Problems with Userspace Threads

• Problems:

  • only one kernel thread, so a process can only use one CPU on MP boxes

  • non-blocking I/O implemented via O_NONBLOCK

    • file-level flag: shared between processes after fork(), etc

    • whether a process uses the thread library affects other processes

  • some blocking syscalls can't be polled for: open(FIFO)

  • some weren't: kevent(), socket timeouts

  • some block anyway: disk I/O