7.1 Silberschatz, Galvin and Gagne 2005 Operating System Concepts Chapter 7: Deadlocks The Deadlock Problem System Model Deadlock Characterization Methods.
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Legislation passed in Kansas early in the 20th century “When two trains approach each other at a crossing, both shall
come to a full stop and neither shall start up again until the other has gone.”
In a multiprogramming environment several processes compete for a given number of resources. A Process requests for resources; if the resources are not available at that time the process remains blocked until it acquires the requested resource.
How about this law: “All vehicles approaching a 4-way stop sign shall come to a full stop. The vehicle that reached the stop sign first has the right to cross the intersection first.”
Mutual exclusion: Only one process is allowed to use a resource at a time.
Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes.
No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task.
Circular wait: there exists a set {P0, P1, …, Pn} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.
Deadlock can be prevented if any one of the above four conditions can be prevented
Deadlock can arise if the following four conditions hold simultaneously.
Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources.Since we cannot really make all resources sharable, mutual exclusion is a desirable property and we cannot prevent mutually exclusive access to all types of resources.
Preventing Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources. Require process to request and be allocated all its
resources before it begins execution, or allow a process to request resources only when the process has none allocated to it.
Disadvantage of this approach: Low resource utilization; starvation possible.
Preventing No Preemption – If a process that is holding some resources requests another
resource that cannot be immediately allocated to it, then all resources currently being held are released.
Preempted resources are added to the list of resources for which the process is waiting.
Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting.
Preventing Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration. Prove how this will prevent circular wait
Deadlock Avoidance - Safe StateDeadlock Avoidance - Safe State When a process requests an available resource, system must
decide if immediate allocation leaves the system in a safe state.
A definition of safe state: System is in safe state if there exists a safe sequence of all processes. Sequence <P1, P2, …, Pn> is safe if for each Pi, the
resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j<i.
If Pi’s resource needs are not immediately available, then Pi can wait until all Pj (j<i) have finished.
When Pj s (j<i) have finished, Pi can obtain needed resources, execute, return allocated resources, and terminate.
When Pi terminates, Pi+1 can obtain its needed resources, and so on.
Resource-Allocation Graph AlgorithmResource-Allocation Graph Algorithm Assumption: There is only one unit of each resource type in the system. The algorithm for deadlock avoidance (it tries to prevent circular wait):
Claim edge Pi Rj indicates that process Pi may request a resource of type Rj
in the future. It resembles a request edge but represented by a dashed line. Claim edge converts to request edge when a process requests a resource. Request edge converts to assignment edge when the requested resource is
allocated. When a resource is released by a process, the corresponding assignment edge
reconverts to a claim edge. Resources must be claimed a priori in the system. When a process makes a request, request can be granted only if the resulting
assignment edge will not result in a cycle in the resource allocation graph. The algorithm applies only to systems with one unit of each resource type
Resource-Allocation Graph For Deadlock AvoidanceResource-Allocation Graph For Deadlock Avoidance
Consider the resource allocation graph. If P2 makes a request for R2, it cannot be assigned to P2 eventhough it is available, because it will create a cycle.
safety requirement and hence the requested resources can be allocated. Can request for (3,3,0) by P4 be granted? Can request for (0,2,0) by P0 be granted?
Basic idea behind Banker’s algorithmBasic idea behind Banker’s algorithm
When a process requests for resources: The requested resources are allocated
to the process only if after allocating the resources, the resulting state will be a safe state; otherwise the process has to wait for acquiring the resources requested until this condition is satisfied.
Deadlock Detection in a System with Deadlock Detection in a System with Several Instances of a Resource TypeSeveral Instances of a Resource Type
Basic idea Behind the Deadlock detection algorithm: Assumptions:
A process does not have to declare how many units of each resource it will need during its life time
All processes that do not have outstanding requests will finish and release the resources they are holding soon.
Processes that are currently requesting for resources will not request for any more resources than they are requesting currently
Periodically check for the following: With the above assumptions, check if the current requests of all the
processes can be satisfied in some order. (Note that we do not worry about future requests). If so, then there is no deadlock; otherwise, processes whose requests cannot be satisfied are assumed to be deadlocked and a deadlock is declared.
Deadlock Detection in a System with Deadlock Detection in a System with Several Instances of a Resource Type Several Instances of a Resource Type
(cont…)(cont…)
Available: A vector of length m indicates the number of available resources of each type.
Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process.
Request: An n x m matrix indicates the current request of each process. If Request [i,j] = k, then process Pi is requesting k more instances of resource type Rj.
This algorithm takes an optimistic attitude and assumes that the process whose current requests can be satisfied will not require any more resources to complete its task and thus will soon return its resources. If this assumption is incorrect, it may result in deadlock later which will be detected at the later invocation of the deadlock detection algorithm.
Algorithm requires an order of O(m x n2) operations to detect whether the system is in deadlocked state.
When, and how often, to invoke the deadlock detection algorithm depends on: How often a deadlock is likely to occur? How many processes will need to be rolled back?
one for each disjoint cycle?
If deadlock detection algorithm is invoked arbitrarily, there may be many cycles in the resource allocation graph and so we would not be able to tell which of the many processes on the cycles “caused” the deadlock.
Recovery from Deadlock: Process TerminationRecovery from Deadlock: Process Termination Abort all deadlocked processes.
Abort one process at a time until all the deadlock cycles are eliminated.
In which order should we choose to abort? Use priority of the process. How long a process has computed, and how much longer to
completion. Resources the process has used. Resources process needs to complete. Minimum number of processes that need to be terminated. Is the process chosen for termination interactive or batch?