Linux 5.11 Is Regressing Hard For AMD Performance With Schedutil
Written by Michael Larabel in Software on 25 December 2020. Page 6 of 6. 32 Comments

When taking the geometric mean of the various Ryzen 9 5900X benchmarks conducted, moving to Linux 5.11 in its early development state on Schedutil led to about a 14% decline in performance overall. The other tested governors were stable from Linux 5.10 to 5.11.

Seeing as more environments are now defaulting to the Schedutil governor by default, these early Linux 5.11 results are frightening in leading to measurably lower performance across a wide range of tests from Linux gaming to Intel oneAPI workloads, AI, and more.

Fortunately, this has been spotted early enough with Linux 5.11-rc1 not even coming out until the weekend that it will ideally be addressed before the Linux 5.11 stable release expected in February. But it does once again raise questions how well the new code was tested in the first place. On every AMD Zen 2 or newer (where frequency invariance is supported via CPPC), I've been able to find performance hits with little effort. Hopefully this change will get reverted until being properly dealt with or otherwise improved upon in short order. For those concerned, you can always force the "performance" governor which arguably would love to see more distributions use as the default.

This is also another example of where AMD could benefit from more upstream, bleeding-edge kernel testing/QA. Intel is well known for running various kernel test bots (including various PTS tests) albeit as far as I know there all of Intel's testing efforts are obviously focused just on Intel hardware. AMD given their available resources particularly on the desktop side have previously indicated they are mostly focusing on testing of LTS/enterprise distributions/kernels and at least previously less in the way of close upstream Git testing, which besides this Schedutil snafu there was the Zen 2 RdRand situation and similar incidents in the past. Hopefully in 2021 given their continuing successes from mobile to server they will be able to ramp up in this area and their other timely open-source contributions. But at least thanks to AMD's support I have plenty of desktop/server hardware around and am habitually benchmarking the latest Linux code even through holidays.

The good news though that once the Schedutil regressions are addressed, Linux 5.11 should actually be a great kernel for AMD Linux users. Linux 5.11 brings the AMD Sensor Fusion Hub driver, the AMD SoC PMC driver and work on S2idle for improving the mobile experience, Zen/Zen2/Zen3 PowerCap RAPL support, EPYC Zen 3 support in the energy driver, and over on the graphics side new Radeon GPU support and other improvements for their leading open-source dGPU support.

Now back to some Christmas benchmarking looking at some other Linux 5.11 changes being investigated...

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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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