Deadlock,Basic concept, Deadlock characterization,resource allocation graph

1. Deadlocks
Prepared By: Mr. Sangram A. Patil
Assistant Professor PVPIT,Budhgaon

2.  Definition
Deadlock is a situation where a set of processes are blocked because each
process is holding a resource and waiting for another resource acquired by some
process.
 Example
 System has 2 disk drives.
 P1 and P2 each hold one disk drive and each needs another one.
 Example
 semaphores A and B, initialized to 1
P0
wait (A);
wait (B);
P1
wait(B)
wait(A)

3. System Model
 System consists of
1. Finite number of resources shared between number of competing processes.
Resources are partitioned into several types such as CPU cycles, files and IO
devices(printers and drives) .
Each resource type consisting of several instances.
2. process must request a resource before using it and release a resource after using it.
process can request as many as resources required to it to carry out designated
task
number of resource request cannot exceed.
Process request a resource allocation of any instance of resource satisfies the
request.

4.  Each process utilizes a resource as follows:
 request
 use
 release

5. Deadlock Characterization
 Deadlock can arise if four conditions hold simultaneously.
1. Mutual exclusion: only one process at a time can use a resource.
2. Hold and wait: a process holding at least one resource is waiting to
acquire additional resources held by other processes.
3. No preemption: resources can not be preempted a resource can be
released only voluntarily by the process holding it, after that process has
completed its task.
4. 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.

6. Resource-Allocation Graph
 A set of vertices V and a set of edges E.
 V is partitioned into two types:
 P = {P1, P2, …, Pn}, the set consisting of all active processes in the
system.
 R = {R1, R2, …, Rm}, the set consisting of all resource types in the system.
 request edge – directed edge Pi  Rj
 assignment edge – directed edge Rj  Pi

7. Resource-Allocation Graph (Cont.)

8. Example of a Resource Allocation Graph
• Consists of sets P,R,E
P={P1,P2,P3}
R= {R1,R2,R3,R4}
E={P1->R1,P2->R3,R1->P2,R2->P2,R2->P1,
R3->P3}
• Resource instances:
• Process states
If graph contains no cycle then no process in the
system is deadlocked
If graph does contain a cycle then deadlock may
exists.

9. Resource Allocation Graph With A
Deadlock
 Two cycles
P1->R1->P2->R3->P3->R2->P1
P2->R3->P3->R2->P2

10. Graph With A Cycle But No Deadlock
 There is cycle but no deadlock
P1->R1->P3->R2->P1

11. Basic Facts
 If graph contains no cycles  no deadlock.
 If graph contains a cycle 
 if only one instance per resource type, then deadlock.
 if several instances per resource type, possibility of deadlock.