Memory Subsystems Compared

On the memory subsystem side, there’s quite a few big changes for both the Snapdragon 865 as well as the Exynos 990, as these are the first commercial SoCs on the market using LPDDR5. Qualcomm especially is said to have made huge progress in its memory subsystem, and we’re now able to verify the initially promissing results we saw on the QRD865 back in December with a production device.

And indeed, the news keeps on getting better for Qualcomm, as the new Galaxy S20 showcases even better memory results than we had measured on the reference device. The improvements over the Snapdragon 855 are just enormous and Qualcomm not only manages to catch up but very much now is able to beat the Exynos chips in terms of memory subsystem performance.

Arm very famously quotes that an improvement of 5ns in memory latency corresponds to an increase of around 1% in performance. And if that’s the case, Qualcomm will have had a ~12% improvement in CPU performance just by virtue of the new memory controller and SoC memory subsystem design. Our structural estimate in the memory latency falls in around 106 vs 124ns – most of the improvement seems to be due to how Qualcomm is now handling accesses to the DRAM chips themselves, previously attributing the bad latencies on the Snapdragon 855 due to power management mechanisms.

Samsung’s Exynos 990 also improves in memory latency compared to the Exynos 9820, but by a smaller margin than what the Snapdragon 865 was able to achieve. All latency patterns here are still clearly worse than the Qualcomm chip, and there’s some oddities in the results. Let’s zoom in into a logarithmic graph:

 

Comparing the Exynos 990 results vs the Exynos 9820, it’s now quite visible that the L2 cache has increased dramatically in size, similar to what we’ve described on the previous page, corresponding to the doubling of the available cache to a core from 1MB to 2MB. Samsung’s cores still have some advantages, for example they’re still on a 3-cycle L1 latency design whereas the Arm cores make due with 4-cycle accesses, however in other regards, the design just falls apart.

The TLB issues that we had described last year in the M4 are still very much present in the M5 core, which results in some absurd results such as random accesses over a 2MB region being actually faster than at 1MB. Cache-line accesses with TLB miss penalties now actually have lower access latencies in the L3 than in the L2 regions, and I have no idea what’s happening in the 16-64MB region in that test as it behaves worse than the 9820.

Examining the A76 cores of the Exynos 990, we see a much cleaner set of results more akin to what you’d expect to see from a CPU. Here we also see the 2MB SLC cache hierarchy in the 1-3MB region, meaning the Arm core cluster does have access to this cache, with the M5 cores bypassing it for better latency. Last year I had noted that the A76’s prefetchers had seen some massive improvements, and this is again evident here in the result sets of the two CPUs on the same chip as the middle cores actually handle some access patterns better than the M5 cores.

Samsung has had large issues with its memory subsystem ever since the M3 design, and unfortunately it seems they never addressed them, even with the more recent M5 core.

The Snapdragon 865 here is quite straightforward. The biggest difference to the 855, besides the improved DRAM latency, is the doubling of the L3 from 2 to 4MB which is also immediately visible. It still pales in comparison to the Apple A13’s cache hierarchy, but we do hope that the Arm vendors will be able to catch up in the next few years.

The Exynos 990 SoC: Last of Custom CPUs SPEC2006: Worst Disparity Yet
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  • Reflex78 - Friday, April 3, 2020 - link

    I live in Europe, I like Samsung and have S9 at the moment.
    But I will never pay +1000€ for lower quality Exynos S20 version in Europe!
    This is a big mistake from the company management to allow such a difference between this 2 variants at the same price!
    And I just read that they have chosen to sell Snapdragon version even for their home country:
    https://www.phonearena.com/news/Samsung-chip-divis...
    Reply
  • twtech - Friday, April 3, 2020 - link

    The edge design of Samsung's more recent releases is just not good. There are no cases that can both properly protect the screen, and avoid blocking any of it. My older phones typically lasted for years without any significant damage. These new ones are one slip of the hand away from being garbage fodder. Reply
  • FunBunny2 - Friday, April 3, 2020 - link

    "These new ones are one slip of the hand away from being garbage fodder."

    rube!! :) it's a feature, not a bug. going back to Lotus/MS conflict: "DOS ain't done til 1-2-3 won't run."
    Reply
  • Harysviewty - Friday, April 3, 2020 - link

    Totally wrong calculation. It's 7.1mp when you crop 3x3. It's only possible to do full 12mp resolution with the help of Super resolution algorithm, which adds up to 75% more detail to 'normal bayer' setup Reply
  • Andrei Frumusanu - Friday, April 3, 2020 - link

    I don't know what you're talking about. The sensor is 108MP at 12000 x 9000. 3x3 binning results in 4000 x 3000, which is 12MP. Reply
  • krazyfrog - Friday, April 3, 2020 - link

    The 3x3 binning only happens on the 108MP sensor, not on the 64MP sensor. Reply
  • s.yu - Saturday, April 4, 2020 - link

    You said "crop 3x3" which confuses people, usually we just say 3x crop, but yes, the digital zoom doesn't provide native 12MP in any sense at 3x. Reply
  • JDSP - Friday, April 3, 2020 - link

    Image links on the iPhone are wrong, Wide links to zoom and night sight links to normal Reply
  • Andrei Frumusanu - Friday, April 3, 2020 - link

    Corrected that, thanks. There's probably a few other link issues there I'll keep an eye out for that. Reply
  • Quantumz0d - Friday, April 3, 2020 - link

    Great analysis, will go through it slowly but looking at that Exynos 990, Seriously WTF is that. Higher power consumption, lower GPU performance, higher throttling. Very unfortunate. And losing to that copycat Chinese Huawei Kirin trash (EMUI garbage with LZ Play backdoor, Read only EROFS Filesystem and Google copying that into Pixel 4 and the proprietary garbage NMSDslot no 3.5mm jack, No Play Store, lies and deception) Samsung should be ashamed of themselves removing all their genius whatever PR ads for milking customers and offering mediocrity.

    The only good part for Exynos is unlockable bootloader. Since SD versions in US are locked as hell and useless for customization esp how they depreciated the SpO2 sensor in this phone HW and from SW side also in S10 and previous phones to promote bullshit Smartwatches.

    This phone sucks bad, their S10 has better features and looks better as well on top this phone camera sucks, no 3.5mm jack and ugliest design ever with insane price tag on top, LG's V60 is looking very good in comparison from Camera to Audio and other features/specs plus price vs this phone or even that Chinese OP8 Pro (despite lacking SD slot and 3.5mm jack as at-least it is cheaper and has hassle free Bootloader unlock), since Samsung dropping HW feature set which defines the all in one phones from Samsung they want greed and money from that shitty Buds and other garbage.

    S10+ Exynos is a better choice all rounder as it has BL unlock as well. On top I'd like to mention how DJ Koh also was removed from the Samsung mobile CEO office, I presume it also played an important role in S.LSI even if independent and the financial results of the conglomerate, esp how it impacted the design philosophy for sure as Note 10 provided a platform for removing features and cost cutting similar aspect in S20 a bit more worse.

    On the Foundry aspects, TSMC 7NP is not EUV and Samsung 7nm LPP is EUV since N7+ is the EUV one. Not sure 20% to 30% is valid ? I do not know since the uArch is garbage doesn't mean that the node is trash, esp last rumor was Ampere would be on Samsung node with EUV and Nvidia wouldn't afford a stupid decision tbh, and a big shame is S.LSI getting hacked. I think the PR marketing team and the budgets ruined them or such, we may never know. Shame indeed.
    Reply

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