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Curbing Your GPU's Power Use: Is It Worthwhile?

The First Experiment: AMD’s Radeon HD 5870

The First Experiment: AMD’s Radeon HD 5870

First, we tried our luck with a Radeon HD 5870. However, we quickly noticed something odd. When we changed clocks, the operating voltage also changed. The resulting setting (1.125 V) is actually higher than the default voltage at the card’s default clock speed (1.063 V). In addition, idle clock speeds also changed. Power measurements show the card using more power at idle with these changes.

At first we blamed the utility we were using. So, we tried several different utilities, such as the Overdrive tab in AMD’s Catalyst Control Center, the company’s own GPU Clock Tool, and even third-party utilities like ATI Tool and Afterburner. No dice.

So, we decided to tackle this the hard way: by modifying the card’s BIOS to force our clock settings. To limit the chances of failure, leaving us with a non-functional card, we only changed clock settings for UVD mode. One thing we noticed while testing the Radeon HD 5870 was that its core and memory clocks stay constant when UVD is active. This behavior is applicable to most AMD graphics cards. Now we have a makeshift intermediary P-state for GPUs.

We need to remind you that running your graphics card outside of the vendor's specification is considered overclocking, thus voiding your warranty. Overclocking can cause crashes, system instability, data loss, and even permanent hardware damages. If you choose to do so, continue at your own risk.

Performing The Operation

Before we begin, there are some preparations in order. Although we’re making limited modifications to the UVD clock settings, there's still a chance something could go wrong. Since we don't have any PCI graphics cards to fall back on in case of a bad flash, we have to rely on the AMD 790GX’s integrated graphics engine. This turned out to be a good choice, since the flash tools weren't able to flash its BIOS.

We extracted the Radeon HD 5870’s BIOS with GPU-Z and loaded it into Radeon BIOS Editor (RBE).

The default clocks and voltages on our Radeon HD 5870 sample

A look at the clock settings in RBE revealed some interesting facts. There are two options listed in the voltage drop-down list: 0.95 V and 1.063 V. This means we can try to undervolt the card as well.

Changes were limited to just DXVA clocks and voltages

We ran several tests at the default clocks and voltage (400/900 MHz @ 1.063 V), along with two undervolted settings (400/900 and 600/900 MHz @ 0.95 V). For reference purposes, we also ran the tests at full clocks and voltages (925/1225 MHz @ 1.063 V), then added results from a Radeon HD 5770 and HD 5670. Both mainstream cards were running at their default clocks, too.

We chose 600 MHz because power consumption numbers measured at this setting are similar to default UVD clock and voltage. Memory clock settings were left untouched, because higher frequencies require more voltage and lower clocks may introduce screen flickering.