It also performs badly due to to all the bitmask twiddling, and copying of those bitmasks to the kernel and back.
If you want to select on file descriptor 1000, the bitmask has to include zeros for descriptors 0 through 999. Those get copied to the kernel and iterated over dutifully.
If you want to select on file descriptor 1000, the bitmask has to include zeros for descriptors 0 through 999. Those get copied to the kernel and iterated over dutifully.