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Review: AMD Radeon HD 7950
by Damien Triolet
Published on March 6, 2012

Overclocking and undervolting
Not all Tahiti GPUs are the same and not all samples with certain specifications have an identical profile, a result of the manufacturing variability mentioned above. Thus, certain GPUs need higher voltages to maintain the desired clock, while others, with big current leakages, have to be run at relatively low voltage to remain within the thermal envelope.

In order to equal things out, AMD hasn’t attempted to reduce energy consumption beyond this thermal envelope. In other words if a Tahiti GPU is given as being clocked at 925 MHz within a TDP of 250W with a high voltage, AMD will not look to find out if the GPU could in reality be validated with a lower voltage and therefore consume less power. This sort of stringent selection process would only make any sense on a product such as the Radeon HD 7990 or a mobile version of the Radeon HD 7900s, which however is unlikely to appear.

There are several issues to be raised here: on one hand, the overclocking potential will be significant on many samples, though exceeding the given TDP by a little or a lot depending on the sample, and on the other it will be possible to undervolt some samples, reducing their voltage to reduce power consumption.

Don’t put too much faith in the GPU quality rating from GPU-Z as it means nothing clear here. As AMD obviously has no ‘quality’ parameter, this is simply a rapid interpretation made of the value of a register by the authors of GPU-Z. In practice this value is more of a measure of the current leakage during validation but doesn’t measure the quality of a GPU, which is moreover a subjective notion.

The voltage on a Tahiti GPU can vary on the Radeon HD 7970 as much as on the Radeon HD 7950. We have seen samples of Radeon HD 7970s at 1.174V and samples of Radeon HD 7950s at 1.013V and 1.093V.

The Radeon HD 7950 supplied by AMD was clocked at 800 MHz for 1.093V. We managed to clock it up to 1050 MHz without changing the voltage, to 1125 at 1.150V, to 1150 MHz at 1.200V and 1175 MHz at 1.225V. We managed to clock the memory up to 1525 MHz. Beyond this the system crashed.

We also managed to reduce the voltage of our sample from 1.093V to 0.900V at its original clocks, which allowed us to reduce energy consumption significantly as we’ll see later in this report.

Note that we’re talking about the voltages requested at the power stage, which manages to maintain them during high load to a greater or lesser extent, depending on its quality,. In practice they vary and are on average lower by between 0.030 and 0.080V, according to the GPU-Z monitoring.

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