PowerTune and ZeroCore PowerOf course Tahiti includes support for the PowerTune technology introduced with Cayman. To recap, this corresponds to some extent to Turbo on a CPU, allowing you to benefit from the available thermal envelope to a maximum. There is a small difference all the same. The GPU starts from the principle that unless its sensors are telling it something different, it can always function at its maximum clock. AMD therefore talks only about this core clock speed, though it isn't guarantee to be the actual clock in all cases.
Without a technology such as PowerTune, its graphics cards would be likely to see their clocks fall significantly, by up to 30% according to AMD. Without a similar technology, NVIDIA finds itself in the position of having to cobble together an approximate software alternative for its GeForce GTX 500s that consists in monitoring the energy consumption into the card.
PowerTune is based on a multitude of load sensors in the various GPU blocks, with readings then compared in an energy consumption correspondence table. PowerTune doesn’t measure to see if the GPU has passed its authorised limit but rather estimates as precisely as possible whether in the worst of cases it is going to do so. AMD fixes what this ‘worst of cases’ is: a GPU with a lot of leakage and which runs at a high temperature.
A GPU with maximum energy consumption fixed at 250W won’t be limited when it attains 250W but rather beforehand, when the worst of sample models would have reached 250W under the same conditions. This means that all graphics cards are limited in the same way and therefore will perform in the same way. Please note that the GPU clock is used as a parameter to estimate power consumption but GPU voltage isn't.
The Radeon HD 7970, like the Radeon HD 6970, is limited to 250W. However it has more sensors, which allows the GPU to estimate its energy consumption more precisely and thus avoid reducing its clock unless really necessary. You can still modify the limit by +/- 20% in the Overdrive control panel.
With Tahiti and the Radeon HD 7970, AMD has introduced a new technology: ZeroCore Power. This puts the GPU into long idle when it no longer has to power the display, typically when there's a blank screen. The graphics card then consumes less than 3W according to AMD, a value we can confirm following our tests (we measured it at just 1.8W). The fan is then also turned off.
ZeroCore Power comes in useful when a system isn’t being used constantly but needs to stay on. For example, an HTPC that is used for both gaming and file storage will benefit from the technology with a significant reduction in energy consumption. Another example could be with a supercomputer that is equipped with GPU. When the GPUs are not being used they can be switched to long idle.
ZeroCore Power will also be useful in multi-GPU systems as it goes much further than the Ultra Low Power State mode used on previous GPUs, which only reduce their consumption be a few Watts when they aren’t sollicited.