AH Tech Talk: Why Snapdragon 820's Quad Core Doesn't Matter

The majority of applications written for consumer mobile operating systems use just one application core in order to complete their tasks. A small number use two processor cores and very few use additional cores. When multiple applications are running, there is potentially a situation where more processor cores are used, but again relatively few applications have a meaningful demand on system resources when running in the background. Yet many mobile system-on-chips, SoCs, have eight or more application processor cores. This means that for most of the time, the majority of application cores on these SoCs are shut down to conserve power. Modern processors are designed to complete their given task as quickly as possible so that they may then be shut down until the next task comes along.

It's important to introduce the big.LITTLE processor architecture into the discussion at this point. Here, one set of processor cores is designed for low workloads, running at low clock speeds and using little power. This is our LITTLE core arrangement. There's a big core arrangement: application processor cores designed to be more efficient when worked harder. Many system-on-chips can combine the two core types in use, so we could have one LITTLE application core and one big application core running at the same time. We've seen MediaTek introduce three tiers of processor cores, with LITTLE, medium and big cores combined. These chipsets are ultimately about blending efficiency with performance: it's the marketing department that makes performance more of an issue. Ten cores sounds more impressive, right? We've seen some strong performing dual (application) core handsets and tablets in recent years, such as the 2012 Google Nexus 10, 2013 Moto X and 2014 Google Nexus 9, not to mention every dual core iOS processor since the iPhone 4S.

And then there's Qualcomm, who have traditionally designed their own application cores rather than use the ARM reference design. Qualcomm's 32-bit custom core, Krait, shied away from a big.LITTLE architecture until the Android industry followed Apple's jump to 64-bit mobile processors. This forced Qualcomm's hand: their 64-bit custom core was not ready for the market, so they switched to using ARM's reference core designs and that necessitated a big.LITTLE design. With hindsight, it is perhaps no surprise that the 2015 high end Snapdragon portfolio is less optimized than we are used to from Qualcomm's stable. However, the next high end Snapdragon processor, the 820, will move back to a custom design application core, the Kyro. Qualcomm are also dropping big.LITTLE from their high end SoCs and reverting back to a quad (equal) core design, although their 2016 mid-range processors still appear to be based around an eight core, big.LITTLE architecture.

Qualcomm's marketing literature promises significant improvements in performance and energy consumption of the 820 compared with the 810. The less energy the processor consumes, the less heat is dumped into the device chassis. The Snapdragon 820 will also be built on a smaller processor die, 14nm compared with 20nm of the Snapdragon 810, and at a smaller size the electronics require less voltage, which reduces power consumption. The change in architecture and of course the customized application core design of the Snapdragon 820 will make for an interesting comparison with the likely big.LITTLE designs many of its competitors are working on. However, the 820 is not simply four application cores - it contains embedded radios (mobile, WiFi, Bluetooth, NFC), the Graphics Processor Unit (or GPU) plus a digital signal processor (DSP), the Hexagon 680. In previous generations of Qualcomm SoC, the DSP has been used to offload certain tasks away from the application cores, such as MP3 playback. For the Snapdragon 820, the Hexagon 680 will also be used for the low power sensor suite and high performance, low power live image processing.

When the first devices running the Snapdragon 820 are released, we can expect a slew of benchmarking scores showing how the 820 underperforms the competition in certain tests. This will be because the 820 has four application cores rather than eight, ten or more. A more relevant comparison will be to compare how well the chipset runs through composite benchmarks designed to reflect real world performance, such as PCMark, and of course how well batteries last in the hands of consumers. It will also be interesting to see how well marketing departments adapt to the change to a quad core processor from a generation of octa core application processors, especially when mid-range processors continue to have eight processor cores - even Qualcomm's in-house designs.

We will have to wait and see if or when Qualcomm allow their custom Kyro core design to filter down into their mid-range System-on-Chip lineup. Or perhaps we will see a slightly diluted dual core, lesser clock speed version of the Snapdragon 820, as we've seen with the Snapdragon 810 and 808? But for 2016, if you are considering buying a flagship device, don't let that the Snapdragon 820 only has four cores sway your decision.

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