The Snapdragon 865 SoC: Beating Expectations

We’ve covered the Snapdragon 865 extensively over the last few months, and more recently did a performance preview of the chip on the Galaxy S20 Ultra:

It’s safe to say that Qualcomm managed to beat our expectations in terms of power efficiency improvements. Which is something we’ll go over in more detail in this piece as well.

At the heart of the Snapdragon 865 we find Arm’s newest Cortex-A77 CPU cores. The new microarchitecture is said to bring a 20-25% IPC improvement over its predecessors, and that’s where the new SoC derives most of its performance improvements from, as the clock frequencies of the cores are identical to that of last year’s Snapdragon 855.


Snapdragon 865 CPU Topology

One aspect where Qualcomm did improve the design is in doubling the shared L3 cache of the CPU cluster, going from 2MB to 4MB. Not only does this further improve the performance of the CPUs by allowing for more data to be cached on-chip, but Qualcomm has explained that one of the primary reasons for this was to also improve power efficiency of the SoC by reducing how often the SoC has to access the DRAM, which is a relatively power-expensive operation.

The chip still has a 3MB system level cache that serves the various IP blocks on the SoC – it’s again meant to not only improve performance but also improve power efficiency as it avoids external memory accesses. The memory subsystem here is smart and detects when to bypass this cache when there’s latency-sensitive workloads, and in general we’ll see some massive memory subsystem improvements on the part of the Snapdragon 865 in a later dedicated section.

All the CPUs being in the same cluster and cache hierarchy means that the core-to-core latencies are relatively uniform, only differing based on their frequencies and lower level cache access latencies. It’s not too much of an exciting metric here, but it’s important context to have as we’ll consider the Exynos 990’s CPU topology in just a bit.

Again, we’ve covered the Snapdragon 865 quite extensively in the above linked articles so I recommend reading them again for other details on other parts of the new chip, such as the new ISP, DSP, and GPU details. However, one aspect that’s very defining for the flagship Qualcomm chipset this year is that the company is separating the modem from the SoC – essentially making the SoC just an application processor for this generation.

The external nature of the X55 modem has a few implications: first of all, there’s an additional component on the motherboard which vendors will have to make space for, which means additional cost. Secondly, there’s the big question of how power efficiency will be affected by the external modem. We’ve seen Apple devices perform excellently over the years while never having an integrated modem, and I feel like the Snapdragon 865 and X55 also fall into this classification, as I haven’t seen any major differences in efficiency due to the external nature of the modem.

Design, Continued: An Ultra Mega Phone The Exynos 990 SoC: Last of Custom CPUs
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  • Andrei Frumusanu - Friday, April 3, 2020 - link

    No, there's no software application notion of displaying something at a given refresh rate - things just render as fast as possible unless. 3D games might have an FPS cap, but that's not refresh rate. Reply
  • FunBunny2 - Friday, April 3, 2020 - link

    this is what I mean.

    "If you can run a game at 100 frames per second, you may see a tangible benefit from playing it on a monitor that can refresh that many times per second. But if you’re watching a movie at a classic 24 FPS (frames per second), a higher refresh rate monitor won’t make any difference."

    here: https://www.digitaltrends.com/computing/do-you-nee...

    IOW, unless the processor sending either video or coded application images does so 120 per second, all the 120hz screen does is re-scan each image multiple times. how can the refresh rate create modified images, between those sent by the processor? or do 90/120hz screens do just that?

    do you disagree with that author?
    Reply
  • krazyfrog - Friday, April 3, 2020 - link

    The screen refreshes at a set rate regardless of the content being sent to it. In this case, it always refreshes at 120Hz. If the content is in 24fps, each frame of the video persists for 5 refreshes of the display. To the eye, it looks no different than watching the same 24fps video on a 60Hz display. Reply
  • surt - Saturday, April 4, 2020 - link

    Not true. It does not look the same to your eye, and the difference is the latency from the time that information is ready to display to the time it reaches your eye. The 120hz display will show that transition from e.g. the 23rd to the 24th frame significantly faster. Reply
  • FunBunny2 - Sunday, April 5, 2020 - link

    " It does not look the same to your eye"

    that's a may be. years ago I worked in a manufacturing plant, no windows and only florescent lights. one of the guys I worked with wore glasses that looked like very weak sunglasses, but no prescription. I asked him about them and he said his eye doctor prescribed them for his constant headaches. turns out that some folks rectify the 60hz flash of florescent light, and it hurts. the same phenomenon would occur with monitors. if you're not among the rectifiers, it's hard to see how you would see different at 120hz.
    Reply
  • surt - Sunday, April 5, 2020 - link

    And yet, it's not hard to see at all. Response tests are undeniable. People's reactions are unquestionably faster on 120hz. Whether you notice the difference or not, it exists. Reply
  • surt - Saturday, April 4, 2020 - link

    It matters to any game. If your game updates at 30fps, the 120hz display will get that information to your eye a fraction faster than the 60hz display, because the 'time to next frame' + 'time to display next frame' is always smaller on the 120hz. Reply
  • eastcoast_pete - Friday, April 3, 2020 - link

    Great review, thanks Andrei! Question: just how much power draw does the 5G modem add, especially the mm ones for us in the US? Along those lines, can the 5G function disabled in software, so not just deselected, but actually shut off? I imagine that the phone hunting for mm connectivity when it's not there could eat quite a bit of battery life. Reply
  • Andrei Frumusanu - Friday, April 3, 2020 - link

    I don't even have 5G coverage here so I wouldn't know!

    Yes, 5G can be disabled in the options. I would assume that actually shuts off the extra RF. Similarly, I don't know how the mmWave antenna power management works.
    Reply
  • eastcoast_pete - Friday, April 3, 2020 - link

    Thanks for the reply! mm 5G coverage is supposedly "available" in some places here in the US, but I don't believe the carriers here have set up anywhere near enough cells for it to be viable. Plus, even if I'd get Gb download rates, they still have caps on their plans, unless one shells out for the premium unlimited ones. And those make the 20 Ultra's price tag look like a bargain (: Reply

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