deadlock problem & charatrization

1. ORIENTAL INSTITUTE OF SCIENCE AND TECHNOLOGY,
BHOPAL
Department of Information Technology
SELF STUDY
Presentation
On
Deadlock
Presented By:-
DEEPAK YADAV
0105IT131027
Semester V09/03/15 1
Oriental Institute Of Science & Technology, Bhopal

2. The Deadlock Problem
EXAMPLES:

"It takes money to make money".

You can't get a job without experience; you can't get
experience without a job.
BACKGROUND:
The cause of deadlocks: Each process needing what another
process has. This results from sharing resources such as
memory devices, links.
Under normal operation, a resource allocations proceed like
this:

Request a resource (suspend until available if necessary.

Use the resource.

Release the resource.
09/03/15
2Oriental Institute Of Science & Technology,
Bhopal

3. Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.

Mutual exclusion: only one process at a time can use a
resource

Hold and wait: a process holding at least one resource is
waiting to acquire additional resources held by other processes

No preemption: a resource can be released only voluntarily by
the process holding it, after that process has completed its task
 Circular wait: there exists a set {P0, P1, …, Pn} of waiting
processes such that P0 is waiting for a resource that is held by
P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is
waiting for a resource that is held by Pn, and Pn is waiting for a
resource that is held by P0.09/03/15 3Oriental Institute Of Science & Technology,
Bhopal

4. Methods for Handling Deadlocks

Ensure that the system will never enter a
deadlock state – deadlock prevention

Allow the system to enter a deadlock state
and then recover

Ignore the problem and pretend that
deadlocks never occur in the system; used
by most operating systems, including UNIX
09/03/15 4Oriental Institute Of Science & Technology,
Bhopal

5. Do not allow one of the four conditions to occur.
Mutual exclusion:
a) Automatically holds for printers and other non-sharables.
b) Shared entities (read only files) don't need mutual exclusion
(and aren’t susceptible to deadlock.)
c) Prevention not possible, since some devices are intrinsically
non-sharable.
Hold and wait:
a) Collect all resources before execution.
b) A particular resource can only be requested when no others
are being held. A sequence of resources is always collected
beginning with the same one.
c) Utilization is low, starvation possible.
Deadlock Prevention
09/03/15 5Oriental Institute Of Science & Technology,
Bhopal

6. Example (Cont.)

The content of the matrix Need is defined to be Max –
Allocation
Need
A B C
P0 7 4 3
P1 1 2 2
P2 6 0 0
P3 0 1 1
P4 4 3 1
 The system is in a safe state since the sequence < P1,
P3, P4, P2, P0> satisfies safety criteria
09/03/15 6
Oriental Institute Of Science & Technology,
Bhopal

7. NOTE: All deadlocks are unsafe, but all unsafes are
NOT deadlocks.
SAFE
DEADLOCK
UNSAFE
Only with luck will
processes avoid
deadlock.
O.S. can avoid
deadlock.
Deadlock Avoidance
09/03/15 7Oriental Institute Of Science & Technology,
Bhopal

8. 
Single instance of a resource type

Use a resource-allocation graph

Multiple instances of a resource
type

Use the banker’s algorithm
Avoidance algorithms
09/03/15 8Oriental Institute Of Science & Technology,
Bhopal

9. Recovery from Deadlock:
Process Termination

Abort all deadlocked processes

Abort one process at a time until the deadlock cycle is
eliminated

In which order should we choose to abort?

Priority of the process

How long process has computed, and how much
longer to completion

Resources the process has used

Resources process needs to complete

How many processes will need to be terminated

Is process interactive or batch?
09/03/15 9Oriental Institute Of Science & Technology,
Bhopal

10. Recovery from Deadlock:
Resource Preemption

Selecting a victim – minimize cost

Rollback – return to some safe state,
restart process for that state

Problem: starvation – same process may
always be picked as victim, include number
of rollback in cost factor
09/03/15 10Oriental Institute Of Science & Technology,
Bhopal

11. 09/03/15 11Oriental Institute Of Science & Technology,
Bhopal