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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  • katakuri4744 - Wednesday, April 8, 2020 - link

    Hi Andrei,
    Appreciate such a detailed article. I have one question, how did you pull the voltage curve, is there a command?
    I had S10 exynos then upgraded it to S20+ exynos, both the articles had this voltage curve though the Snapdragon variant does not, I would like to check the same for my device as well.
    Reply
  • helloworld_chip - Monday, April 6, 2020 - link

    Though 865's compute efficiency loks pretty good, from some other data points I heard that its infrastructure power (impacting real user-daily activities, light-loaded scenarios) is quite BAD. Would be good if more such comparisons are available to confirm this, since it determines how long people can actually use a phone. Reply
  • s.yu - Monday, April 6, 2020 - link

    Idle is relatively inefficient and the numbers here show. Reply
  • helloworld_chip - Monday, April 6, 2020 - link

    Gotcha thx.

    I am thinking we should calculate bat-capacity / hours instead of just hours to really show how is the SoCs overall efficiency.

    Bigger and Bigger battery really make us feel heavier, we should push the designer harder to make these more efficiency instead of just using larger and larger battery.
    Reply
  • FunBunny2 - Tuesday, April 7, 2020 - link

    "Bigger and Bigger battery really make us feel heavier, we should push the designer harder to make these more efficiency instead of just using larger and larger battery. "

    last time I looked, most of the battery goes to driving the screen. so resurrect your original iPhone. the notion that batteries will have increasing power density this millennium is fantasy. Li is the smallest source of electrons available. well, unless you're willing to carry around a Hindenburg in your pocket.
    Reply
  • s.yu - Tuesday, April 7, 2020 - link

    Technically I'm with you but I just don't trust all developers to sufficiently optimize their code. You see that Peachncream guy who always claims that his dinosaur fossils run everything smoothly but it just doesn't happen to me. The way it is my S6E(backup device) that literally runs 3 apps in all(not even simultaneously, AFAIK) with animations turned off in dev mode is much slower than his...some really old low end phone, same thing with the battery, a large capacity gives me peace of mind over any optimization because I know that nothing could eat through the battery too fast. Reply
  • watzupken - Monday, April 6, 2020 - link

    This is the reason why I will not consider any Samsung Galaxy phone that is running on Exynos SOC. If it is on the low/ mid end, I can't complain about it. But when shelling out for a flagship, I don't see why one would pay the same amount for the slower variant and you don't have the choice to get the superior version locally. Reply
  • andyfrut - Monday, April 6, 2020 - link

    Is the S20+ (855) comparable with the S20 Ultra (855) in battery life in 120hz? Reply
  • andyfrut - Monday, April 6, 2020 - link

    snapdragon 865* Reply
  • Ayaan_G - Tuesday, April 7, 2020 - link

    The 5G addition to the Samsung Galaxy series will really give it a boost in another level ! Reply

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