> Why use the operating system's async I/O system when you can simply burn a thread and do blocking I/O? </snark>
mmap isn't non-blocking; page faults are blocking, no different from a read or write to a (non-direct I/O) file using a syscall.
Until recently io_uring literally burned a thread (from a thread pool) for every read or write regular file operation, too. Though now it finally has hooks into the buffer cache so it can opportunistically perform the operation from the same thread that dequeued the command, pushing it to a worker thread if it would need to wait for a cache fault.[1]
[1] Technically the same behavior could be implemented in user space using userfaultfd, but the latency would likely be higher on faults.
mmap isn't non-blocking; page faults are blocking, no different from a read or write to a (non-direct I/O) file using a syscall.
Until recently io_uring literally burned a thread (from a thread pool) for every read or write regular file operation, too. Though now it finally has hooks into the buffer cache so it can opportunistically perform the operation from the same thread that dequeued the command, pushing it to a worker thread if it would need to wait for a cache fault.[1]
[1] Technically the same behavior could be implemented in user space using userfaultfd, but the latency would likely be higher on faults.