Operating System Concepts and

Techniques Lecture 15

Deadlock and starvation-1

M. Naghibzadeh

ReferenceM. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.

To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com

Deadlock and StarvationDeadlock is an standstill situation in which two

or more processes are unable to proceed because of each process is waiting for

something from another process and a closed wait-for cycle is formed

For example, if two people one has a pencil and the other one a sheet of paper and each one needs the other’s resource which will never

receive a deadlock occurs

2

Deadlock conditionsThere are three necessary conditions for deadlock to

occur1. Mutual exclusion: there are resources in the system for

which mutual exclusion is enforced to prevent race condition

2. Hold and wait (or partial allocation): A situation in which a process is holding one or more resources and is also waiting for one or more other resources that are not

available for the time being3. Non-preemptable resources: A resource that cannot be

forcibly taken away from the holder process without imposing any intolerable harm to the process

One sufficient condition for deadlock1. The existence of a wait-for closed cycle, when this

happens it means the three necessary conditions also hold

3

Deadlock modelingTo graphically show deadlock opposite

symbols are usedThe following closed wait-

for graph shows a deadlock

4

A process

A resource

A process waiting for a resource

A resource is taken by a process scanner

A

B

CD drive

Solutions for deadlock problem

There are four policies to attack deadlock Ignoring Deadlock

Deadlock Prevention Deadlock Avoidance

Deadlock Detection and Recovery

Ignoring deadlock (or ostrich algorithm): we are not actually suggesting a solution, but rather

erasing a problem statement It does not mean deadlock will not occur

The side effects of such an action could be very harmful

It is not wise to ignore deadlock in sensitive

environments

5

Deadlock PreventionFor deadlock prevention, we can arrange that

at least one of the conditions is not satisfied By not enforcing mutual exclusion, we allow race

conditions to occur; this is not acceptable

Removing the hold-and-wait is possible; do not allow a process to wait for a new resource while holding

other resources (preemptable resources are exempted)

Assign all the needed resource of a process at start or not accept this process

Disadvantages: Inefficient use of resources

Must declare all its needed resources at the start

6

Deadlock Prevention…Removing the non-preemptability

Not possible, some resources are naturally non-preemptable

Removing the circular-wait One method is to use resource ordering

All (non-preemptable) resources are clustered into n ordered classes

If a process is given a resource from class i, it can only request resources from classes of a higher order

Resources possessed by a process must be returned in a descending order

Disadvantages: Inefficient use of resources but much better than

removing hold and wait Must declare all its needed resources at the start

7

Deadlock Prevention…Semi-Proof of deadlock freeness by showing a two-process

caseExample: a Process will need resources 5, 18, 13, 20, 22, 28,

32, 45, during its execution1- Resource number 13 is requested // OS will allocate resources 5,

8, and 13 2- Resource number 22 is requested // OS will allocate resources 20

and 223- Resource number 22 is released // OS will take back this resource4- Resource number 45 is requested // OS will allocate resources 22,

28, 32, and 455- Resource number 28 is released // OS will not take back this

resource because the process still possesses resources 32 and 456- Resources number 32 and 45 are released // OS will take back

resource numbers 45, 32, and 287- Resource number 22 is released // OS will take back this resource

8

Deadlock AvoidanceDeadlock avoidance ensures that deadlock will

not occurDeadline avoidance is based on careful

analysis of every request by examining what will happen in the future if the requested

resource is allocated The resource is allocated if the system will be safe and will not be allocated otherwise. If not allocated,

the requesting process withdraws its request and resubmits it at a later time

One of the most respected deadline avoidance methods is the banker’s algorithm formalized

by Dijkstra

9

Deadlock Avoidance - Banker’s algorithm

Think of a bank branch that has certain amount of cash capital (resources) and an intelligent manager

Its capital will not increaseThe banker grants credit to customers who need a

loan The total sum of all credits is greater than the

bank’s capital, otherwise the system is safe anyways

Customers may claim all their credit at once or in a number of installments

The banker is not allowed to forcibly collect the money from a customer (non-preemptable)

10

Deadlock Avoidance - Banker’s algorithm…

The banker carefully checks every request to make sure by lending the amount he will not

encounter deadlockA deadlock will occur when lending a requested amount leads to a situation in which there is no

way to complete total needs of all customers, no matter what the future ordering of the

requests would beA safe request is one which will not cause

deadlockAn unsafe request will not be granted

11

Banker’s algorithm1. If the number of requested resources is greater than the

number of available resources the request is not valid; return from this algorithm; otherwise continue with the

next step2. Suppose that the required resources are given to the

process (it is not actually given)3. See if there is a process that can complete its job with the

available resources; if there is one, mark it as completed; suppose that all of its resources are taken back and are

added to available resources; repeat this step for unmarked processes until there are no more unmarked

processes or there is no process left that can complete its job with the available resources

4. If all processes are marked, it is safe to allocate the requested resources to the process; otherwise declare

that the allocation is unsafe and return

12

Banker’s algorithmExample:

Suppose the number of simultaneously opened files is restricted to 128

four processes in the system that need to simultaneously open, in the worst case, 55,

95, 10, and 80 files, respectively Total need is 55+95+10+80=240 Next slide is a scenario of requests

13

Banker’s algorithm-example

1. Process p1 needs to open 20 files2. Process p3 needs to open 2 files

3. Process p1 needs to open 15 files4. Process p4 needs to open 25 files

5. Process p3 needs to open 3 files6. Process p2 needs to open 47 files

7. Process p4 needs to open 7 files8. Process p3 needs to open 5 files

9. Process p2 needs to open 33 files10. Process p1 needs to open 20 files11. Process p4 needs to open 30 files12. Process p4 needs to open 18 files13 Process p2 needs to open 15 files

14

Banker’s algorithm-example…

15

Step no. Process Request Given,so far

Will needin the future

Available,overall

Safety

0 128 Safe

1 P1 20 0 35 128 Safe

2 p3 2 0 8 108 Safe

3 p1 15 20 20 106 Safe

4 p4 25 0 55 91 Safe

5 p3 3 2 5 66 Safe

6 p2 47 0 48 63 Safe

7 p4 7 25 48 16 Unsafe

SummaryControlled usage of shared resources, by

guaranteeing mutual exclusion to prevent race condition, causes an important undesirable

side effects called deadlockThere are four different policies for attacking

deadlocks: ignoring deadlock, deadlock prevention, deadlock avoidance, and deadlock

detection and recoveryThe first policy erases the problem statement

altogetherThe second and third policy forestall deadlock

from happening, even though the system usually pays a high price for this

16

17

Find outThe exact difference between deadlock

prevention and deadlock avoidance

As many non-preemptable resources as you can

As many preemptable resources as you can

Whether there could be a closed circular wait which includes main memory when memory

management is virtual memory

There cannot be a deadlock with only one process

Other policies for preventing deadlock, like wait-wound

18

Any questions?