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Windows 11 Adversely Affects Scheduling in Some AMD CPUs, Increases L3 Latency

As with most newly launched applications, Windows 11 comes with its fair share of features bugs. In addition to reducing gaming and content creation performance by a smidge, it turns out that it also makes the thread scheduling worse on some AMD processors. Furthermore, the L3 cache latency (which is very important in gaming workloads) may increase by as much as 3x, resulting in performance drops varying from 3-5% and even more in gaming.

Known Discrepancy Impact  Fix
Measured and functional L3 cache latency may increase by ~3X.  Applications sensitive to memory subsystem access time may be impacted. Expected performance impact of 3-5% in affected applications, 10-15% outliers possible in games commonly used for eSports. A Windows update is in development to address this issue with expected availability in October of 2021.  
UEFI CPPC2 (“preferred core”) may not preferentially schedule threads on a processor’s fastest core.Applications sensitive to the performance of one or a few CPU threads may exhibit reduced performance. Performance impact may be more detectable in >8-core processors above 65W TDP. A software update is in development to address this issue with expected availability in October of 2021.

Unfortunately, not many details have been provided regarding the affected processors or architectures. I’m guessing it’s likely the Zen+/Zen 2 parts due to the dual-CCX layout. AMD warns of “detectable” performance deficits in 8 core+ SKUs. This means that Windows 11 might have trouble figuring out the fastest thread in chiplet (MCM) based processors. The higher-end Ryzen 9 CPUs from the Zen 3 lineup may also be affected as a result. 

This may be why the performance drops in benchmarking applications on the AMD side and rises while improving on Intel’s. Luckily, both Microsoft and AMD are working on fixes which are expected to be land later this month. As such, we advise against upgrading to Windows 11, especially if you’re on AMD hardware till these bugs are ironed out.

Source: AMD

Areej

Computer Engineering dropout (3 years), writer, journalist, and amateur poet. I started my first technology blog, Techquila while in college to address my hardware passion. Although largely successful, it was a classic example of too many people trying out multiple different things but getting nothing done. Left in late 2019 and been working on Hardware Times ever since.
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