CMPSCI 377 Operating Systems Fall 2008
Lecture 13: October 16
Lecturer: Prashant Shenoy Scribe: Shashi Singh
13.1 Banker’s Algorithm
The Banker’s algorithm is a resource allocation and deadlock avoidance algorithm developed by Edsger
Dijkstra that tests for safety by simulating the allocation of pre-determined maximum possible amounts
of all resources, and then makes a ”safe-state” check to test for possible deadlock conditions for all other
pending activities, before deciding whether allocation should be allowed to continue. The Banker’s algorithm
is run by the operating system whenever a process requests resources. The algorithm prevents deadlock by
denying or postponing the request if it determines that accepting the request could put the system in an
unsafe state (one where deadlock could occur).
13.1.1 Resources
For the Banker’s algorithm to work, it needs to know three things:
1. How much of each resource each process could possibly request
2. How much of each resource each process is currently holding
3. How much of each resource the system has available
Some of the resources that are tracked in real systems are memory, semaphores and interface access.
13.1.2 Safe and Unsafe States
A state is considered safe if it is possible for all processes to finish executing (terminate). Since the system
cannot know when a process will terminate, or how many resources it will have requested by then, the
system assumes that all processes will eventually attempt to acquire their stated maximum resources and
terminate soon afterward. This is a reasonable assumption in most cases since the system is not particularly
concerned with how long each process runs (at least not from a deadlock avoidance perspective). Also, if a
process terminates without acquiring its maximum resources, it only makes it easier on the system. Given
that assumption, the algorithm determines if a state is safe by trying to find a hypothetical set of requests
by the processes that would allow each to acquire (one-by-one) its maximum resources and then terminate
(returning its resources to the system). Any state where no such set exists is an unsafe state. In a safe
state, at least one process should be able to acquire its maximum possible set of resources, and proceed to
termination.
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