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Power Consumption

Our tests suggest that the Kaveri isn’t a large improvement over the Richland in terms of performance. But it is expected to be better in terms of heat dissipation and power consumption. AMD specifies a lower TDP for it. Then, the Kaveri is manufactured on a more advanced tech process. And the clock rates of the new A10-class APUs are lower than those of their predecessors. So we hope that the new APUs will be competitive in terms of energy efficiency. Let’s check this out.

The graphs below (unless specified otherwise) show the full power draw of the computer (without the monitor) from the wall socket. It is the total power consumption of all system components. The PSU's efficiency is taken into account but our Corsair AX760i is a highly efficient 80 PLUS Platinum product, so its effect on the result is very small. The x86 cores are loaded by running the 64-bit version of LinX 0.6.5 utility with support for AVX, AVX2 and FMA instructions. The graphics core is loaded by running Furmark 1.13.0. Moreover, we enable Turbo technology and all power-saving technologies to correctly measure the computer's power draw: Intel’s C1E, C6, Enhanced Intel SpeedStep and AMD’s Cool’n’Quiet.

Modern processors don’t consume much power when idle, so the numbers in the diagram are indicative of the power consumption of the whole platform rather than of the APUs. And we don’t see much difference between the LGA1150, Socket FM2 and Socket FM2+ platforms. All of them are economical at zero load.

When the processor has some work to do, we see a typical picture: AMD's APUs need more power than their Intel opponents but deliver lower performance. In other words, the Kaveri's x86 performance per watt is still inferior to the Haswell's. On the other hand, there’s some obvious progress as the A10-7850K consumes 11 watts less than the flagship Richland model.

We have the same picture at graphics load. The A10-7850K consumes more power than Intel’s Haswell-based products but less than its Richland predecessor. Lower power consumption rather than higher performance seems to have been the main goal for the Kaveri developers.

We get the most impressive picture when all of the APU's resources are loaded concurrently.

Here, the A10-7850K is more energy efficient not only than its predecessor but also than the quad-core i5-4430. Moreover, the senior quad-core Kaveri is close to the dual-core Haswell in terms of power draw.

Well, it turns out that the A10-7850K needs about the same amount of power when there’s high load on its x86 cores irrespective of whether the graphics core is loaded or not. How is it possible? The fact is the Kaveri's peak power draw is limited, so when all of the APU's resources are in use, the CPU and GPU clock rates are dropped down, and quite heavily so.

The CPU section is clocked at 3.0GHz whereas the graphics core frequency is periodically lowered from the default 720 MHz to 650 MHz. That’s why the peak power consumption of the Socket FM2+ platform with A10-7850K is limited to 116 watts in our testing.

The clock rate reduction helps keep the APU's appetite within specified limits but the peak heterogeneous performance suffers. It looks like AMD’s claims of the peak combined performance at 856 gigaflops are not true because the A10-7850K can’t work with its x86 and graphics cores all working at the default clock rates. The APU’s actual performance is about 760 gigaflops due to the clock rate drop.

This effect is going to be persistent because heterogeneous computing is about using all of an APU’s resources simultaneously.

 
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