Undervolting & Underclocking

By Barry Pangrle
Last month we looked at record-breaking clock frequencies accompanied by voltage levels over 2V for some high-speed x86 processors. This month we’re going to go in the opposite direction—reducing the voltage and clock frequency to reduce power.

Our processor of choice is the AMD A8-3850, a 100W, 2.9 GHz, quad-core, x86 processor that also incorporates 400 “Radeon cores” (Radeon HD 6550D Graphics) on the same piece of silicon. AMD also makes a 2.4 GHz (2.7 GHz Turbo Core) A8-3800 that’s rated at 65W but has been very hard to obtain. The planned application is a Home Theatre Personal Computer (HTPC). Since this is a box that will be used to record broadcasts, it will be on 24×7, and it’s imperative that it be rock stable.

For my local power service provider, it costs me about $1.58 a year for 1W that is on 24x7x52. So, for example, a 35W reduction would translate into approximately a $55/year savings.

Figure 1. AMD A8-3850 Processor In Box

The components used in this system were the following:
• AMD A8-3850 Llano 2.9GHz Socket FM1 100W Quad-Core Desktop APU with DirectX 11 Graphic AMD Radeon HD 6550D AD3850WNGXBOX
• GIGABYTE GA-A75M-UD2H FM1 AMD A75 (Hudson D3) SATA 6Gb/s USB 3.0 HDMI Micro ATX AMD Motherboard
• CORSAIR Vengeance 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1866 (PC3 15000) Desktop Memory Model CMZ8GX3M2A1866C9B
• SeaSonic X series SS-400FL Active PFC F3 400W ATX12V Fanless 80 PLUS GOLD Certified Modular Active PFC Power Supply
• (2 x) HITACHI Deskstar 5K3000 HDS5C3020ALA632 (0F12117) 2TB SATA 6.0Gb/s 3.5″ Internal Hard Drive
• SONY Black Blu-ray Burner SATA BD-5300S-0B – OEM
• Ceton InfiniTV 4 Quad-tuner Card for Watching Digital Cable TV on the PC, PCI-Express x1 Interface
• LIAN LI Black Aluminum PC-C39 Micro ATX Media Center / HTPC Case

Power measurements were made with a P3 International P4460 Kill A Watt EZ Electricity Usage Monitor.

Marcus Pollice wrote an article for Brightsideofnews [1] and used Prime95 and MSI Kombustor to stress test his system for stability and max power usage. I’ve used the same two programs for testing here. Probably the trickiest part of this setup was finding the necessary DirectX libraries to run Kombustor and you can find those here. Marcus reported that he was able to get a 32% reduction in power by lowering the voltage from 1.4 V to 1.136 V and under stress testing the peak power dropped from 209 W to 142 W. That’s a whopping 67 W reduction in power at load.

Figure 2. “Husky” 32 nm x86 Core [2]

Figure 3. Llano Die Photo with Annotations [3]

Figures 2 and 3 show photos for the x86 “Husky” core and the Llano die, respectively. The GPU portion runs on its own separate voltage plane. Using the IFFT stress test to load all 4 cores simultaneously, I was only able to reduce the voltage to 1.225 V from the 1.4 V stock voltage. Reducing the voltage further to 1.2 V caused errors to occur during the Prime95 test.

Figure 4. 32 nm Cores: Frequency vs Voltage [4]

Figure 4 shows the relationship between frequency and voltage for the Bulldozer core and a “legacy” 32nm core (presumably our “Husky” core for Llano). It’s clear that reducing the clock frequency should enable a lower stable voltage. In fact, by reducing the clock frequency to 2.4 GHz it was possible to get a stable system at 1.2 V. Another really nice benefit was that under Prime95 testing the reported CPU temperature had now dropped from ~ 74°C to ~51°C. This also meant that the stock heat-sink fan didn’t have to spin at its max RPM, keeping the system quieter.

So, what was the impact on the final power? At the wall plug, the system routinely idles around 47W with CPU temperatures in the 22 to 23 degrees Celsius range. Just running the Prime95 stress testing, the power increases to about 105W, and adding the Kombustor GPU stress to the mix pushes the power up to about 125W max. So, while I didn’t get the voltage reduction reported by Pollice, the overall power is lower (125W vs. 142W). There are probably numerous reasons for this and perhaps primary is a more efficient power supply unit. If the PSU is 8% more efficient, that would be good for about 10W. The approximately 20% lower clock frequency should also help to keep the power lower. On the other hand, this system does have two hard drives, a cable tuner card and another 4 GB of RAM that is not part of the Brightside system.

The GPU ran at the stock 1.15 V for the graphics logic. Max GPU temperature was reported to be only about 40°C. The reported system temperature was 48°C and didn’t seem to vary much whether the system was idling or being stressed.

Figure 5 shows the HTPC case with all of the components mounted. The Ceton card is hidden underneath the center support for the case.

I suspect that we’ll see more of these types of systems being discussed and Semiaccurate [5] has even recently reported about tweaking software for AMD’s Brazos platform here.

Figure 5. Components Mounted Inside of HTPC Case

References:

[1] Marcus Pollice, AMD APU Undervolting: Reduce Power Consumption by 32%!, Bright Side of News, 7/6/2011.

[2] Jotwani, R.; Sundaram, S.; Kosonocky, S.; Schaefer, A.; Andrade, V.F.; Novak, A.; Naffziger, S.; An x86-64 Core in 32 nm SOI CMOS, Solid-State Circuits, IEEE Journal of, Volume: 46 , Issue: 1, 2011, Page(s): 162 – 172

[3] Denis Foley, Maurice Steinman, Alex Branover, Antonio Asaro, Ljubisa Bajic, Swamy Punyamurtula and Greg Smaus, AMD’s Llano Fusion APU, Hot Chips 23, August 19, 2011.

[4] McIntyre, H.; Arekapudi, S.; Busta, E.; Fischer, T.; Golden, M.; Horiuchi, A.; Meneghini, T.; Naffziger, S.; Vinh, J.; Design of the Two-Core x86-64 AMD “Bulldozer” Module in 32 nm SOI CMOS, Solid-State Circuits, IEEE Journal of, Volume: 47 , Issue: 1, 2012 , Page(s): 164 – 176

[5] Thomas Ryan, Spotlight: BrazosTweaker, An Under-volting Tool for AMD APUs, SemiAccurate, Jan 9, 2012.

–Barry Pangrle is a solutions architect for low-power design and verification at Mentor Graphics.