Digital Twin

The digital twin is Joulie’s core predictive engine. It is a lightweight O(1) parametric model that predicts the impact of scheduling and power-cap decisions on node thermal and power state, without running a full simulation for each scheduling decision.

What the digital twin computes

For each managed node, the twin produces three scores stored in NodeTwin.status:

The twin also computes:

• SchedulableClass: performance, eco, or draining (transition state). The scheduler extender uses this to filter and score nodes.
• EstimatedPUE: derived from cooling stress, between 1.05 and 1.40. Published for observability; not read by the scheduler today.
• PowerMeasurement: a block of measured and derived power values consumed directly by the scheduler for projected headroom scoring.

Every power-derived score starts from one number: measuredNodePowerW. The twin is not a cap-based estimator. It reads what the node is drawing and compares it against budgets it derives from hardware.

PowerMeasurement output

The powerMeasurement block in NodeTwin.status provides the scheduler with the real-time power data it needs for projected headroom scoring:

The scheduler uses measuredNodePowerW + pod marginal power to compute projected headroom relative to nodeCappedPowerW. The powerTrendWPerMin feeds the trend bonus, capped at 25 points in either direction. See Scheduler Extender for details.

Where the power reading comes from

The controller manager resolves measuredNodePowerW before calling the twin. NODE_POWER_SOURCE selects the source:

Because the default is static, an unconfigured cluster reports 0 watts for every node: headroom reads 100 and cooling stress reads 0 everywhere. Configure a source before treating the scores as a ranking signal.

kepler and utilization are reserved source values; no component writes them yet. When a reading exceeds twice the node’s TDP the controller manager logs a one-shot warning, because that is the signature of a cumulative joules counter being read as watts.

Node TDP and the capped budget

Both power formulas need the node’s hardware limits, derived from NodeHardware.status:

cpuTdp  = cpuMaxWattsPerSocket * sockets
gpuTdp  = gpuMaxWatts * gpuCount          (0 when no GPU is present)
nodeTdp = cpuTdp + gpuTdp

cpuCappedPower  = cpuTdp * cpuCapPct/100
gpuCappedPower  = gpuTdp * gpuCapPct/100
nodeCappedPower = cpuCappedPower + gpuCappedPower

cpuCapPct and gpuCapPct come from the resolved NodeTwin.spec intent, defaulting to 100 when unset or non-positive.

When the socket count is unknown but a per-socket maximum is known, the twin assumes a single socket. A zero socket count would multiply the per-socket maximum down to zero, and a zero budget reads as unlimited headroom further down, so a busy node would look like the emptiest one in the cluster. Assuming one socket underestimates the budget on a multi-socket machine, which is the safe direction: the node looks fuller than it is rather than infinitely free.

Reference node used in the examples below

A node with two AMD EPYC 9654 sockets (360 W per socket) and four NVIDIA H100 NVL GPUs (400 W each), running eco at a 60% cap on both CPU and GPU:

• cpuTdpW = 360 * 2 = 720 W
• gpuTdpW = 400 * 4 = 1600 W
• nodeTdpW = 2320 W
• cpuCappedPowerW = 720 * 0.6 = 432 W
• gpuCappedPowerW = 1600 * 0.6 = 960 W
• nodeCappedPowerW = 1392 W

CoolingStress formula

CoolingStress is a per-node metric. It answers: “how close is this node to its cooling limit?”

tempMultiplier = 1 / max(0.5, 1 - (ambientTempC - 20) * 0.02)
coolingStress  = clamp((measuredPower / nodeTdp) * tempMultiplier * 100, 0, 100)

Stress is normalized against uncapped TDP, not against the capped budget, because the cooling system is sized for the hardware that is installed, not for the cap currently applied to it. Capping a node lowers its measured power and so lowers its cooling stress, which is the intended behaviour.

If nodeTdp is 0 (no hardware information) the score is 0: the twin emits no stress signal rather than a fabricated one. If the ambient temperature is 0 or below it is treated as unknown and the multiplier is 1.0. Facility metrics are off by default (ENABLE_FACILITY_METRICS=false), so an unknown ambient is the common case, and a genuine sub-zero ambient reads the same as no reading at all.

The multiplier at a few ambient temperatures:

Example: the reference node drawing a measured 1160 W at 25C ambient:

• measuredPower / nodeTdp = 1160 / 2320 = 0.5
• tempMultiplier = 1 / max(0.5, 1 - (25 - 20) * 0.02) = 1 / 0.9 = 1.111
• coolingStress = 0.5 * 1.111 * 100 = 55.6

The same node with an unknown ambient temperature scores 0.5 * 1.0 * 100 = 50.0.

Why this model

It is an algebraic proxy. It avoids CFD or thermal RC simulation and runs in O(1) per node. It is deliberately conservative (it overestimates stress relative to real cooling capacity) because its job is to provide a ranking signal for the scheduler, not an exact thermal prediction.

Power headroom

Power headroom is the fraction of the node’s capped power budget that is still unused:

headroom = min(100, (nodeCappedPower - measuredPower) / nodeCappedPower * 100)

It is clamped at 100 at the top but not at the bottom: a node drawing more than its budget scores negative, which ranks it below an idle node instead of tying with one. If nodeCappedPower is 0 (unknown hardware) the score is 100, a neutral value that avoids penalizing nodes the twin knows nothing about.

Example: the reference node (1392 W capped budget) drawing a measured 1160 W:

• headroom = (1392 - 1160) / 1392 * 100 = 16.7

Example: the same node drawing 1600 W, over its eco budget:

• headroom = (1392 - 1600) / 1392 * 100 = -14.9

The scheduler prefers its own projected headroom, computed from powerMeasurement plus the candidate pod’s marginal watts, and falls back to this predictedPowerHeadroomScore only for a node with no powerMeasurement block.

PSUStress formula

PSUStress answers: “how close is this rack to its power supply limit?”

rackPower = rackTotalPowerW, or clusterTotalPowerW when no rack total is known
psuStress = clamp(rackPower / 50000 * 100, 0, 100)

When neither power figure is available the score is 0. Both come from the facility metrics poller, which is disabled by default, so PSU stress is 0 on a default install.

PSUStress is per-rack when rack topology is available and cluster-wide otherwise. In the cluster-wide case every node reports the same score, which is intentional: a PDU brownout affects every node behind it, not just the one drawing the most power.

The scheduler does not read this score today. It is published for observability and reserved for rack-topology-aware placement.

Example: a rack drawing 30 kW:

• psuStress = 30000 / 50000 * 100 = 60

Estimated PUE

estimatedPUE = 1.05 + (coolingStress / 100) * 0.35

A node at zero cooling stress reports 1.05 (best-case datacenter overhead) and one at full stress reports 1.40 (stressed cooling, high overhead). It is a restatement of cooling stress on a PUE scale, not an independent measurement, and the scheduler does not read it.

Example: the reference node at cooling stress 55.6 reports 1.05 + 0.556 * 0.35 = 1.24.

How it feeds the scheduler

The twin controller runs in the controller manager on each reconcile tick (~1 minute) and writes NodeTwin.status per managed node. The scheduler extender caches these NodeTwin CRs with a 30-second TTL and uses them in its filter and score logic:

twin controller (controller manager)
  → writes NodeTwin.status
    → scheduler extender cache (30s TTL)
      → filter: rejects eco and draining nodes for performance pods
      → score: headroomScore*0.7 + (100-coolingStress)*0.15 + trendBonus + profileBonus + pressureRelief

Of the twin’s outputs, the scheduler reads schedulableClass, predictedCoolingStressScore, the whole powerMeasurement block, and predictedPowerHeadroomScore as a fallback. predictedPsuStressScore and estimatedPUE are not part of the score.

This keeps scheduling decisions lightweight (one cache lookup per node per scheduling attempt) while reflecting the latest thermal and power state of the cluster.

How it feeds the controller manager

The twin also drives controller manager decisions:

• Transition guard: when a node is transitioning from performance to eco, the twin sets schedulableClass to draining until all performance pods have completed or been drained.

Implementation

The twin is implemented in pkg/controller/twin/twin.go. Key types and functions:

• Input: everything needed to compute twin state for one node (hardware, profile, cap percentages, measured power, power trend, facility and topology signals).
• Output: the computed NodeTwin.status fields.
• Compute(Input) Output: the main computation function. Derives TDP and capped budgets, then calls the helpers below.
• computePowerHeadroom, computeCoolingStress, computePSUStress: the three score formulas.

There is no cooling model interface and no alternative cooling implementation. Cooling stress is the single closed-form expression above; replacing it with a higher-fidelity thermal model means changing computeCoolingStress.

What to read next

1. Scheduler Extender
2. Joulie Controller Manager
3. CRD and Policy Model
4. Hardware Modeling: reference power profiles used by NodeHardware and the twin’s TDP derivation