Why is Octa-core still slow on cell phones?

Many people look at cell phones and want to see the number of cores in the processor, with the belief that multi-cores perform well and will not be slow. But when they buy eight-core phones and still feel they are slow, then they complain, “Why is eight-core still slow?”

Here is the answer broken down for you.

1. What processor is not slow?
In fact, a smoothly running system is not closely related to having multiple cores. Here, smoothness includes two parts. First is the system itself, actually the smoothness of the application starter. Second is the smoothness of operation of different applications.

What affects these two things is not how many cores you have, but rather the performance of individual cores and memory bandwidth.

In most cases, our starter and applications can only use one or two cores, can rarely use four, and to use eight is extremely rare.

So is the deciding factor in speed not having multiple cores, but individual core performance?

The market now offers A7, A9, A15, various editions of Snapdragon, as well as Intel processors, with large differences in performance.

If you are using A7, eight-core is not even as good as an A15 dual-core. When it comes to smoothness, the frequency and type of individual cores is more important, having a larger number of cores is useless.

2. Memory bandwidth is actually more crucial.
Smart phones and tablets are all SOC, meaning the graphics core is integrated, sharing memory bandwidth with the CPU, with no independent memory. And resolution is getting better for phones and tablets, this puts a great demand on memory bandwidth.

Programs need to run inside the internal memory. If the processor is quick, and the memory access speed can not keep up, the system will still be slow. And as resolution increases, so will the demand on memory bandwidth.

When memory bandwidth is not enough and resolution is too high, no matter how fast the processor is the system will still be slow.

In the early days of Android, WVGA resolution was only 800*600=480,000 pixels, not much pressure on memory bandwidth.

But with soaring Android resolution, bandwidth became a problem. So how much bandwidth is needed? Let’s refer to Apple.

The New iPad is a very interesting short-lived product. Its  processor and resolution can be used as a reference.  A5X is actually only a dual-core A9 processor, the host frequency is not high, on the level of MT6577.

Supporting the New iPad’s smooth operation is the powerful A5X graphics performance and memory bandwidth reaching 12.8GB/s.

The New iPad’s resolution is 2048*1536=3,140,000 pixels. That is to say, Apple believes that 4GB/s of bandwidth for every 1,000,000 pixels can guarantee basic smoothness.

At the same time as the A5X, the A9 quad-core bandwidth was only 4.2GB/s and 6.4GB/s. They used a resolution of 1280*720=920,000 pixels. Every 1,000,000 pixels uses 4GB/s of bandwidth, so fundamentally it can guarantee smoothness.

And now with resolution of 1920*1080=2,070,000 pixels, only the 8064 with 8.3GB/s bandwidth can manage to support it.

That is why the first generation 1080P smart phones all used APQ8064.

The latest Qualcomm 801 (8974AC) memory bandwidth reaches 14.9GB/s, supporting 2560*1440=3,680,000 pixels. That is just over the 4GB/s per 1,000,000 pixel bottom line, but is still fundamentally smooth. That is why several of the latest phones dare to support this fundamental resolution.

But, several other processors on the market are not so optimistic. The MT6592 bandwidth is about 5.3GB/s, to support 720P is not a problem; but to support 1080P, each 1,000,000 pixels only gets 2.56GB/s. No matter how optimized the starter is, it will be hard to be smooth, that is why eight-core is still slow.

Similarly, the Huawei Ascend K910 bandwidth is 6.4GB/s, like the MT6592, but strains to support 1080P.

Lately, test reports shows that an Intel ATOM Z3735D tablet, 3,140,000 pixels with only 10.6GB/s of bandwidth, and it felt slow.

Memory bandwidth is actually more critical than the processor.