Transcript Class4 - UWC Computer Science - University of the Western Cape

```OPERATING SYSTEMS
DESIGN AND IMPLEMENTATION
Third Edition
ANDREW S. TANENBAUM
ALBERT S. WOODHULL
Yan hao (Wilson) Wu
[email protected]
University of the Western Cape
Computer Science Department
The Producer-Consumer Problem
Description: Producer and consumer share a buffer with N slots
l
Producer: produce one data and put into a empty slot at one time.
Block itself if no empty slot available
Consumer: consume one data a time. Block itself if no data
available
Consume
r
Producer
Buffer
0 1 2
In = In % N
In Out
Out = Out % N
N
-1
The Producer-Consumer Problem (1)
Figure 2-13. The producer-consumer problem with a fatal
race condition.
The Producer-Consumer Problem (2)
Figure 2-13. The producer-consumer problem
with a fatal race condition
The Producer-Consumer Problem
Mutual exclusion
Mutex: only one process or thread can operate a slot at a time
Synchronization
empty : indicate how many slots is empty now
full
: indicate how many slots is full now
relationship: when “empty = empty -1”, “full = full +1” (P V)
Monitors (1)
Figure 2-15. A monitor.
Monitors (2)
Figure 2-16. An outline of the producer-consumer problem with monitors. Only one monitor
procedure at a time is active. The buffer has N slots
The Producer-Consumer Problem (3)
Figure 2-14. The producer-consumer problem using semaphores.
Note:when buffer is full, producer will block at the second down with mutex =0 -> Deadlock:
Producer block by empty semaphore, Consumer block by mutex semaphore
The Producer-Consumer Problem (4)
Figure 2-14. The producer-consumer problem
using semaphores.
Message Passing (1)
Figure 2-17. The producer-consumer problem with N messages.
Message Passing (2)
The Dining Philosophers Problem (1)
There are 5 philosophers sitting at a round
table. Between each adjacent pair of
philosophers is a chopstick. In other words,
there are five chopsticks. Each philosopher
does two things: think and eat. The
philosopher thinks for a while, and then
stops thinking and becomes hungry. When
the philosopher becomes hungry, he/she
cannot eat until he/she owns the chopsticks
to his/her left and right. When the
philosopher is done eating he/she puts down
the chopsticks and begins thinking again.
Figure 2-18. Lunch time in the Philosophy Department.
The Dining Philosophers Problem (2)
All the programs continue to run indefinitely but fail to make any progress is called starvation
Figure 2-19. A nonsolution to the dining philosophers problem.
The Dining Philosophers Problem (3)
The Dining Philosophers Problem (4)
The Dining Philosophers Problem (5)
Figure 2-20. A solution to the dining philosophers problem.
The Readers and Writers Problem (1)
The Readers and Writers Problem (2)
Process Behavior (1)
Figure 2-22. Bursts of CPU usage alternate with periods of waiting for
I/O.
(a) A CPU-bound process. (b) An I/O-bound process.
When to Schedule
When scheduling is absolutely required:
1. When a process exits.
2. When a process blocks on I/O, or a semaphore.
When scheduling usually done (though not
absolutely required)
1. When a new process is created.
2. When an I/O interrupt occurs.
3. When a clock interrupt occurs.
Scheduling Algorithms (1)
Scheduling Algorithms (2)
Figure 2-24. An example of shortest job first scheduling.
(a) Running four jobs in the original order.
(b) Running them in shortest job first order.
Shortest Job First
0
P1
P2
P3
P4
4
8
12
16
20
24
Turnaround time: P1 P2 P3 P4
average time:
Three Level Scheduling (1)
Figure 2-25. Three-level
scheduling.
Three Level Scheduling (2)
Criteria for deciding which process to
choose(memory scheduler):
How long has it been since the process was
swapped in or out?
How much CPU time has the process had
recently?
How big is the process?
How important is the process?
Round-Robin Scheduling
Figure 2-26. Round-robin scheduling.
(a) The list of runnable processes.
(b) The list of runnable processes after B uses up its quantum.
Priority Scheduling
Figure 2-27. A scheduling algorithm with four priority classes.
m
∑
i=1
Ci
Pi
<=1
Real time scheduling
Schedulable
A:20ms 10ms
B:50ms 25ms
A
B
0
50
100