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Samsung’s Exynos 5 Octa not as efficient as advertised?

It seems Samsung's implementation of ARM's new big.LITTLE heterogeneous computing architecture isn't entirely what people (and perhaps Samsung too) hoped it would be.

Samsung has been advertising that their newest flagship Galaxy S4 packs an octa core processor. While technically true, the implementation isn't. At best only four of the eight available cores are working, making the S4 another quad core running phone. Samsung justified the use of an octa core design for higher energy efficiency, and, thus, leading to longer battery life. Popular android kernel developer AndreiLux is convinced, however, that Samsung's implementation is simply awful. In the words of the wise developer:

“Basically there are three modes of operation of big.LITTLE:

Cluster migration: When load rises, just like when a phone increases is frequency per DVFS, all the cores will jump from the LITTLE processor to the big one. You either have 4 A7 cores or 4 A15 cores online.

Core migration: When load rises, it will only jump those cores from the LITTLE processor whose load exceeds the capacity of the A7 cores: You can have a single A15 online with 3 A7′s on. Or any other combination of a total of 4 processors.

HMP (Heterogeneous Multi-Processing): This is a vastly more complex working mechanism, and its implementation is also an order of magnitude more sophisticated. It requires the kernel scheduler to actually be aware of the differentiation of between the A7 and A15 cores. Currently, the Linux kernel is not capable of doing this and treats all CPUs as equals. This is a problem since we do not want to use the A15 cores when a task can simply me processed on an A7 core with a much lower power cost.”

The problem here is more driver related than anything. Currently, the linux kernel doesn't differentiate between the different architecture based cores and thus treats them all equally. As a result, big.LITTLE's MP mode (it's most powerful running mode) is broken, although Samsung's Exynos 5 Octa silicon supports it. If implemented correctly, all the physical cores (true octa core phone in that case) will work in tandem, with threads having higher priority or computational intensity simply allocated to the bigger, and more powerful cores.

Perhaps a new Linux kernel and associated updates from Samsung might change this over the months, and the Galaxy S4 might actually end up showing big performance gains and battery life improvements as a result.

Source: XDA & SamMobile

Preetam Nath
A 3rd year engineering student, Preetam Nath has a keen interest in following the smartphone and tablet sphere. He plays games on his favorite old PC when he's not doing anything. In the mornings, you can often find him at the gym.

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