Help confirming Windows bug with Intel big.LITTLE CPUs (>= 12th Gen.)

[kwalisch]

Hi,

I am pretty confident I found a Windows bug (both Windows 10 & 11 are affected) with Intel big.LITTLE CPUs (>= 12th Gen.) that is severely impacting the performance of my primesieve program. However, I am unsure whether all Windows users are affected by this bug or only me. This Windows bug was not present on my PC about a year ago and could potentially have been triggered by me frequently enabling/disabling the efficiency CPU cores of my i5-12600 CPU in the BIOS. The problem is that Windows (GetLogicalProcessorInformationEx from kernel32.dll) does not report the cache sizes of the efficiency CPU cores, instead it only reports the cache sizes of the performance CPU cores.

If you own an Intel big.LITTLE CPUs (>= 12th Gen.) you can confirm the issue using the commands below:

winget install primesieve
primesieve --cpu-info

This should print information about the efficiency CPU cores with 32 KiB L1 data cache but instead prints information about the performance CPU cores with 48 KiB L1 data cache. Below is the incorrect output on my PC:

12th Gen Intel(R) Core(TM) i5-12600K
Logical CPU cores: 16
L1 cache size: 48 KiB
L2 cache size: 1280 KiB
L3 cache size: 20 MiB
L1 cache sharing: 2 threads
L2 cache sharing: 2 threads
L3 cache sharing: 16 threads

Another option for confirming this bug is to open the Windows Subsystem for Linux and print the different L1 data cache sizes recognized by WSL. For 12th & 13th gen. Intel big.LITTLE CPUs this should print 48KiB & 32 KiB but only prints 48KiB on my PC.

for i in {0..64}; do cat /sys/devices/system/cpu/cpu$i/cache/index0/size 2>/dev/null; done | uniq
48 KiB

If confirmed, this Windows bug is not only affecting primesieve but many of CPU Info libraries that are using GetLogicalProcessorInformationEx such as e.g. pytorch/cpuinfo and all of the programs that depend on these libraries. I have also opened opened an issue in the primesieve bug tracker for this: https://github.com/kimwalisch/primesieve/issues/129

[kwalisch]

My Windows bug disappeared after updating to the latest Windows 11 version 22H2. However the WSL2 bug is stil present (but this is less critical).