A cycle detection algorithm For each node N in the graph Initialize L to the empty list and designate all edges as unmarked Add the current node to L and check to see if it appears twice.
Short of killing the process that is preempted, how might this problem be addressed. Either of the methods described above can lead to starvation if a process requires one or more popular resources. Circular waiting strings these length-2 paths together into a cycle. The text describes the bankers algorithm but then points out that it is essentially impossible to implement because of this assumption.
For example, in the keyboard-and-display situation, each process could declare that it may want at some point 1 keyboard and 1 display.
Graph theorists have developed a number of algorithms to detect cycles in a graph. The first approach may be severely limiting for example, an interactive program can't get exclusive access to the keyboard and the display at the same time.
What happens to a process if resources it is holding are preempted. This can be wasteful of system resources if a process needs one resource early in its execution and doesn't need some other resource until much later.
If such a process is found, then some of their resources may get preempted and added to the list of resources for which the process is waiting.
Dots inside the resource nodes indicate specific instances of the resource. Processes may or may not correspond to full-blown processes as used elsewhere. R1 is now the current node so we add that to L, checking to confirm that it is not already in L. If a resource category contains more than one instance, then the presence of a cycle in the resource-allocation graph indicates the possibility of a deadlock, but does not guarantee one.
The 4 Necessary Conditions for Deadlock Exclusive access mutual exclusion only one process may use a resource at a time Wait while holding hold-and-wait A process can continue to hold a resource while requesting another No preemption A process cannot be forced to give up resources before it chooses to give them up Circular wait There is a cycle of hold-and-wait relationships In order for there to be a deadlock, all of the above conditions must be true.
Deadlock detection is fairly straightforward, but deadlock recovery requires either aborting processes or preempting resources, neither of which is an attractive alternative.
Is this a safe state. Mutual Exclusion - At least one resource must be held in a non-sharable mode; If any other process requests this resource, then that process must wait for the resource to be released.
For more details see SilberschatzGalvinGagne Chapter 7.
A pipe is a connection between two processes in which one process writes data to the pipe and the other reads from the pipe. This is similar to no-hold-and-wait. We'll imagine each process puts its outgoing data in a buffer guarded by a semaphore, and blocks when the buffer fills up.
It's not hard to see that the Banker's Algorithm computes a safe sequence. However note also that request edges point to the category box, whereas assignment edges emanate from a particular instance dot within the box.
When an interrupt occurs, the processor stops executing instructions in the current running process and executes an interrupt handler function in the kernel.
What are examples of strategies for prevention. Deadlock is when two or more tasks never make progress because each is waiting for some resource held by another process. Release - The process relinquishes the resource. Suppose the resource is a printer and a print job is half completed.
In general, if we number the resource nodes and walk around a cycle in the resource allocation graph, there must come a point where we go from a higher-numbered resource to a lower numbered resource. Some categories may have a single resource. If process P grabs the keyboard while process Q grabs the display, both may be stuck waiting for the other resource.
One big challenge in this scheme is determining the relative ordering of the different resources 7. Look for a row that is componentwise less than or equal to A.
Consider, for example, Figures 7. Exclusive access mutual exclusion redesign to eliminate the need for mutual exclusion Can you think of an example. Some process must be holding one resource while waiting for another.
Reading from the pipe and writing to the pipe are done with the read and write calls that you have seen and used before. If deadlock does occur, it may be necessary to bring the system down, or at least manually kill a number of processes, but even that is not an extreme solution in most situations.
Resource Allocation Graph • Deadlock can be described through a resource allocation • Running the deadlock detection algorithm often will catch deadlock cycles early – Few processes will be affected – Note: there is no single process that caused the deadlock – May incur large overhead.
The figure shows the deadlock that is possible for the two pseudocode processes above, modeled by a (single-unit) resource allocation graph.
This is the simplest of several graph models for resource allocation we will consider below. Two processes want to write a file to a print spool area at the same time and both start writing.
if there is only one instance of each resource, it is possible to detect deadlock by constructing a resource allocation/request graph and checking for cycles. Here is the resource request/allocation graph. The algorithm needs to search each.
Lecture Note: Deadlocks 2 Resource Allocation qExamples of computer resources ßprinters ßtape drives qSolution: Build a graph, called Resource Allocation Graph (RAG) ßThere is a node for every process and a node for every resource qDeadlock is the result of granting a resource.
qBanker’s algorithm ßDeny potentially unsafe requests Resource Allocation Graph • Deadlock can be described through a resource allocation hard model to write applications for – Circular wait: impose an ordering (numbering) on the • Running the deadlock detection algorithm often will catch deadlock cycles early – Few processes will be affected.
Resource-Allocation Graph A set of vertices Vand a set of edges E. Banker’s Algorithm for Resource Allocation with Deadlock Avoidance Data Structures: • Max: n x mmatrix.
Combined Approach to Deadlock Handling • Combine the three basic apppp proaches (prevention, avoidance.Write a note on resource allocation graph algorithm