Availability Vector
2 3
Give 1 2 to P1:
Current allocation matrix
P1 2 5
P2 4 1
P3 1 2
P4 2 0
Current request matrix
P1 0 0
P2 4 3
P3 1 7
P4 5 1
Availability Vector
1 1
P1 finishes, releases 2 5:
Current allocation matrix
P2 4 1
P3 1 2
P4 2 0
Current request matrix
P2 4 3
P3 1 7
P4 5 1
Availability Vector
3 6
Now there is nothing satisfyable. Deadlocked
Scenario 3: Is the system deadlocked if the availability is
2 4
Current allocation matrix
P1 1 3
P2 4 1
P3 1 2
P4 2 0
Current request matrix
P1 1 2
P2 4 3
P3 1 7
P4 5 1
Availability Vector
2 4
Give 1 2 to P1
Current allocation matrix
P1 2 5
P2 4 1
P3 1 2
P4 2 0
Current request matrix
P1 0 0
P2 4 3
P3 1 7
P4 5 1
Availability Vector
1 2
P1 ends, releasing 2 5
Current allocation matrix
P2 4 1
P3 1 2
P4 2 0
Current request matrix
P2 4 3
P3 1 7
P4 5 1
Availability Vector
3 7
Run P3 (releases 1 2):
Current allocation matrix
P2 4 1
P4 2 0
Current request matrix
P2 4 3
P4 5 1
Availability Vector
4 9
Run P2 (releases 4 1):
Current allocation matrix
P4 2 0
Current request matrix
P4 5 1
Availability Vector
8 10
Then P4 can run. System isn't deadlocked.
General approach:
a) look for a process whose requests <= the availability.
we know we can then satisfy the requirements for this process, so we assume it gets its resources required
and terminates. Thus, we can
31. Assuming the operating system detects the system is deadlocked, what can the operating system do to recover from deadlock?
32. What must the banker's algorithm know a priori in order to prevent deadlock?
