hm. in linux you have MAP_POPULATE which forces a prefetch where macos relies on page faults and lazy loading. if MADV_WILLNEED doesn't help, maybe readv to vector read directly or mmap+writev (write to a dummy fd, with an iovec for each page, hopefully resulting in a one-syscall-big-pagefault for mmap.) maybe also experiment with a loop that just reads one byte from each relevant page after mmap but before real computation?