The Snapdragon Galaxy S4 Is Much Preferred Over The Exynos Version

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I have not been a fan of Samsung, but I was still excited when Galaxy S4 was announced. The Exynos 5410 Octa CPU was the first commercial implementation of ARM’s new big.LITTLE concept, which if done right, has much potential. Granted Nvidia was the first to come up with the concept (Tegra 3 comes with 5 CPU cores – 4 high clocked cores + 1 low clocked “companion” core, which NV dubbed as 4-PLUS-1), but it is still different from ARM’s concept of pairing 2 architectures of CPU into 1.

However it really puzzled me that Samsung went with 4 Cortex-A7 cores with the Octa. Common sense would tell us that more cores = more power consumption. Cortex-A7 has better performance-per-watt compared to previous generations of  ARM architecture (Snapdragon S1 or Hummingbird kind of performance), but still, using 4 such cores to handle light tasks is really overkill. I have always felt that Nvidia’s implementation is much more logical with only 1 core handling light loads (albeit a high performance but low clocked core). A dual core Cortex-A7 at the most is already more than sufficient, considering there are many devices with even less capable hardware but still humming Android 4.x nicely. What puzzled me more, was that 70% of the Galaxy S4 shipped runs on Snapdragon instead. LTE compatibility does not seem to be the main reason since Korea gets Octa + LTE.

The missing pieces of puzzle are filled up by AndreiLux, a famous Android kernel developer in XDA forum. Let’s have a short recap of big.LITTLE first.

Samsung has been using the above picture when promoting Exynos 5410 Octa, which demonstrates the possible modes of working of big.LITTLE CPU.

To summarize them up:

1. Cluster Migration: The simplest mode to implement. Each A7 core is paired to a A15 core, and work from each A7 core can only be passed on to the paired A15 core. Either all 4 of the A7 cores working, or all 4 of the A15 cores working, which also mean all 4 cores are running even if you need only 1.
2. CPU Migration: The most flexible mode to work. Say there are 4 threads of work running, they will all start from the A7 cores. When load of 1 thread increases but the other 3 remains light, the kernel will move the one heavier work  from the A7 to the A15, while the rest remains in the A7. You can have any combination of the cores working (three A7 + one A15, or two A7 + two A15, and so on).
3. MP mode: The highest performing mode, with all 8 cores working simultaneously. But this is currently impossible in Android since Linux kernel treats all CPU cores as equal, which can lead to scenarios where light work assigned to A15 but heavy ones assigned to A7.

AndreiLux has dug deep into the kernel of Galaxy S4, and found that the Octa currently works in Cluster Migration mode only. This is a problem with power consumption and heat too since only 1 or 2 cores are needed in most times, and yet all 4 cores are running at any given time. AndreiLux tried to flash a kernel with CPU Migration mode driver, which is much more preferred in terms of power consumption, but interestingly it refuses to boot. Further digging reveals that it is a hardware limitation, meaning, the Exynos 5410 Octa can only work in Cluster Migration mode forever. This is pretty much turning from a bad product design to fiasco. We are talking about things false advertisement and products that are not up to spec. It seems more like Samsung has been cutting corners amd rushing through with its A15 CPU so as to beat the rest of the CPU makers and that it can come out in time for the Galaxy S4. Now with such a huge flaw in the Exynos CPU, it makes sense to look for alternatives, and the closest is Snapdragon. LTE compatibility seems to be more of a cover up instead. Bottom line is: you only get 4 cores at any time, and Exynos 5410 Octa is not running at the most optimal power efficiency, and it never will.

Now with Nexus 4 (and possibly the new Nexus 7) running on Qualcomm’s Krait cores and Adreno GPU, it will be logical for Google to prioritize Snapdragon while coding Android. Double that with the plethora of Snapdragon devices being released in the last few months, Galaxy S4 with Snapdragon will only run better with each update. LTE or not, look for GT-I9505 and you will get the Snapdragon version.

Sources: XDA(1), XDA(2), SamMobile

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