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PhanSpeed auto-tuner — design

A closed-loop calibrator that drives a lever, stress-tests each setting, measures the result, and converges on the best stable operating point — so the machine tunes itself instead of you turning knobs by hand.

Objective: adaptive (AC vs battery)

The tuner finds and stores two operating points and phanspeed switches between them automatically on plug/unplug:

State Objective Converges at
AC max sustained performance the performance knee — lowest cap that still reaches the best clock under the thermal ceiling (extra watts past it buy no clock)
Battery best perf-per-watt the knee of the curve — max MHz delivered per watt

The loop (state machine)

IDLE ──start──▶ CALIBRATING ──converged──▶ PROBATION ──survives reboot──▶ COMMITTED
                   │  set lever → load → dwell → measure → judge → step
                   └─ instability / over-ceiling ─▶ revert to last-known-good
  1. Lever + search. Levers are 1-D and monotonic, so the search is simple:
    • Power (RAPL): ramp PL1 up until the package reaches the ceiling under load; settle just below (AC) or pick the MHz/W knee (battery). Safe — RAPL can only make the chip slow, never wrong. This is Tier 1, available today.
    • Undervolt: step the offset down until instability, back off with a safety margin. Gated — see Safety below.
  2. Load. A repeatable all-core burst per step, long enough to reach thermal steady state (--dwell, default 60 s).
  3. Measure. Per step: steady-state package temp, package watts, and achieved clock (the average of the last N samples, after the transient settles).
  4. Judge. AC: the performance knee — the lowest cap that still reaches (within 1% of) the best clock seen under the ceiling, so it's max performance at minimum heat/headroom rather than blindly the highest cap. Battery: maximize MHz/W. Undervolt adds a stability gate (below).

Coordination with the daemon

The calibrator does not stop the daemon — that would disarm the thermal failsafe during a stress run. Instead it sets power_limit_w = 0 over the socket (daemon stops managing CPU power) and drives the RAPL node directly. The daemon's emergency branch still overrides to base TDP if a sensor crosses emergency_temp, so the failsafe stays armed throughout calibration.

Safety (what makes auto-undervolt acceptable — Tier 2)

An undervolt search deliberately drives the CPU to the edge of instability, so two guardrails are mandatory before it ships:

  • Self-checking workload. An unstable undervolt produces silently wrong answers, not just crashes. The load must compute a known-answer workload (deterministic hash/matrix) and verify the result each step; a mismatch — or a WHEA/MCE in dmesg — counts as instability even with no crash.
  • Probation / commit + boot watchdog. A candidate is applied live only, never written as the boot default until it survives the full pass and a reboot confirm. The daemon writes an "attempting X" flag before applying at boot; if it finds the flag still set on next boot (i.e. the last boot crashed), it reverts to last-known-good and refuses X. Self-healing against the tuner's own search.

Status

  • Tier 1 — RAPL power auto-tune: implemented in bin/phanspeed-tune (--target ac|battery|both). Safe; no new daemon capability.
  • Tier 2 — undervolt auto-tune: blocked on (a) verifying undervolt works at all on this firmware (Plundervolt/SGX may swallow offsets) and (b) the self-checking + boot-watchdog harness above.