# Z840 Power Consumption Study

A before/after power study documenting the Z840's evolution from a dual-socket Xeon workstation into a dedicated local AI inference box. Measured and documented 2026-05-18 for posterity — this was the early days of running local AI at home on recycled enterprise iron, before it became normal.

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## The Machine

HP Z840 Workstation. Bought second-hand. Repurposed as a local AI inference box running Ollama, Open WebUI, and Hermes monitoring. 128GB ECC DDR4 — retained because RAM prices made starting fresh impractical.

See z840 for full hardware spec and service documentation.

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## Phase 1 — Baseline (2026-05-18, pre-upgrade)

CPUs installed: 2× Intel Xeon E5-2698 v3 @ 2.30GHz - 16 cores / 32 threads per socket - TDP: 135W each — 270W combined TDP - Architecture: Haswell-EP (v3), LGA 2011-3

GPU installed: NVIDIA Quadro M4000 (original workstation card, placeholder) - 8GB GDDR5 - Not suitable for modern AI inference

RAM: 128GB DDR4 ECC Registered @ 2133 MT/s (8× 16GB DIMMs across both sockets)

### Measured idle power draw (RAPL, system at rest, load avg ~0.00)

Note: RAPL measures CPU package power only. Wall draw will be higher due to
motherboard, storage, fans, and PSU inefficiency. Estimated wall idle: 150–180W

### AI inference capability at this stage

- Ollama running but limited by Quadro M4000 (8GB VRAM, no modern tensor cores) - Models constrained to ~7B parameter quantised - No tensor parallelism across cards

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## Phase 2 — CPU Upgrade (arriving 2026-05-19)

CPUs incoming: 2× Intel Xeon E5-2620 v4 @ 2.10GHz - 8 cores / 16 threads per socket - TDP: 85W each — 170W combined TDP (vs 270W before — saving 100W TDP) - Architecture: Broadwell-EP (v4), LGA 2011-3 - Cost: £24 for the pair (eBay)

Why this makes sense: The GPU does the AI heavy lifting. CPU cores are largely idle during inference — they're just feeding data and managing I/O. Dropping from 16-core to 8-core per socket costs nothing in real-world AI performance.

### Expected power improvement

Measured figures to be recorded after swap — see Phase 2 update below.

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## Phase 3 — GPU Upgrade (cards arriving, date TBC)

GPUs incoming: 2× NVIDIA GeForce RTX 3060 - 12GB GDDR6 each — 24GB total VRAM across both cards - TDP: 170W each — 340W combined at full load - Architecture: Ampere — proper tensor cores, CUDA compute 8.6 - Cost: TBC

### AI capability step-change

### Power at full AI load (estimated)

Z840 PSU is 1125W — plenty of headroom.

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## Phase 4 — Power Mitigation Scripts (planned)

Once GPUs are installed, a set of scripts and Hermes integrations will manage power dynamically to reduce idle draw:

### GPU power capping (nvidia-smi)

At idle, cap each 3060 to ~50W. On job start, restore full power limit:

```bash # /usr/local/bin/gpu-power-idle.sh nvidia-smi -i 0 -pl 50 nvidia-smi -i 1 -pl 50

# /usr/local/bin/gpu-power-full.sh nvidia-smi -i 0 -pl 170 nvidia-smi -i 1 -pl 170 ```

### CPU core parking (single socket at idle)

When no heavy job is queued, offline all cores on socket 1:

```bash # /usr/local/bin/cpu-park-socket1.sh for cpu in $(cat /sys/devices/system/node/node1/cpulist | tr ',' ' '); do

echo 0 > /sys/devices/system/cpu/cpu${cpu}/online

done

# /usr/local/bin/cpu-unpark-socket1.sh for cpu in $(cat /sys/devices/system/node/node1/cpulist | tr ',' ' '); do

echo 1 > /sys/devices/system/cpu/cpu${cpu}/online

done ```

### Hermes integration (planned)

- Add GPU wattage to Hermes Z840 stats endpoint (`nvidia-smi –query-gpu=power.draw`) - Alert if combined GPU draw exceeds 400W unexpectedly - Dashboard panel showing real-time power consumption - Auto-cap GPUs after 30 minutes of idle inference

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## Phase 2 Results — POST CPU SWAP (measured 2026-05-19, ~2 hours post-install, 34°C ambient)

``` CPU socket 0 (E5-2620 v4): 15.2W CPU socket 1 (E5-2620 v4): 14.0W DRAM socket 0: 1.9W DRAM socket 1: 0.9W GPU (Quadro M4000): 42.3W Total measured (RAPL + nvidia-smi): ~74W Estimated wall draw: ~130–150W Saving vs Phase 1 (CPU package only): ~12W ```

RAPL sampled at idle (load avg ~0.00), 5-second window per domain.
DRAM figures slightly higher than Phase 1 sample — within normal variance.
CPU package power: 41.1W → 29.2W combined (-29% reduction).

## Phase 3 Results — POST GPU SWAP (to be filled in after cards arrive)

``` CPU socket 0 (E5-2620 v4): _W CPU socket 1 (E5-2620 v4): _W 2× RTX 3060 at idle: _W Total measured idle: _W Estimated wall idle: ___W

2× RTX 3060 at full inference load: _W Total measured load: _W Estimated wall load: _W Saving vs Phase 1 idle: _W Monthly cost at UK avg (28p/kWh, idle 18h/day + load 6h/day): £___ ```

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## Cost Context (UK electricity, May 2026)

UK average electricity price: ~28p/kWh

Moral: the CPUs are not the enemy. The GPUs at full load are. Smart scheduling
and power capping matter more than the CPU swap — but every watt counts.