By definition, all the resources within a category are equivalent, and a request of this category can be equally satisfied by any one of the resources in that category. For example, "printers" may need to be separated into "laser printers" and "color inkjet printers". Some categories may have a single resource. In normal operation a process must request a resource before using it, and release it when it is done, in the following sequence:
This strategy detects deadlock when it occurs, and takes appropriate measures to break the deadlock. This is accomplished by Re-computing the resource allocation graph each time a resource is requested and released. Aborting processes until deadlock cycles are broken. An alternate, weaker strategy just kills processes that have blocked for a long time on a resource.
Not so good for daemons that process infrequent requests. According to Tanenbaum, deadlock detection is a strategy employed on mainframes, where batch jobs can be easily re-started if aborted to break deadlocks. Prevention involves denying one of the four deadlock conditions. Mutual Exclusion This strategy involves making all resources either sharable, or spooling to resources like printers that are not readily shared.
The printer daemon gets exclusive access to the printer, but it is the only resource it holds. Hold and Wait Require processes to request all at the start of execution. Those processes that cannot obtain all of their required resources are not allowed to run.
A resource might not be used until the end of the process, in which case it is being held idle. A process might not know what resources it needs until it is well under way Emacs.
Two-phase Locking In the database world, a transaction is a unit of computation that is guaranteed by the dbms to either commit execute completely and write the result permanently or abort quit with no trace. Two-phase locking is employed to ensure correct locking of records by concurrent transactions, so that the results are the same as if the transactions were executed sequentially.
There are two phases to a transaction, hence the name two-phase locking: The growing phase, during which a transaction may only obtain locks resourcesnot release them. The shrinking phase, during which a transaction may only release locks, not obtain them.
This does not prevent deadlock by itself: The reason this is workable is that transactions don't commit until the end, when all resources are released. Thus, aborting t2 does not affect the permanent state of the database. No-preemption Allow the kernel to take resources away from processes that hold them.
This is not a good strategy: Circular Wait Two strategies: A process may only hold one resource like the printer daemon above. Not realistic in general. Assign an ordering to resources, and require that processes always request resources in ascending order.Here is the resource request/allocation graph.
The algorithm needs to search each node; let's start at node P1. We add P1 to L and follow the only edge to R1, marking that ashio-midori.com Resource-Allocation Graph Algorithm • Suppose that process Pi requests a resource Rj • The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation ashio-midori.com://ashio-midori.com · (regenerated from [OSC8] Fig.
on p. ) If a resource allocation graph contains no cycles, then no process is deadlocked. If a resource allocation graph contains a cycle, then a deadlock may exist.. Therefore, a cycle means deadlock is possible, but not necessarily present..
A cycle is not sufficient proof of the presence of ashio-midori.com /cps/lecture_notes/ashio-midori.com Chart and Diagram Slides for PowerPoint - Beautifully designed chart and diagram s for PowerPoint with visually stunning graphics and animation effects.
Our new CrystalGraphics Chart and Diagram Slides for PowerPoint is a collection of over impressively designed data-driven chart and editable diagram s guaranteed to impress any ashio-midori.com · Each set V i is called a stage in the graph.
Consider the resource allocation problem: Given n units of resources to be allocated to k projects. For 1 resource to project i. To write a simple algorithm, Note: Here in step 1 we can select the node 8 because ashio-midori.com /dynamic-prog/multistage-graphs.
If there is a cycle in the Resource Allocation Graph and each resource in the cycle provides only one instance, then the processes will deadlock.
For example, if process 1 holds resource A, process 2 holds resource B and process 1 is waiting for B and process 2 is waiting for A, then process 1 and 2 process will be deadlocked.