=1 is the conservative first arm -- a stream
takes its token from the chunk step and one from the held step (~2 tokens per
182 ms instead of one per 166 ms); the cost lands on the prefilling prompt's
TTFT and on requests still queued, not on anything decoding.
Rollback: PREFILL_INTERLEAVE=0. Verification: the flag must reach argv (the
serve script only passes it when > 0), engine_steps_held_total must leave zero,
then the §9 bucket pair must move before N rises.
Frank
aa75b45543