% Class11: Smarter Scheduling
% David Evans
% 2013-10-03
## 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 there is
another team scheduled after you and you wasted time by not having
things ready to go at your appointed start time, we will cut your demo
short so the next team does not need to wait. 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.
Start making contributions to the Norvig Numbers. You
should claim the number you want to work on by posting in the
forum, but your contribution should be a link to a github repository
containing your code and a README file that explains why the number is
important and how your code estimates it. Everyone should have made
some meaningful contribution to this by the end of Thanksgiving break,
but if you use a sensible scheduling algorithm you will do it sooner!
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).
## Revised Course Schedule
PS3 (zhtta) will be due by 8:59pm on Monday, 28 October (not 10 October
as originally scheduled).
There is no PS4. After PS3, you will work on an open ended project due
the last day of class (5 December). **Note:** if you already have a
great idea for a project that will be more worthwhile for you than PS3,
you can make a case for doing that instead of PS3 also.
## Peter Norvig's Numbers Every Programmer Should Know
Original essay: [Teach Yourself Programming in Ten
Years](http://norvig.com/21-days.html) (from 2001)
> _So it may be that 10,000 hours, not 10 years, is the magic
number. (Henri Cartier-Bresson (1908-2004) said "Your first 10,000
photographs are your worst," but he shot more than one an hour.)
Samuel Johnson (1709-1784) thought it took even longer: "Excellence in
any department can be attained only by the labor of a lifetime; it is
not to be purchased at a lesser price." And Chaucer (1340-1400)
complained "the lyf so short, the craft so long to lerne." Hippocrates
(c. 400BC) is known for the excerpt "ars longa, vita brevis", which is
part of the longer quotation "Ars longa, vita brevis, occasio
praeceps, experimentum periculosum, iudicium difficile", which in
English renders as "Life is short, [the] craft long, opportunity
fleeting, experiment treacherous, judgment difficult." Although in
Latin, ars can mean either art or craft, in the original Greek the
word "techne" can only mean "skill", not "art"._
Approximate timing for various operations on a typical PC:
| Operation | Time |
| --- | --- |
| execute typical instruction | 1/1,000,000,000 sec = 1 ns |
| fetch from L1 cache memory | 0.5 ns |
| branch misprediction | 5 ns |
| fetch from L2 cache memory | 7 ns |
| Mutex lock/unlock | 25 ns |
| fetch from main memory | 100 ns |
| send 2K bytes over 1Gbps network | 20,000 ns |
| read 1MB sequentially from memory | 250,000 ns |
| fetch from new disk location (seek) | 8,000,000 ns |
| read 1MB sequentially from disk | 20,000,000 ns |
| send packet US to Europe and back | 150 ms = 150,000,000 ns |
To make a continually updated and useful list:
- Define and justify a number that you think it is important for
programmers to know.
- Write a program that can estimate that number (e.g., Wil's _Kernel
Timer Interval_ program)
You may work alone, or in teams as large as you want (so long as the
value of what you do scales with at least the square root of the team
size).
## Premptive Priority Scheduling
- Always run the _highest priority process_ that is ready to run
- _Round-robin_ schedule among equally high, ready to run, highest-priority
processes
What are the advantages and disadvantages of preemptive priority
scheduling?
#
What really happened on Mars? [Glen Reeve's account](http://research.microsoft.com/en-us/um/people/mbj/mars_pathfinder/authoritative_account.html)
#
Is priority inheritance a good solution?
#
## Lottery Scheduling
Carl Waldspurger's PhD dissertation: [_Lottery and Stride Scheduling:
Flexible Proportional-Share Resource
Management_](http://www.waldspurger.org/carl/papers/phd-mit-tr667.pdf)
(If you are considering going to grad school and want a good model of
what a PhD involves, this is the best example I know of a dissertation
worth reading.)
> _Number one will be your dissertation. Almost everyone hates their
dissertation by the time they're done with it. The process inherently
tends to produce an unpleasant result, like a cake made out of whole
wheat flour and baked for twelve hours. Few dissertations are read with
pleasure, especially by their authors.
> But thousands before you have suffered through writing a
dissertation. And aside from that, grad school is close to
paradise. Many people remember it as the happiest time of their
lives. And nearly all the rest, including me, remember it as a period
that would have been, if they hadn't had to write a dissertation._
> Paul Graham,
[Undergraduation](http://www.paulgraham.com/college.html). (Bonus
sticker to anyone who can tell me who his footnote for this is about.)
## Stride Scheduling
% \clearpage