Incomplete /sys/device/system/cpu folder causes software fail to detect hyper-threading

[zhihaoy]

Please fill out the below information:

• Your Windows build number: (Type ver at a Windows Command Prompt)

Microsoft Windows [Version 10.0.17134.590]

• What you're doing and what's happening: (Copy&paste the full set of specific command-line steps necessary to reproduce the behavior, and their output. Include screen shots if that helps demonstrate the problem.)

My machine has Intel i7 8750H, a 6 core CPU, but when using NumPy or simple OpenBLAS programs, they attempt to use 12 threads. The performance is horrible.

• What's wrong / what should be happening instead:

12 is the number of hyper-threads, but for numeric software it has no use, therefore they avoid schedule threads on the same physical core. But in WSL OpenBLAS is not doing that because WSL's /sys/devices/system/cpu/cpu%d/cache/index%d/shared_cpu_map is missing; the cpu folders are incomplete, and OpenBLAS is checking for this https://github.com/xianyi/OpenBLAS/blob/ce3651516f12079f3ca2418aa85b9ad571c3a391/driver/others/init.c#L112

setenv OPENBLAS_NUM_THREADS 6 or lower gets reasonable performance.

[therealkenc]

Ref #3736 which is objectively different but spirituality related in the sense /proc diverge on CPU core/thread details (I gather you are having the "opposite" problem).

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