The Coming War: ARM versus x86

Power consumption
While the x86 microprocessors in this comparison enjoy a clear overall performance advantage, ARM CPUs are renowned for their power usage thriftiness. It is very difficult to compare power usage among the four CPUs under test for this report. The AMD and VIA systems are inappropriate for power comparisons because they are based on desktop hardware.

The chart below contrasts power consumption between the Intel Atom N450 and the ARM Cortex-A8 while running miniBench. The power curves were generated from system power usage adjusted downwards so that idle system power was discarded. For the Atom, idle power was 13.7W with the Gateway netbook’s integrated panel disabled while the idle power for the Pegatron system was only 5.4W.

Be aware that the Pegatron prototype does not implement many power management features. ARM representatives note:

The Pegatron development board was designed as a software development tool and does not have a commercial production software build so it does not have many of the power management features found in ARM-based mobile devices. Production systems would expect to have aggressive power management implemented, lowering the ARM power consumption.

Given this information, the results we show here likely represent an energy consumption condition considerably worse than would be encountered with a similarly configured, commercial, ARM Cortex-A8-based system. Subtracting idle power usage should isolate the curves to the power necessary for running miniBench. Note that the Atom reached minimum power usage shortly after startup and never reached that level again. Idle power beyond that point is about 1 Watt higher. Even taking that into consideration, the Atom consumes at least three times the power of the ARM Cortex A8 on the same tests.



It’s particularly interesting to see how power usage compares on the AES tests where both CPUs deliver comparable performance. The first major hump on the Atom curve shows the power consumed on the AES tests. Compared with the ARM Cortex-A8, the Intel Atom N450 required about four times more power while delivering only about 30 percent additional performance - and this is with a 25 percent clock speed advantage.

The sharp peak in Atom power usage occurred on the miniBench floating-point memory bandwidth tests.
The Atom completes miniBench in about one-half the time needed by the ARM Cortex-A8 due to the ARM processor’s very poor floating-point performance. The first major dips in both curves [at 1000s and 2000s] indicate where the two systems complete the benchmark. 

Even though floating-point hardware can draw a lot of power, FP units usually deliver significant energy savings because floating-point operations take much less time to complete with accelerated hardware support. Energy consumed for a task is: E = P * t, where "E" is for "Energy," "P" is for "Power" and "t" is for "time." Good floating point hardware might drive up power demands, but the time to complete FP operations is reduced enough to dramatically reduce the total energy needed for those operations.

Despite the fact that the ARM Cortex-A8 blows away the Intel Atom in power thriftiness, don’t belittle the Atom. It is a resounding success in terms of reducing the power demands of x86 microprocessors. The Intel Atom is currently the only realistic x86 system-on-chip [SoC] design ready to migrate downwards into smartphones.
Doubtlessly inspired by the VIA C7 - which explains why Intel set up shop in Austin, the same town where VIA’s Centaur design team is headquartered  [in fact, a few ex-"Centaurians" worked on the Atom]  - the Intel Atom delivers acceptable performance while sipping power at levels far lower than usually seen in the x86 world. Right now, there is no competing, low-power x86 CPU - let alone SoC - that can match the Atom in terms of performance per Watt, especially on multithreaded applications.