A process in operating systems uses different resources and uses resources in following way.
1) Requests a resource
2) Use the resource
2) Releases the resource
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 other process.
Consider an example when two trains are coming toward each other on same track and there is only one track, none of the trains can move once they are in front of each other. Similar situation occurs in operating systems when there are two or more processes hold some resources and wait for resources held by other(s). For example, in the below diagram, Process 1 is holding Resource 1 and waiting for resource 2 which is acquired by process 2, and process 2 is waiting for resource 1.
Deadlock can arise if following four conditions hold simultaneously (Necessary Conditions)
Mutual Exclusion: One or more than one resource are non-sharable (Only one process can use at a time)
Hold and Wait: A process is holding at least one resource and waiting for resources.
No Preemption: A resource cannot be taken from a process unless the process releases the resource.
Circular Wait: A set of processes are waiting for each other in circular form.
Methods for handling deadlock
There are three ways to handle deadlock
1) Deadlock prevention or avoidance: The idea is to not let the system into deadlock state.
2) Deadlock detection and recovery: Let deadlock occur, then do preemption to handle it once occurred.
3) Ignore the problem all together: If deadlock is very rare, then let it happen and reboot the system. This is the approach that both Windows and UNIX take.
As discussed in the previous post, deadlock has following characteristics.
Hold and Wait.
We can prevent Deadlock by eliminating any of the above four condition.
Eliminate Mutual Exclusion
It is not possible to dis-satisfy the mutual exclusion because some resources, such as the tap drive and printer, are inherently non-shareable.
Eliminate Hold and wait
1. Allocate all required resources to the process before start of its execution, this way hold and wait condition is eliminated but it will lead to low device utilization. for example, if a process requires printer at a later time and we have allocated printer before the start of its execution printer will remained blocked till it has completed its execution.
2. Process will make new request for resources after releasing the current set of resources. This solution may lead to starvation.
Eliminate No Preemption
Preempt resources from process when resources required by other high priority process.
Eliminate Circular Wait
Each resource will be assigned with a numerical number. A process can request for the resources only in increasing order of numbering.
For Example, if P1 process is allocated R5 resources, now next time if P1 ask for R4, R3 lesser than R5 such request will not be granted, only request for resources more than R5 will be granted.
Deadlock avoidance can be done with Banker’s Algorithm.
Bankers’s Algorithm is resource allocation and deadlock avoidance algorithm which test all the request made by processes for resources, it check for safe state, if after granting request system remains in the safe state it allows the request and if their is no safe state it don’t allow the request made by the process.
Inputs to Banker’s Algorithm
1. Max need of resources by each process.
2. Currently allocated resources by each process.
3. Max free available resources in the system.
Request will only be granted under below condition.
1. If request made by process is less than equal to max need to that process.
2. If request made by process is less than equal to freely availbale resource in the system.
Total resources in system:
A B C D
6 5 7 6
Available system resources are:
A B C D
3 1 1 2
Processes (currently allocated resources):
A B C D
P1 1 2 2 1
P2 1 0 3 3
P3 1 2 1 0
Processes (maximum resources):
A B C D
P1 3 3 2 2
P2 1 2 3 4
P3 1 3 5 0
Need = maximum resources - currently allocated resources.
Processes (need resources):
A B C D
P1 2 1 0 1
P2 0 2 0 1
P3 0 1 4 0
In the previous post, we have discussed Deadlock Prevention and Avidance. In this post, Deadlock Detection and Recovery technique to handle deadlock is discussed.
1. If resources have single instance:
In this case for Deadlock detection we can run an algorithm to check for cycle in the Resource Allocation Graph. Presence of cycle in the graph is the sufficient condition for deadlock.
In the above diagram, resource 1 and resource 2 have single instances. There is a cycle R1–>P1–>R2–>P2. So Deadlock is Confirmed.
2. If there are multiple instances of resources:
Detection of cycle is necessary but not sufficient condition for deadlock detection, in this case system may or may not be in deadlock varies according to different situations.
Traditional operating system such as Windows doesn’t deal with deadlock recovery as it is time and space consuming process. Real time operating systems use Deadlock recovery.
1. Killing the process.
killing all the process involved in deadlock.
Killing process one by one. After killing each
process check for deadlock again keep repeating
process till system recover from deadlock.
2. Resource Preemption
Resources are preempted from the processes involved in deadlock, preempted resources are allocated to other processes, so that their is a possibility of recovering the system from deadlock. In this case system go into starvation.
The banker’s algorithm is a resource allocation and deadlock avoidance algorithm that tests for safety by simulating the allocation for predetermined maximum possible amounts of all resources, then makes an “s-state” check to test for possible activities, before deciding whether allocation should be allowed to continue.
Following Data structures are used to implement the Banker’s Algorithm:
Let ‘n’ be the number of processes in the system and ‘m’ be the number of resources types.
Allocationi specifies the resources currently allocated to process Pi and Needi specifies the additional resources that process Pi may still request to complete its task.
Banker’s algorithm consist of Safety algorithm and Resource request algorithm
Let Requesti be the request array for process Pi. Requesti [j] = k means process Pi wants k instances of resource type Rj. When a request for resources is made by process Pi, the following actions are taken:
Considering a system with five processes P0 through P4 and three resources types A, B, C. Resource type A has 10 instances, B has 5 instances and type C has 7 instances. Suppose at time t0 following snapshot of the system has been taken: