Conclusion


Does CPU performance matter? Yes, but compared to what? Does it matter more today than it did 5 years ago? Will it matter more in the future than it does today or less? Will designers dedicate more or less silicon to it compared to other subsystems? Does it make sense to add more and more processor cores? Will it represent more or less of the total power budget?

It certainly matters more today than it did 5 years ago. It was a very big deal in 2009 when the Apple iPhone switched from a 412MHz ARM11 to a 600MHz A8. The increased performance in applications, games, and especially web browsing was a great selling point. The move to 1GHz Dual-Core A9s this year (for Android and probably beyond) is also a big deal. The difference might be slightly smaller and the effect less perceptible, but multi-core makes a real difference for web browsing, gaming, and multitasking - it's a strong differentiator and should be a much bigger selling point than, say, 21Mbps HSPA+ versus 7.2Mbps HSPA. And that is despite the latter costing 10x the silicon of a 2nd Cortex-A9 core (at least with a traditional baseband architecture - we wish we were kidding).

Roadmap: Cost & Power


Will semiconductor manufacturers still push the envelope in a few years? Let's consider the most common ultra-high-end industry roadmap:
  • 65nm->1xCortex-A8 or Scorpion with 512KB L2
  • 40nm->2xCortex-A9 or Scorpion with 1MB L2
  • 28nm->4xCortex-A9 or 2xCortex- A15 with 1MB L2
  • 20nm->4xCortex-A15 with 2MB L2
So the amount of money dedicated to the CPU subsystem is remaining roughly constant (die size is decreasing slightly but wafer costs are also increasing slightly so it evens out). That's pretty good compared to many other things - and the one subsystem which is nearly guaranteed to increase in size is the 3D GPU. That makes sense as the one thing that will always benefit from faster CPUs is gaming... and obviously that benefits even more from faster GPUs.

At the same time, the CPU power budget is actually increasing as power efficiency is definitively no longer improving as fast as die size! Battery size has certainly increased somewhat, but more importantly everyone is now focusing more on average power than peak power (e.g. a dual-core consumes more power at peak but should in theory be more efficient at the same performance level because of lower frequencies/voltages). Peak power cannot increase forever but it doesn't have to: eventually SoCs will monitor their own power consumption and downclock themselves if they're taking too much power for this specific workload (ala AMD PowerTune and not Intel Turbo Boost). In theory this can be done without power monitoring (just limit CPU frequency when other subsystems are active) but that's not quite as effective.

Does performance still matter?


Web browsing is an interesting case. It is supposedly ~50% faster on Android 2.2 with a dual-core processor (3+ threads for Rendering/UI+JavaScript/Flash) and the difference is obviously very close to 2x with heavy multitasking. The benefit of a quad-core processor wouldn't be as obvious although there is some opportunity for further multithreading (per-tab process ala Google Chrome which would make loading multiple pages at the same time faster, background JavaScript compilation ala IE9, and speculative HTML5 parsing ala Firefox). On one hand, you'd be hard pressed not to notice the difference between dual-core 1GHz A9s and dual-core 1.5GHz A15s. On the other hand, waiting 2 seconds instead of 4 is a smaller selling point than 4 instead of 8.

In the end, will consumers care about ever increasing CPU performance? Yes, although probably less than they do today on average, especially after the 20nm generation. And unlike on the desktop, it doesn't make much sense to scale beyond 4 cores (it makes more sense to go for 8 threads via SMT if necessary). However we've been focusing too much on the high-end and what's more interesting here is the low-end: CPU performance historically didn't matter much on feature phones, but in low-end smartphones it should become a very big differentiator for many years to come.

What now?


Hopefully it is now clear that Handheld CPU Architectures are interesting both technically and strategically. But it's not the only interesting architecture in a modern smartphone or tablet. The GPU and baseband are both also worthy of in-depth analysis (although probably much more focused on specific companies) and hopefully we'll get to that soon! (also we may or may not have moved our clocks to Valve Time)

Feedback? Feel free to post it in our forum thread!