Class 10
CS 439
14 Feburary 2013

On the board
------------

1. Last time
2. mutexes/ spinlocks
3. scheduling
    --intro

---------------------------------------------------------------------------

1. Last time

    --threads

    --classification: user v kernel / cooperative-v-preemptive

    --implementation
        swtch()

    --NOTE: we covered the implementation of swtch() when threads
    yield voluntarily

2. mutexes and spinlocks

    --LOCK prefix

    --disable interrupts (only works on uniprocessor system)

    --xchg / spinlocks / etc.

    --ask how we can implement this.


3. Scheduling intro

    A. When do scheduling decisions happen?

                                                   exit (iv)
                                                   |-------->[terminated]
              admitted         (ii) interrupt      |
	[new] -->   [ready]   <-------------- [running]
	              ^        --------------->      |
       I/O or event    \       scheduler dispatch    |
     completion (iii)   \                 ___________|
                         \               /
			      [waiting] v  I/O or event wait (i)


	scheduling decisions take place when a process:

	    (i) Switches from running to waiting state 
	    (ii) Switches from running to ready state 
	    (iii) Switches from waiting to ready 
	    (iv) Exits 

	preemptive scheduling: at all four points

	nonpreemptive scheduling: at points (i), (iv), only (this is the
	definition of nonpreemptive scheduling)

    B. What are metrics and criteria?
    
	--system throughput
	    # of processes that complete per unit time

	--turnaround time
	    time for each process to complete

	--response time
	    time from request to first response (e.g., key press to
	    character echo, not launch to exit)

	--fairness

	    different possible definitions:

		--freedom from starvation

		--all users get equal time on CPU

		--highest priority jobs get most of CPU

		--etc.

	    [often conflicts with efficiency. true in life as well.]

	the above are affected by secondary criteria:

	--CPU utilization (fraction of time CPU is actually working)

	--waiting time (time each process waits in ready queue; this is
	pretty much the same thing as response time)