The ARM vs x86 Wars Have Begun: In-Depth Power Analysis of Atom, Krait & Cortex A15
by Anand Lal Shimpi on January 4, 2013 7:32 AM EST- Posted in
- Tablets
- Intel
- Samsung
- Arm
- Cortex A15
- Smartphones
- Mobile
- SoCs
Determining the TDP of Exynos 5 Dual
Throughout all of our Cortex A15 testing we kept bumping into that 4W ceiling with both the CPU and GPU - but we rarely saw both blocks use that much power at the same time. Intel actually tipped me off to this test to find out what happens if we try and force both the CPU and GPU to run at max performance at the same time. The graph below is divided into five distinct sections, denoted by colored bars above the sections. On this chart I have individual lines for GPU power consumption (green), CPU power consumption (blue) and total platform power consumption, including display, measured at the battery (red).
In the first section (yellow), we begin playing Modern Combat 3 - a GPU intensive first person shooter. GPU power consumption is just shy of 4W, while CPU power consumption remains below 1W. After about a minute of play we switch away from MC3 and you can see both CPU and GPU power consumption drop considerably. In the next section (orange), we fire up a multithreaded instance of CoreMark - a small CPU benchmark - and allow it to loop indefinitely. CPU power draw peaks at just over 4W, while GPU power consumption is understandably very low.
Next, while CoreMark is still running on both cores, we switch back to Modern Combat 3 (pink section of the graph). GPU voltage ramps way up, power consumption is around 4W, but note what happens to CPU power consumption. The CPU cores step down to a much lower voltage/frequency for the background task (~800MHz from 1.7GHz). Total SoC TDP jumps above 4W but the power controller quickly responds by reducing CPU voltage/frequency in order to keep things under control at ~4W. To confirm that CoreMark is still running, we then switch back to the benchmark (blue segment) and you see CPU performance ramps up as GPU performance winds down. Finally we switch back to MC3, combined CPU + GPU power is around 8W for a short period of time before the CPU is throttled.
Now this is a fairy contrived scenario, but it's necessary to understand the behavior of the Exynos 5250. The SoC is allowed to reach 8W, making that its max TDP by conventional definitions, but seems to strive for around 4W as its typical power under load. Why are these two numbers important? With Haswell, Intel has demonstrated interest (and ability) to deliver a part with an 8W TDP. In practice, Intel would need to deliver about half that to really fit into a device like the Nexus 10 but all of the sudden it seems a lot more feasible. Samsung hits 4W by throttling its CPU cores when both the CPU and GPU subsystems are being taxed, I wonder what an 8W Haswell would look like in a similar situation...
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A5 - Friday, January 4, 2013 - link
Even if you just look at the Sunspider (which draws nothing on the screen) power draw, it's pretty clear that the A15 draws more power. There have been a ton of OEMs complaining about A15's power draw, too.madmilk - Friday, January 4, 2013 - link
Since when did screen resolution matter for CPU power consumption on CPU benchmarks? Platform power might change, yes, but this doesn't invalidate many facts like Cortex-A15 using twice as much power on average compared to Krait, Atom or Cortex-A9.Wolfpup - Friday, January 4, 2013 - link
Good lord. Do you have some evidence for any of this? If neither Windows nor Android is the "right platform" for ARM, then...are you waiting for Blackberry benchmarks? That's a whole lot of spin you're doing, presumably to fit the data to your preconceived "ARM IS BETTER!" faith.Veteranv2 - Friday, January 4, 2013 - link
Hahaha, the Nexus 10 has almost 4 times the pixels of the Atom.And the conclusion is it draws more power in benchmarks? Of course, those pixels aren't going to fill itself. Way to make conclusion.
How big was that Intel PR cheque?
iwod - Saturday, January 5, 2013 - link
While i wouldn't say it was a Intel PR, I think they should definitely have left the system level power usage out of the questions. There is no point telling me that a 100" Screen with ARM is using X amount of power compared to 1" Screen with Haswell.It is confusing.
But they did include CPU and GPU benchmarks. So saying it is Intel PR is just trolling.
AlB80 - Friday, January 4, 2013 - link
Architectures with variable length of instruction are doomed. Actually there is only one remains. x86.Intel made the step into a happy past when CISC has an advantage over RISC, when superscalarity was just a theory.
Cortex A57 is coming. ARM cores will easily outperform Atom by effective instruction rate with minimum overhead.
Wolfpup - Friday, January 4, 2013 - link
How is x86 doomed when it has an absolute stranglehold on real PCs, and is now competitive on ultramobile platforms?The only disadvantage it holds is the need for a larger decoder on the front end, which has been proportionally shrinking since 1995.
djgandy - Friday, January 4, 2013 - link
plus effing one!I think some people heard their uni lecturers say something once in 1999 and just keep repeating it as if it is still true!
AlB80 - Friday, January 4, 2013 - link
Shrinking decoder... nice myth. Of course complicated scheduler and ALU dozen impact on performance, but do not forget how decoded instruction queues are filled. Decoder is only one real difference.1. There is fundamental limits how many variable instructions can be decoded per clock. CISC has an instruction cross-interference at the decoder stage. One logical block should determine a total length of decoded instructions.
2. There is a trick when CISC decoder is disintegrated into 2-3 parts with dedicated inputs, so its looks like a few independent decoders, but each part can not decode any instruction.
Now compare it with RISC.
And as I said, what happens when Cortex can decode 4,5,6,7,8 instructions?
Kogies - Friday, January 4, 2013 - link
Don't be so quick to prophesy the death of a' that. What happens when a Cortex decodes 8 instructions... I don't know, it uses 8W?Also, didn't Apple choose CISC (Intel) chips over RISC (PowerPC)? Interestingly, I believe Apple made the switch to Intel because the PowerPC chips had too high a power premium for mobile computers.