Week 6
Exam Reminder
- Exam next week
- review session next Mon, Oct 10
- take-home: Mon
- hand-in: Wed
Lecture Notes
Semaphores
- Two operations:
- wait(s)
- Traditional: P(s)
- Definition:
< await (s > 0) s = s - 1; >
- signal(s)
- Traditional: V(s)
- Definition:
< s = s + 1; >
- wait(s) and signal(s) work the same for binary and counting (general) semaphores
- binary semaphore: value is always either
1
or0
- general semaphore: value is nonnegative
Interleavings
- Formula(number of possible interleavings):
- # interleavings = $\frac{(nk)!} {(k!)^n}$
- where n = # processes and k = # instructions / process
Class exercises
- The Bear and the Honeybees (handout) PDF
- split binary semaphores
- where for binary semaphores s1 and s2:
0 <= s1+s2 <= 1
- technique: pass the baton
Semaphores
- Ben-Ari Ch 6
- slides: PDF
- begin slide 105