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
- 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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  • lilmoe - Friday, April 20, 2018 - link

    That's Microsoft's fault. Instead of focusing on a purely mobile architecture and the "enhancing" it for the desktop after it failed, they should have worked on the desktop experience from the start to be closer to mobile in terms of efficiency and hardware acceleration without sacrificing features, information density and responsiveness, relieving it of its dependence on general cpu compute.

    Instead of going platform agnostic for a native desktop, they went with a *form factor* agnostic approach that focused on the lowest common denominator (mobile) that didn't benefit either.

    If the software is right, and workloads are properly accelerated/deligated, then the CPU of choice wouldn't be nearly as critical to get the optimum experience and performance of the device and the purpose it was is designed to serve.
  • serendip - Saturday, April 21, 2018 - link

    Indeed, you may be on to something there. Microsoft added tablet and mobile features to Windows 10 in a halfhearted way, like how the touch keyboard on Windows tablets is a laggy and inaccurate mess compared to the iPad keyboard. I bring along a folding Bluetooth keyboard with my Windows tablet because using the virtual keyboard is such a painful experience.

    That said, there's probably a ton of Win32 baggage that relies on x86 to run so it could take years for Microsoft to refactor everything.
  • MrSpadge - Saturday, April 21, 2018 - link

    Responsiveness and accuracy of your on-screen keyboard are largely determined by hardware, not MS. E.g. the keyboard on my wifes low-end Lumia 535 is pretty bad, whereas the same software on my Lumia 950 performs great.
  • LiverpoolFC5903 - Monday, April 23, 2018 - link

    The onscreen keyboard on my cube i7 book works as smoothly and quickly as any ipad or android tab. The Core m3 keeps it ticking along nicely. I use it purely as a tablet, even while working on it occasionally, with the type attachment collecting dust in my drawer.

    I think poor quality storage and CPU are the reasons for the bad experience.
  • Alexvrb - Saturday, April 21, 2018 - link

    "relieving of its dependence on general CPU compute"

    Don't smoke crack and post.
  • tuxRoller - Saturday, April 21, 2018 - link

    Otoh, arm servers are looking better all the time.
  • Hurr Durr - Saturday, April 21, 2018 - link

    It`s a pipedream, like loonix on desktop. Pretty sure it`s the same people anyway.
  • tuxRoller - Saturday, April 21, 2018 - link

    Chromeos is a thing.
  • ZolaIII - Sunday, April 22, 2018 - link

    Reed you unlettered kite pipe:
  • tuxRoller - Monday, April 23, 2018 - link

    That was a great post, and it makes SVE look all the more interesting in this space.

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