Dijkstra's Bankers Algorithm

(encountered before in the process scheduling model)

• Process need = Maximum quantity of each resource that process requires during its lifetime.
• Resource Manager (Banker) = needs prior knowledge of all process claims
• Process requests ONE resource at a time
• Current process request is allowed if :
  the request plus current usage of the processes is less than the need
  after allocating the request there exists a sequence in which all processes can run to completion even if they demand their full claim - SAFE STATE
• Users must guarantee to release resources in finite time
• Resource manager guarantees to allocate resources in finite time Process

Example : 12 tape drives, 3 users

User		 Current Loan 		 Max Needed

1		1		4

2		4		6

3		5		8

Available drives = 2
Allocate 2 drives to user 2
user 2 completes and releases 6 drives
allocate 3 drives each to users 1 and 3
ALL users run to completion = SAFE STATE

User		 Current Loan 		 Max Needed

1		8		10

2		2		5

3		1		3

Available = 1
If grant last drive to any user may get deadlock = UNSAFE STATE
Existence of an UNSAFE state DOES NOT imply existence or eventual existence of deadlock, e.g. user 1 may release current loan before user 1 and 2 request extra.