Comparison reviews

Tested: AMD CPU Cache Latency Up to 6x Slower in Windows 11

We put a few of the leading AMD Ryzen chips from our Best CPUs for gaming list through several gaming benchmarks and targeted tests to see the extent of the newly-announced Windows 11 bug that reduces performance on AMD’s processors.

As you’ll see in our extensive CPU Benchmarks below, we found that AMD’s L3 latency can be as much as six times higher in Windows 11 compared to Windows 10, and that L3 bandwidth can be up to 12X higher in Windows 10. We didn’t see such severe impacts in our gaming tests, with our biggest Windows 10 vs 11 differences weighing in at 7% in one game title, while others are far more muted. Notably, we tested with the first Windows 11 update that actually made the bug worse.

AMD’s announcement last week that it and Microsoft were jointly investigating two performance-sapping bugs in Windows 11 was an eye-opener, especially since AMD says they impact all Windows 11-compatible AMD processors and can reduce gaming performance by up to 15% in some eSports titles and 3-5% in desktop PC applications. The bug impacts chips with more than eight cores the most, so we also put the 12-core Ryzen 9 5900X that dominates our CPU Benchmark hierarchy to the test.

Frankly, the bugs couldn’t come at a worse time: Intel is on the cusp of launching its seemingly-potent Alder Lake chips that could swing the advantage back in its favor. Making things worse for AMD, Windows 11 has new scheduler optimizations specifically for Alder Lake, so reviewers will use the new operating system for testing. That raises concerns that the bug could possibly result in unfair comparisons.

It’s surprising that two severe bugs squeezed past both AMD and Microsoft’s QA teams — the first reports of the L3 bug popped up in forums several months ago as enthusiasts tested the pre-release Windows 11 builds. 

(Image credit: AMD)

Additionally, AMD reports that its UEFI CPPC2 (Collaborative Power and Performance Control 2) feature, a technology that helps to steer lightly-threaded work to the fastest cores on the chip, also has issues that can impact lightly-threaded applications (like games). AMD says this bug is more detectable in chips with more than eight cores and >65W TDP.

AMD and Microsoft are jointly investigating, and a software update to fix the CPPC2 issue and a Windows Update to remediate L3 latency problems are in the works. AMD says both should arrive in October 2021 (this month). (Notably, these issues are separate from the performance issues surrounding Microsoft’s recommended VBS and HVCI security settings that have caused an outcry. We put those issues to the test last week.)

Basic versions of the fixes are already in current preview builds of Windows, but they aren’t final yet. We’re putting the Windows 11 issues to the test, and we’ll follow up with post-fix tests when the patches come to the general public.

We can see dramatic changes in our cache latency and bandwidth measurements below, but we didn’t see as profound of an impact in the selection of games we tested. However, it is noteworthy that AMD’s advisory states that the bug impacts both the “measured and functional L3 cache latency,” meaning that the results we measure with microbenchmarks are also indicative of the performance received by some applications. Naturally, the higher latency will impact some AMD processors and games/applications more than others.

AMD is coy with the details of just which applications and games are impacted, and the company even adjusted its advisory to remove its original mention of eSports titles. That makes it a bit tough to narrow down our tests to the impacted games, but we still have plenty of game testing to examine, not to mention cache and Infinity Fabric testing. In either case, we’ll examine where things stand right now, and we’ll reevaluate after AMD and Microsoft issue the patches. 

Windows 10 vs Windows 11 Gaming Performance Benchmarks

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