`ctl bench` has always been able to work out what one replica can serve — it
ramps concurrency until the SLO breaks and reports the last level that held.
It printed that and nothing kept it, so the one number saying what a replica is
worth lived in whoever ran it's scrollback. Everything downstream then had to
guess: admission applies a single global default for healthy queue depth, and
sizing a model against real demand had nothing to size against.
`ctl bench --save` records it; `ctl capacity` reads it back.
Keyed by model and profile. Capacity belongs to the hardware and the engine
arguments as much as to the weights, so the same model on CPU and on an H100
are two numbers, and one key would overwrite the other on every run.
The profile is looked up from the replica rather than asked for, because the
control plane already knows which profile answers on which endpoint and a
measurement filed under the wrong one is worse than none. A target that is not
a replica — the gateway, usually — is refused with that said: benchmarking the
whole path is worth doing and says nothing about the hardware.
The conclusion is stored, not the ramp. Every level and its latencies are worth
reading once while judging whether the run was sound; keeping them would grow
without bound as a model is re-benchmarked.
Re-measuring replaces. A second run is a better answer to the same question,
not another data point, and keeping both would leave a reader choosing between
numbers with no basis for the choice.
The SLO and the date are stored with it. Capacity means nothing without the
target it was measured against, and hardware and engine builds change — a
measurement is an observation with a date on it, not a fact about the model.
A failed save is reported and not fatal: the run has already cost the GPU time.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
`ctl bench` measures what a replica can actually serve, so weight,
target_waiting and kv_ceiling stop being numbers somebody typed. Concurrency
climbs until the SLO breaks and the last level that met it is the capacity:
peak throughput arrives long after latency has become unusable, so throughput
alone overstates what the hardware can serve. Queue depth and KV usage are
read from the engine while it is still meeting the target, which is exactly
what admission should treat as healthy.
Prompts are unique per request by default. With one repeated prompt the
engine's cache answers most of the prefill and the measurement describes the
cache rather than the hardware; --shared-prefix asks that question
deliberately, and it now produces byte-identical prompts, which the first
version did not — it dropped the random header and still picked words at
random, so the cache missed and the flag quietly measured the thing it exists
to avoid.
Running it against the cluster turned up two real defects:
A replica stuck in Starting never timed out. Every other way out of that state
needs the agent to report something, and when it never does the replica sits
there holding GPUs and counting towards the model's replica count while
serving nothing. Two of them had been like that for eight minutes past a
two-minute startup timeout, and nothing would ever have moved them.
They were stuck because adoption lost to a replica we had only placed. Replica
ids come from the model name and a counter, so a control plane that restarts
and places before its agents reconnect builds a different replica under the
same id on a freshly allocated port; the agent's report of the engine that was
actually serving was then discarded as already-known. The cluster pointed at
port 20004 while the engine answered on 20000. The fleet is authoritative
about what is running, so what we merely placed now loses to what an agent
reports — unless our copy has a shard that has actually come up, in which case
two things really are running and the stray is the one we did not ask for.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
