there are three kinds of time delays used for long time delays:

• busy:

  •          unsigned long j = jiffies + jit_delay * HZ;
             while (jiffies < j);
             

  • busy waiting
  • scheduler never interrupts a process running in kernelspace
  • if interrupts disabled, jiffies won't get updated, so you lose control of computer
  • see: /proc/jit*
  • testing: read /proc/jitbusy (busy loops for 1 second):
    example: dd if=/proc/jitbusy bs=1 delays 1 sec each time it reads a character.
  • busy loop also locks all processes on GNUX SMP
• sched:

  •          while (jiffies < j) schedule();
             

  • testing: read /proc/jitsched
  • still not optimal: other processes can run, but if it is the only process, it gets scheduled.
  • therefore still keeps cpu at 100% even if nothing else running.
  • can result in waste of electricity, etc..
  • can still be used as a useful quick hack, e.g. to insert in case a bug hangs your computer (insert delay so printk will finish and print to logfile).
• queue:
  • correct way to put a process to sleep in kernel mode.
  • looks at time every time scheduler runs.

  •          struct wait_queue *wait = NULL;
             current->timeout = j;
             interruptible_sleep_on(&wait);
             

  • testing: read /proc/jitqueue