Kcd226 Sistem Operasi Lecture05

Lecture 5

Deadlocks
5.1. Resource
5.2. Introduction to deadlocks
5.3. The ostrich algorithm
5.4. Deadlock detection and recovery

Sistem Operasi http://fasilkom.narotama.ac.id/
1

Resources

• Examples of computer resources
– printers
– tape drives
– tables
• Processes need access to resources in reasonable order
• Suppose a process holds resource A and requests
resource B
– at same time another process holds B and requests A
– both are blocked and remain so

2

Resources (1)

• Deadlocks occur when …
– processes are granted exclusive access to devices
– we refer to these devices generally as resources
• Preemptable resources
– can be taken away from a process with no ill effects
• Nonpreemptable resources
– will cause the process to fail if taken away

3

Resources (2)

• Sequence of events required to use a resource
1. request the resource
2. use the resource
3. release the resource

• Must wait if request is denied
– requesting process may be blocked
– may fail with error code
4

Introduction to Deadlocks

• Formal definition :
A set of processes is deadlocked if each process in the set is waiting
for an event that only another process in the set can cause
• Usually the event is release of a currently held resource
• None of the processes can …
– run
– release resources
– be awakened

5

Four Conditions for Deadlock

1. Mutual exclusion condition
• each resource assigned to 1 process or is available
2. Hold and wait condition
• process holding resources can request additional
3. No preemption condition
• previously granted resources cannot forcibly taken away
4. Circular wait condition
• must be a circular chain of 2 or more processes
• each is waiting for resource held by next member of the
chain
6

Deadlock Modeling (2)
• Modeled with directed graphs

– resource R assigned to process A
– process B is requesting/waiting for resource S
– process C and D are in deadlock over resources T and U
7


Strategies for dealing with Deadlocks
1. just ignore the problem altogether
2. detection and recovery
3. dynamic avoidance
• careful resource allocation
4. prevention
• negating one of the four necessary conditions

8

A B C

How deadlock occurs 9


(o) (p) (q)

How deadlock can be avoided 10

The Ostrich Algorithm
• Pretend there is no problem
• Reasonable if
– deadlocks occur very rarely
– cost of prevention is high
• UNIX and Windows takes this approach
• It is a trade off between
– convenience
– correctness

11

Detection with One Resource of Each Type (1)

• Note the resource ownership and requests
• A cycle can be found within the graph, denoting deadlock

12


Data structures needed by deadlock detection algorithm

13


An example for the deadlock detection algorithm
14

Recovery from Deadlock (1)

• Recovery through preemption
– take a resource from some other process
– depends on nature of the resource
• Recovery through rollback
– checkpoint a process periodically
– use this saved state
– restart the process if it is found deadlocked

15

Recovery from Deadlock (2)

• Recovery through killing processes
– crudest but simplest way to break a deadlock
– kill one of the processes in the deadlock cycle
– the other processes get its resources
– choose process that can be rerun from the beginning

16

Question/Discussion

17

Kcd226 Sistem Operasi Lecture05

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