Mailinglist Archive: opensuse-factory (439 mails)

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[opensuse-factory] Huge single-core performance loss with spectre v2 mitigation enabled (it's the default)
I use current TW on a Lenovo P72 laptop with a Core i7-8850H CPU.

Some time ago, I decided it to compare Geekbench scores between kernel with (this is the default) and without vulnerability mitigations disabled.

To disable mitigations, I used these kernel parameters:

 noibrs noibpb nopti https://browser.geekbench.com/v4/cpu/compare/12738676?baseline=12738264 nospectre_v1

I was astonished to see that the single-core score mitigated was 4579 vs 5843 the score with mitigations disabled,
resulting in 21.6% performance loss ! Multi-core performance is unaffected.

I finally isolated to nospectre_v2, which if specified is causing that huge bump in performance.
So the spectre v2 mitigation is causing that huge single core performance loss.

Geekbench comparison: default kernel vs kernel with nospectre_v2:
https://browser.geekbench.com/v4/cpu/compare/12738751?baseline=12738676

Today, I tried Geekbench on Fedora 30 beta that got recently released. Without changing any kernel parameter it has
the same performance than TW with nospectre_v2 !:

TW default vs Fedora default:
https://browser.geekbench.com/v4/cpu/compare/12738751?baseline=12738264

TW nospectre_v2 vs Fedora default
https://browser.geekbench.com/v4/cpu/compare/12738676?baseline=12738264

So basically, TW has about 20% less single core performance by default, which in my opinion is some kind of bug.
This seems to me a good candidate to explain why TW trailing in Phoronix recent benchmarks.
And maybe an indication that there may be a performance issue with spectre v2 mitigation only in TW.

For reference, here is the output of spectre-metldown-checker.sh on all systems compared:

TW default: https://termbin.com/8p7z
TW nospectre_v2: https://termbin.com/bb6t
Fedora default: http://termbin.com/0u7o

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