The first Cumulative Update for Windows 11 has worsened the L3 latency issue on AMD Ryzen CPUs rather than fixing it. Unofficially dubbed “patch Tuesday”, Microsoft has scheduled Win 11 updates for the second Tuesday of every month. According to results posted by TPU, the L3 cache latency before the first Tuesday update was 17.1 ns. After the update, this figure grows (by more than 50%) to 31.9 ns!
It’s worth noting that the CPU tested here is the older 2nd Gen Ryzen 7 2700X rather than one of the newer additions to the Ryzen family. It’d be interesting to see if the Zen 2 and Zen 3 parts see the same deterioration in cache latency as Zen+. A while back, it was reported that AMD processors suffer from multiple performance degradation issues in Windows 11. In addition to affecting the thread scheduling on some Ryzen processors (those with multiple active dies), the L3 cache latency (which is very important in gaming workloads) can also increase by as much as 3x, resulting in performance drops varying from 3-5% and even more in gaming.
|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.|
AMD has released a statement on social media that patches for the two issues have been developed, and will be released soon. The patch for the Preferred Cores (UEFI-CPPC2) bug will be released on the 21st of October, while the L3 cache latency bug will be fixed via a future Windows Update scheduled to roll out on the 19th of October.