Power Draw: Test System And Methods
Test System and Power Draw Measurement
We partnered up with HAMEG Instruments (Rohde & Schwarz) to implement a state-of-the-art test system for precise, short interval power and performance measurements.
Only modern lab instruments can keep up with the challenges that AMD’s Power Tune and Nvidia’s GPU Boost present (namely, huge swings in dynamic power consumption). We feed all relevant currents and voltages into a multi-channel 500 MHz oscilloscope (HAMEG HMO 3054), which can be remote-controlled and is able to retain the test data.
We measure the currents with three calibrated DC current clamp probes (HAMEG HZO50). Two of them, 3.3 and 12 V, take their readings at a custom-made riser card, which can reliably pass PCIe 3.0 signals, and one of them at a specially-modified PCIe power cable. All voltages are measured at the single-rail power supply, which we slightly modified to allow better access.
Our time resolution is now a mere 2 ms, which can measure and log all load transients incurred by AMD’s Power Tune and Nvidia’s GPU Boost. In order to keep the volume of data manageable, we limit the duration of a test run to two minutes.
| Test Method | No contact current measurement at the PCIe slot (Riser card) No contact current measurement at the external PCIe power cable Voltage measurement at the PSU |
|---|---|
| Test Equipment | 1 x HAMEG HMO 3054, 500 MHz four-channel oscilloscope 3 x HAMEG HZO50 current probe (1 mA - 30 A, 100 kHz, DC) 4 x HAMEG HZ355 (10:1 probe, 500 MHz) 1 x HAMEG HMC 8012 DSO |
| Test Bench | Microcool Banchetto 101 |
| Power Supply | Corsair AX860i with slightly modified connectors |
What Happens Within 100 ms?
A lot! We log the power draw with three probes at a 2 ms interval and pick a representative 100 ms window. Then we plot the resulting 50 data points in a graph.
Looking at the graph, you almost have to feel sorry for the power supply. Power draw through the PCIe power cables jumps from 140 to 352 W within a few milliseconds. You can't expect just any old generic PSU to cope with that. The two test points at the PCIe riser cards do not exhibit such drastic load changes.
We like that neither AMD nor Nvidia max out the PCIe slot connector's output rating, which is 75 W. Those auxiliary power cables bear the brunt of the load. Nor are there drastic load transients on the motherboard connector. All of this helps ensure system stability, benefiting multi-GPU setups in particular.
Now let's take a look at power consumption in real-world workloads.
