[Question] about when to use `mm_sfence`

[dirtysalt]

Maybe quite naive, but why use mm_sfence if size >= L2 cache size? https://github.com/skywind3000/FastMemcpy/blob/master/FastMemcpy.h#L680

And what if L2 cache size (0x200000) is not actually L2 cache size, is there any impact on performance?