This is the archived Fall 2013 version of the course.
For the most recent version, see

Class 10: Scheduling

Action Items

For your PS2 demo, your team should be ready with your shell running on one of your laptops at the appointed demo time. If your demo is scheduled with Dave, you should come to my office, Rice 507. Thursday and Friday morning demos with Weilin will be held in Rice 514. Friday afternoon demos with Purnam will be in Rice 204.

The midterm exam will be posted on Thursday, 10 October and due on Monday, 14 October. It will be open resources (except for other humans) and untimed. You should not be surprised if questions from the notes appear on the midterm. It is fine (and encouraged!) to discuss those questions in the course forum before the midterm is posted (but not during the midterm).


What are the two main decisions the supervisor needs to make on each kernel timer interrupt?

Max Ehrmann's "Desiderata":

Go placidly amid the noise and haste, and remember what peace there may be in silence. As far as possible without surrender be on good terms with all persons. Speak your truth quietly and clearly; and listen to others, even the dull and the ignorant; they too have their story. Avoid loud and aggressive persons, they are vexations to the spirit... Exercise caution in your business affairs; for the world is full of trickery... And whether or not it is clear to you, no doubt the universe is unfolding as it should... whatever your labors and aspirations, in the noisy confusion of life keep peace with your soul. With all its sham, drudgery, and broken dreams, it is still a beautiful world. Be cheerful. Strive to be happy.

Why is there a fundamental tradeoff between maximizing resource utilization and fairness in scheduling?

Scheduling Strategies

First Come, First Served (FIFO): processes run to completion, in the order they are started.

Round-Robin: each process gets to run for a given time slice (or until it finishes or is blocked).

What are the advantages and disadvantages of Round-Robin scheduling compred to First Come, First Served?

Pre-emptive Priority Scheduling: each process is assigned a priority (in Unix, higher priority is indicated by a lower priority value) and the scheduler always runs the highest priority process that is ready. If there are multiple equally-high ready to run processes, they are scheduled using round-robin scheduling.

What can go wrong with pre-emptive priority scheduling?

Kernel Timer Interrupts

Wil Thomason has won the kernel timer interrupt challenge!

His code is here:

See the slides for updates on remaining and new challenge problems.