Constant FUTEX_QUEUE_BUCKETS 

const FUTEX_QUEUE_BUCKETS: usize = 256;

Number of independent hash buckets. Must be a power of two.

256 buckets × 32 slots = 8 192 distinct futex addresses system-wide. Increasing this reduces hot-bucket probability at the cost of 256 × sizeof(SpinLock) of static memory (~16 KiB on x86-64).