sec/sec-reasoning crash-bounced twice in ten minutes, not once in thirteen days.
My earlier read of "rare, not chronic" came off a RestartCount of 1 and was
wrong; the operator pushed back and the second and third failures arrived while
that recommendation was still on screen. The memory entry making that call is
replaced rather than left standing.
Cause is unchanged from the first diagnosis: mog-sec at 0.52 plus gen at 0.38
reserve 0.90 of the card, leaving about 4.6 GiB, and vLLM's utilization figure
covers weights and the KV pool but not transient activation memory. A request
about 151,700 tokens deep scheduling a further 15,700-token chunk asked for 1.04
GiB with roughly 600 MB free.
Dropping utilization alone does not work, and fails in a worse way: a single
420,000-token sequence needs 17.88 GiB of KV, and at 0.50 the pool is 17.4 to
17.5 GiB, so vLLM refuses to start at all and the seat crash-loops during startup
instead of during a request. The context length and the crash were directly
coupled -- 420k was only reachable at the utilization that left no transient
headroom. So both moved: 0.50 and 393,216.
384k rather than vLLM's suggested maximum, deliberately. It estimated 406,352 on
one boot and 409,840 on the next, because the available-KV figure drifts about
0.1 GiB boot to boot; pinning the edge value fails to start on an unlucky boot.
393,216 sits 3% under the lower estimate and leaves roughly 0.7 GiB of the pool
unspent, which is the transient headroom the change exists to buy.
Verified after: KV 405,612 tokens, concurrency 1.03x at 393,216, and both sec and
sec-reasoning return 200 through the gateway.
num_speculative_tokens is documented as NOT the lever. The crash window logged
17.6% draft acceptance with positions 5 through 7 at 1.5 to 4.9 percent, which
reads as an obvious cut from 7 to 3; across 180 samples the median acceptance
length is 3.12 of 7 and median draft acceptance is 30.4%, so the crash window sat
near the minimum and cutting would cap the workloads accepting nearly the full
draft.
Cost: 384k of context instead of 420k, an 8.5% reduction on a seat whose crashes
were happening at 151k.
vh
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