Deadlock is a situation that occurs in computing systems where two or more processes or threads are blocked, waiting for each other to release the resources necessary to continue their execution. Deadlocks are a common problem in operating systems, and they can cause system crashes or other unwanted behavior. Therefore, various techniques have been devised over the years to handle deadlocks efficiently.
In this article, we'll take a closer look at deadlock handling methods in operating systems, along with code examples.
Deadlock Prevention
Deadlock prevention is a technique that involves designing the system in such a way that deadlocks cannot occur in the first place. Here are a few ways to achieve this –
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Resource Ordering: This method involves defining an order in which resources should be acquired. In this way, each process will request resources in a specific order, which will prevent cycles from forming. For example, if there are two resources, A and B, then all processes must acquire resource A before they request resource B.
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Resource Allocation Graph: This method involves representing the system as a graph where vertices represent processes and resources. Edges represent requests and allocations. If there is a cycle in this graph, then there is a deadlock. If a request would create a cycle in the graph, then the request is denied.
Here is an implementation of Resource Ordering.
void request_resources(int process_id, int resource_id){
//If the current request will break the order of resource allocation, deny the request
if(check_ordering(process_id, resource_id) == 0){
printf("Request Denied. Cannot break ordering");
return;
}
//If the order is maintained, grant the request.
grant_resources(process_id,resource_id);
}
Deadlock Avoidance
Deadlock avoidance is a technique that involves checking the system state before executing a request to see if it will cause a deadlock. Here are a few ways to achieve this –
- Banker's Algorithm: The Banker's algorithm is a resource allocation and deadlock avoidance algorithm. The algorithm is used to determine if a request for resources can be granted safely. If the request would result in an unsafe state (a state where deadlock is possible), the request is denied.
Here is an implementation of Banker's Algorithm –
int available[N];
int max[N][M];
int need[N][M];
int allocation[N][M];
void is_safe(){
int count = 0;
int safe_sequence[N];
int work[M];
for(int i=0;i<M;i++){
work[i] = available[i];
}
int finish[N] = {0};
while(count < N){
int found = 0;
for(int i=0;i<N;i++){
if(finish[i] == 0){
int j=0;
for(j=0;j<M;j++){
if(need[i][j] > work[j]){
break;
}
}
if(j==M){
for(int k=0;k<M;k++){
work[k] += allocation[i][k];
}
safe_sequence[count++] = i;
finish[i] = 1;
found = 1;
}
}
}
if(found == 0){
printf("System is not in safe state");
return;
}
}
printf("System is in safe state. Safe sequence is: ");
for(int i=0;i<N;i++){
printf("%d ",safe_sequence[i]);
}
}
Deadlock Detection and Recovery
Deadlock detection and recovery is a technique that involves periodically checking the system state for deadlocks and then recovering from them if they occur. Here are a few ways to achieve this –
-
Timeout: This method involves setting a timeout for each process or thread, after which the process or thread will give up its resources and try again later. If a process or thread is stuck waiting for resources for too long, it is assumed to be deadlocked and is terminated.
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Resource Preemption: Resource preemption involves forcibly taking resources away from a process or thread. This can be a complex process because it requires identifying the resources being used and then reallocating them to other processes or threads.
Here is an implementation of Timeout based deadlock detection –
int resource[N][M];
int request[N][M];
int finished[N];
void check_deadlock(){
int count=0;
int deadlock = 0;
while(count < N){
int possible_deadlock = 1;
for(int i=0;i<M;i++){
if(request[count][i] > resource[count][i]){
possible_deadlock = 0;
break;
}
}
if(possible_deadlock && !finished[count]){
finished[count] = 1;
for(int i=0;i<M;i++){
resource[count][i] += request[count][i];
}
count = 0;
}else{
count++;
}
if(count == N){
for(int i=0;i<N;i++){
if(!finished[i]){
deadlock = 1;
break;
}
}
}
if(deadlock == 1){
printf("Deadlock Detected");
}else{
printf("No Deadlock Detected");
}
}
}
Conclusion
Deadlock handling is an essential aspect of operating system design. Various techniques can be used to prevent, avoid, detect, and recover from deadlocks. This article has explored these methods in detail, along with code examples. By understanding deadlock handling methods, developers can create more robust and reliable operating systems that are less prone to crashes and other issues.
Deadlock handling is a crucial aspect of operating system design as it affects the overall reliability and efficiency of the system. It is, therefore, essential to handle deadlocks efficiently to minimize the impact on system performance and user experience.
Deadlock Prevention
Deadlock prevention techniques aim to prevent deadlocks from occurring in the first place. This is achieved by designing the system in such a way that cycles cannot form. One such technique is resource ordering, where an order is defined in which processes should acquire resources. This ensures that processes only request resources in a specific order, thereby preventing cycles from forming.
Resource allocation graphs are another commonly used method for deadlock prevention. In this approach, the system is modeled as a graph in which resources and processes are represented as vertices, while requests and resource allocations are represented as edges. Checks are performed to detect cycles in the graph. If a request would create a cycle in the graph, the request is denied.
Deadlock Avoidance
Deadlock avoidance focuses on preventing deadlocks by checking the system state before executing requests that might cause a deadlock. The objective is to ensure that requests will not result in a deadlock state. Banker's algorithm is an example of a deadlock avoidance technique. The algorithm checks for safety by analyzing the available resources and the outstanding documents needed. If a request will not result in an unsafe state, the request is granted.
Deadlock Detection
Deadlock detection is a technique that involves periodically checking the system state for deadlocks. If a deadlock is detected, measures are taken to recover the system. Timeout is a common technique for deadlock detection. In this approach, each process is given a specific period to complete its task. If a process doesn't release its resources within the specified period, it is terminated, and its resources are reclaimed.
Other methods used in deadlock detection include the use of wait-for graphs, where a graph is formed consisting of all the resources allocated and waiting for the requests of each process. If a cycle is detected in the graph, a deadlock exists. Another technique for deadlock detection is resource preemption, where resources are forcibly taken from one process and allocated to another in a bid to break the deadlock.
Deadlock Recovery
Deadlock recovery techniques focus on recovering the system from a deadlock state. When a deadlock is detected, measures are taken to rectify the situation. Several methods can be used to recover from a deadlock, depending on the specific scenario. For instance, one can release all allocated resources and request them again or allocate additional resources.
Moreover, processes can be terminated when a deadlock is detected, and resources can be reclaimed and allocated to processes to prevent future deadlocks. Other methods like resource preemption can be applied to recover from deadlocks.
Conclusion
Effective deadlock handling is crucial in preventing system crashes, improving system performance, and enhancing user experience. Different techniques can be used to handle deadlocks, including prevention, avoidance, detection, and recovery. The best approach depends on several factors, including the system requirements and the specific scenario. With the right approach to deadlock handling, developers can create more robust systems that can handle deadlocks efficiently.
Popular questions
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What is deadlock prevention, and what are some common techniques used in preventing deadlocks?
Answer: Deadlock prevention is a technique that prevents deadlocks from occurring by designing the system in such a way that cycles cannot form. Some common techniques used in preventing deadlocks include resource ordering and resource allocation graphs. -
What is the Banker's algorithm, and how does it prevent deadlocks?
Answer: The Banker's algorithm is a deadlock avoidance technique that checks the system state before executing requests that might cause a deadlock. It analyses the available resources and the outstanding documents needed to ensure that requests will not result in an unsafe state. -
How can timeouts be used for deadlock detection?
Answer: Timeout is a common technique used for deadlock detection. In this approach, each process is given a specific period to complete its task. If a process doesn't release its resources within the specified timeout period, it is terminated, and its resources are reclaimed. -
How can resource allocation graphs be used to prevent deadlocks, and what happens when a cycle is detected in the graph?
Answer: Resource allocation graphs are a common method used in deadlock prevention. In this approach, resources and processes are represented as vertices, while requests and resource allocations are represented as edges. When a cycle is detected in the graph, it indicates that a deadlock exists, and the request is denied. -
What is resource preemption, and how can it be used for deadlock recovery?
Answer: Resource preemption is a technique that forcibly takes resources away from one process and reallocates them to another. In the event of a deadlock, resource preemption can be used to recover the system by reallocating resources from one process to another, thereby breaking the deadlock.
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