[big.LITTLE size hybrid architecture]
The concept of the Exynos 5 Octa processor is based on the big.LITTLE architecture proposed by ARM and is the first practical demonstration of this technology. As a senior partner of ARM, Samsung is at the forefront, but other manufacturers are also conducting research, such as Renesas also announced similar products.
So, this processor architecture is not designed by Samsung, but from ARM, nor is it a frivolous conceptual hype, but a serious technology.
The core of the big.LITTLE architecture is to both take advantage of the ultra-high efficiency of the Cortex-A7 and take advantage of the ultra-high performance of the Cortex-A15 architecture. The performance of the A7 is not as good as that of the A9, but its power consumption is much lower. The performance of the A15 is unbeatable, but at the cost of high power consumption, it is at least unacceptable in the current process.
Qualcomm, who likes its own intrusion architecture, took another approach and redesigned it based on the ARM instruction set, but sacrificed some of its performance and functionality in exchange for better energy efficiency.
As for how to measure performance and energy efficiency, there are two key indicators:
IPC (instructions per clock cycle): represents the performance level of an architecture, A15 is undoubtedly the best, and far ahead, followed by Krait 400, Krait 300, Krait 200, A9, A7, A8.
Perf/W (performance per watt): The performance-to-power ratio represents energy efficiency, preferably A7, followed by A9, Krait, and A15.
Samsung has undoubtedly announced the details of the architecture and specifications of Exynos 5 Octa: 28nm HKMG process manufacturing, of which the Cortex-A15 has an area of ​​19mm2, 32KB of instruction/data cache, 2MB of L2 cache, and a frequency of 200MHz. 1.8+GHz; Cortex-A7 has an area of ​​3.8mm2, 32KB of instruction/data cache, 512KB of L2 cache, and clock frequency of 200MHz to 1.2+GHz.
The area of ​​the A7 part is only a quarter of the A15, and the power consumption is only 17%.
[In-Kernel Switcher/IKS]
Although Exynos 5 Octa has eight physical cores, only four are presented to users. Each A15 has a corresponding A7 as a partner, but this link cannot be seen on Samsung's published kernel photos.
IKS plays the role of dispatcher, but it is not in charge of frequency switching but switching between different processors. When the load becomes high, it is as simple as going from one energy efficiency curve to another energy efficiency curve.
What makes this switch is a very simple kernel driver, which determines the processor's operating state based on the measured workload. For example, video playback, document processing, and Internet surfing are all given to the A7. After entering the game, they are handed over to the A15 and quit and returned to the A7.
Here are a few:
- You don't need or can have eight cores at the same time. In fact, at most there will only be four at the same time.
- If the load is light and no multi-threading is required, big.LITTLE will be turned off and no power will be wasted.
- Each paired A15-A7 can be switched independently, not all four A15s and all four A7s. For example, you can have one A15, two A7s working, and the fourth pair both resting.
- Four A15s and four A7s have their own frequency layers and can operate at different frequencies. That is, A15 and A7 can have different frequencies, but the A15 core has only one frequency at the same time.
Heterogeneous Multi-Processing (HMP)
This is another feature of the big.LITTLE architecture, but it has not yet been implemented. With it, all eight cores can be started at the same time.
This is an extremely complex working state, and the complexity is several orders of magnitude higher than a simple switch. It requires the system kernel scheduler to distinguish between the A15 and A7 cores. At present, the Linux kernel can not do this, and it will only treat all cores equally.
The Linaro team has completed the first phase of the HMP design and will continue to patch them into the Linux 3.8 Kernel. Their job is to make the kernel scheduler smarter, to track the load of each process, to intelligently distribute the load to the A7 or A15, and the latency of the switch is very, very low, thus maximizing the maximum power efficiency of all cores.
The Linaro Connect Summit will be held once every few months. Developers concerned with this progress may wish to take note of the progress.
The final conclusion is: Samsung Exynos 5 Octa is indeed an eight-core processor, but it is not so used, it can only be said to be 4 + 4 core processor.
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