1. The Banker’s Algorithm for A Single Resource
Granting the request leads to an unsafe state?
True: deny the request
False: carry the request out
Has
Max A 6 B 5 C 4 D 7
Has
Max A 1 6 B 5 C 2 4 D 7
Has
Max A 1 6 B 2 5 C 4 D 7
Free: 10
Free: 2
Free: 1
Safe
Safe
Unsafe!

2. Dealing With Multiple Resources
Tape drives
Scanners
CD ROMs
Process
Plotters
Tape drives
Scanners
CD ROMs
Process
Plotters
E=( )
A=( )
P=( )
E=A+P A 3 1 B C D E A 1 B 2 C 3 D E
C: Resources assigned
R: Resources still needed

3. Algorithm
Find a row (process) in R whose unmet resource needs are all smaller than or equal to A
No such row  the system will possibly deadlock (unsafe)
Mark the process as terminated, add all its resources to the A vector
Repeat 1 and 2 until either all process are marked terminated (safe state), or until a deadlock occurs (unsafe state)

4. Problem of Deadlock Avoidance
Wonderful algorithm in theory
Essentially useless in practice
Can you tell in advance what your maximum resource need for running your browser is?
Number of process is dynamic
Resources can break
Information about future requests is unknown!
Kind of familiar?

5. Outline Resources and deadlock The ostrich algorithm
Deadlock detection and recovery
Deadlock avoidance
Deadlock prevention
Related issues

6. Deadlock Avoidance Vs. Prevention
Avoidance (a police directing traffic)
The system dynamically consider every request and decides if it is safe to grant it at this point
Allow more concurrency
Prevention (a red traffic light)
Constrain how requests for resources can be made and how they are handled
Ensure at least one of the necessary conditions for deadlock cannot hold.
Four necessary conditions for a deadlock: mutual exclusion, hold and wait, no preemption and circular wait

7. Eliminate Mutual Exclusion
Spooling technique
Good for printer, plotter, etc.
Only printer daemon can access the physical printer and it requires no other resource.
Not all resources can be spooled. Examples?
Competition on spooling space
Daemon only prints after the complete output file is available.
General idea to reduce deadlocks
Assign a resource only when it is absolutely necessary
As few processes as possible may actually claim the resource

8. Break Hold And Wait Condition
Request all resources before execution
Resource requirement is unclear before execution
Inefficient usage of resources
Used in some mainframe batch systems
Much more instances of each I/O device
Many jobs are CPU-bound
Improvement: when requesting a new resource, temporarily release all resource, then request all at once

9. Destroy Circular Wait One process at most one resource at any moment
Must release the first one when requiring the second one.
How to copy a huge file from a tape to a printer?
Global numbering of all resources
Processes request resources in numerical order
Hard to find an order good for everyone
Different systems may have different orders  a program may have to be modified before it runs!
RAG i A j B
1. Scanner Plotter 3. Tape driver 4. CD-ROM driver OK! illegal!
i<j, B not allowed to require i i>j, A not allowed to require j

10. Approaches to Deadlock Prevention
Condition
Approach
Mutual exclusion
Spooling
Hold and wait
Request all resources before execution
No preemption
Take resources away
Circular wait
Order resources numerically

11. Outline Resources and deadlock The ostrich algorithm
Deadlock detection and recovery
Deadlock avoidance
Deadlock prevention
Related issues

12. Two-phase Locking in Databases
An excellent special purpose algorithm
Scenario: lock on several records, update all of them and release.
Phase I: try to lock all the records it needs, one at a time
If succeed (get all the records), go to Phase II
Else, release all locks, start Phase I again
Problems? Refer to dining philosophers problem
Phase II: do updates, release the locks
Not a general strategy
No process pending/start over in real-time system
Many operations cannot be safely repeated.

13. Starvation Deadlock free is not enough for OS
No starvation: make sure each process getting service.
Example: allocation of CPU
Shortest job first: maximize throughput
Long job may starve to death in a busy system!
First-come-first-serve: fair, no starvation
Lower throughput

14. Summary
Deadlock: all processes are blocked and no progress can be made
Mutual exclusion
Hold and wait
No preemption
Circular wait
Deadlock detection & recovery, avoidance, and prevention