Tips on Writing the Preemptive Minithreads System

Here are some basic tips (as usual, passed on through generations of CSE 451 TAs) for writing the preemptive minithreads system.

• The clock ticks will not be called until you call minithread_clock_init and install an interrupt service routine. This routine will not be called until you enable interrupts with enable_interrupts().
• Once enabled, the clock interrupt handler can be called at ANY TIME (to change the clock period to something less than the 500 microseconds it is currently set to, change the #define in clock.h).
• Consequently, you will need to ensure that everything you do while scheduling threads, updating global structures etc. must be protected in some way, either by disabling interrupts or by using mutex's. However, note that enabling or disabling interrupts too many times, is bad, so avoid doing so. Use some synchronization primitives to protect kernel structures wherever possible.
• You may find it useful to implement spinlocks. Spinlocks are really test-and-set locks where you continually test and set the lock until you've acquired it. This means that the caller blocks until they acquire the lock.
• Remember that semaphore_P and semaphore_V need to run atomically.
• printf's will get messy (unless you put a semaphore around them, but if you're trying to debug semaphores...). Remember to fflush(stdout) after each printf, otherwise things will get even more confusing because the printf's won't necessarily print out stuff when you expect them to.
• Run your program a LOT of times, because each run will be different, and new bugs may manifest themselves.