Following our review of the Galaxy S9 there’s been a lot of discussion about both the performance and battery life of Exynos 9810 variants of the Galaxy S9. In the original review I had identified a few key issues with the platform for which I had deemed to be the most negatively attributing to the bad characteristics of the phone. In a first piece following the review I did a few minor changes to the kernel which already seemed to have benefited battery life in our web browsing test, and slightly changing the performance characteristics of the phone for the positive.

In that previous article I noted that there’s a lot to be done to improve the performance of the phone further and trying to optimise battery life. Especially on the performance side of things there were in my opinion very low-hanging fruit in terms of possible changes that would benefit the user-experience.

Focusing on Performance

For this second part I set about trying to recover the best performance possible and matching the Snapdragon 845 variant of the Galaxy S9, while still keeping an eye on battery life.

Samsung Galaxy S9 (E9810)
Kernel Comparison and Changelog
Version Changes and Notes
Official Firmware As Shipped - Stock setup and behavior
- Single Core M3 at 2704 MHz
- Dual Core M3 at 2314 MHz
- Quad Core M3 at 1794 MHz
'CPU Limited
Mode'
- Optional Samsung-defined CPU Mode in Settings
- CPU limited to 1469 MHz
- Memory controller at half-speed
- Conservative Scheduler
Custom Config 1 - Start with 'As Shipped' Firmware
- Remove hotplugging mechanism
- Limit M3 frequency peak to 1794MHz at any loading
Custom Config 2
(Kernel source)
- Raise little core frequency to 1950MHz
- Raise big core minimum frequency to 962MHz
- Adapt EAS cost tables based on measured perf & power
- Merge scheduler patches to 4.9-eas-dev (Up to Jan18)
- Backport PELT util_est and use it 
- Backport PELT decay rate change to 16ms
- Adapt/disable no longer needed Samsung sched(util) mods
- Minor custom modifications for tuning
Custom Config 3 - Raise big core frequency to 2314MHz & relevant adjustments

As a starting point we’re continuing on where we left off in part 1, which was extremely straightforward as the only changes were the removal of all boost frequencies above 1.8GHz on the M3 cores and disabling the online core / hotplugging driver.

In the original review the most evident issue that I identified in terms of badly affecting performance of the phone was the way the device was extremely slow in terms of scaling up in frequency, as well as migrating threads onto the big cores. The original values I described were around 410ms for a steady state continuous workload to actually reach the maximum frequency of the big cores. This was a great contrast to the 65ms of the Snapdragon 845 variant. Setting all other things aside this is what was limiting the interactive performance of the Exynos 9810 the most, so naturally it’s what we want to fix first and foremost.

Scheduling history around EAS

As a little backstory, ever since big.LITTLE’s introduction several years ago the biggest goal for ARM has been to have SoC vendors run the heterogeneous CPUs with a smart scheduler that would be aware of the various CPU’s performance and energy characteristics. This was a fine goal to have but the road to get there has been in my opinion nothing short of a mess. ARM’s approach was to try to do the work in the upstream Linux kernel or within the Linaro workgroup kernel. Unfortunately over the years and delays a lot of the hype that energy aware scheduling (EAS) would bring ended up with a fizzle when it came to shipping commercial devices. I think Qualcomm was on the ball here as even as early as 2015 for the Snapdragon 810, and we’ve covered extensively what the company was trying to do to resolve issues relating to EAS.

A key component to enabling scheduling across heterogeneous CPUs is the ability for the scheduler to actually know the activity and load of individual tasks, instead of only knowing the general CPU utilisation. If you know an individual task’s load, then you can make batter scheduling decisions on which CPU cores to place it. This was originally implemented through the PELT mechanism (Per-entity load tracking) into the Linux kernel and is what was used for migration decisions both in HMP and EAS scheduling.


Exynos 9810 Floor Plan. Image Credit TechInsights

Another long-running goal of Arm and the Linux community was to integrate CPU frequency selection logic within the scheduler, instead of it being a separate mechanism. This was first attempted in a project called schedfreq, and is now fully integrated into a new governor called schedutil. Again the implementation time-scale we’re talking about here was several years, while at the same time we’re seeing several device generations being shipped with a myriad of solutions.

S.LSI’s Exynos chipsets were playing it safe, and up to the Exnyos 9810 the company just chose to stick to a HMP scheduler with a separate interactive cpu frequency governor. Huawei Kirin chipsets ship with EAS, however here even with the latest devices such as the P20, the company foregoes the scheduler CPU frequency governors and falls back to a traditional interactive one (with very good results). Meanwhile Qualcomm has advanced their custom implementation and taken another approach called WALT (Window-assisted load tracking) that is far more responsive to PELT. On the Snapdragon 835 and 845 this is the core mechanism that assures the best performance in terms of scheduling and CPU frequency selection.

Scheduler mechanisms: WALT & PELT
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  • zepi - Friday, April 20, 2018 - link

    Andrei, how come you are still with Anandtech and not working for one of the big smartphone manufacturers, Qualcomm or maybe ARM? Reply
  • zepi - Friday, April 20, 2018 - link

    Excellent piece, like always btw. Reply
  • tipoo - Friday, April 20, 2018 - link

    (smartphone manufacturers, please don't take him too! :P ) Reply
  • Manch - Friday, April 20, 2018 - link

    AndreiTech Reply
  • Morawka - Friday, April 20, 2018 - link

    We need good writers, he's fine where he's at. He can always do consulting like Anand did. This is a great investigative piece and i'm glad Anandtech has linux guru's who can make their own OS through a patchwork of kernal modifications. Reply
  • RaduR - Tuesday, April 24, 2018 - link

    Andrei was working for ImgTech if i'm not mistaking. Unfortunately since Apple move ImgTech without MIPS I dont't think will ever come out with a SOC.

    Real one not just on paper.

    So unfortunate that MIPS+PowerVR was never to become a successful competition to ARM.
    Reply
  • juicytuna - Friday, April 20, 2018 - link

    Great stuff. Reads like a job application to the S.LSI BSP team. Reply
  • fishjunk - Friday, April 20, 2018 - link

    Excellent investigation. Samsung designed the M3 core with wider decode, lower frequency, and potentially better integration with its own hardware yet still could not match the performance and efficiency of ARM A75. Why did they not do their internal testing of A75 before deciding to go with M3? Reply
  • eastcoast_pete - Friday, April 20, 2018 - link

    My strong guess is that Samsung has the same idea that Apple has for its future MacBooks - Intel outside. In mobile systems (phones), well executed hardware and software designs of wide and deep cores (Apple: yes, Samsung: not really) can offer great peak performance but have to throttle heavily due to thermal and power constraints. Same core designs in a laptop with good heat management and a much larger battery, those restraints are loosened significantly. Samsung's attempt failed mostly due to their typical report card - hardware: A- or B+, software: D or F.
    That being said, Apple's drive towards in-house chips plus the ongoing Windows 10 on Snapdragon 835/845 initiative by Qualcomm and MS doesn't augur well for Intel's almost-monopoly in the ultraportable laptop and 2-in-1 market, especially once Samsung & Co. get their act together.
    Reply
  • serendip - Friday, April 20, 2018 - link

    I used to share your optimism on ARM muscling into Intel territory but now I'm not so sure. Snapdragon Win10 PCs have been announced but nobody's buying them - this really does smell like WinRT and Surface RT all over again. Microsoft, Qualcomm and PC OEMs are adopting a wait and see approach when they should be going all-out on ARM. Reply

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