EECS 3221:
OPERATING SYSTEM FUNDAMENTALS
Hamzeh Khazaei
Department of Electrical Engineering and Computer Science
Week 9, Module 1:
Deadlocks
March 8, 2021
EECS3221: Operating System Fundamentals 8.1 Deadlocks
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Chapter 8: Deadlocks
! System Model
! Deadlock in Multithreaded Applications
! Deadlock Characterization
! Methods for Handling Deadlocks
! Deadlock Prevention
! Deadlock Avoidance
! Deadlock Detection
! Recovery from Deadlock
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System Model
! System consists of resources
! Resource types R1, R2, . . ., Rm
CPU cycles, memory space, I/O devices
! Each resource type Ri has Wi instances.
! Each process utilizes a resource as follows:
! request ! use
! release
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Deadlock in Multithreaded Application — Example
! Two mutex locks are created an initialized:
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Deadlock in Multithreaded Application
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Deadlock in Multithreaded Application
! Deadlock is possible if thread 1 acquires first_mutex and thread 2 acquires second_mutex. Thread 1 then waits for second_mutex and thread 2 waits for first_mutex.
! Can be illustrated with a resource allocation graph:
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Deadlock Characterization
Deadlock can arise if four conditions hold simultaneously.
! Mutual exclusion: only one process at a time can use a resource
! 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.
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Resource-Allocation Graph
A set of vertices V and a set of edges E.
! V is partitioned into two types:
! P = {P1, P2, …, Pn}, the set consisting of all the processes in the system
! R = {R1, R2, …, Rm}, the set consisting of all resource types in the system
! request edge – directed edge Pi ® Rj
! assignment edge – directed edge Rj ® Pi
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Resource Allocation Graph Example
! One instance of R1
! Two instances of R2
! One instance of R3
! Three instance of R4
! T1 holds one instance of R2 and is waiting for an instance of R1
! T2 holds one instance of R1, one instance of R2, and is waiting for an instance of R3
! T3 is holds one instance of R3
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Resource Allocation Graph With A Deadlock
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Graph With A Cycle But No Deadlock
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Basic Facts
! If graph contains no cycles Þ no deadlock
! If graph contains a cycle Þ
! if only one instance per resource type, then deadlock
! if several instances per resource type, possibility of deadlock
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Methods for Handling Deadlocks
! Ensure that the system will never enter a deadlock state:
! Deadlock prevention
! Deadlock avoidance
! Allow the system to enter a deadlock state and then
recover
! Ignore the problem and pretend that deadlocks never occur in the system.
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Deadlock Prevention
Invalidate one of the four necessary conditions for deadlock:
! Mutual Exclusion – not required for sharable resources (e.g., read-only files); must hold for non-sharable resources
! 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 process to request resources only when the process has none allocated to it.
! Low resource utilization; starvation possible
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Deadlock Prevention (Cont.)
! 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
! Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration
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Circular Wait
! Invalidating the circular wait condition is most common.
! Simply assign each resource (i.e. mutex locks) a unique number.
! Resources must be acquired in order.
! If:
first_mutex = 1
second_mutex = 5
code for thread_two could not be written as follows:
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Deadlock Avoidance
Requires that the system has some additional a priori information available
! Simplest and most useful model requires that each process declare the maximum number of resources of each type that it may need
! The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular- wait condition
! Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes
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Safe State
! When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state
! System is in safe state if:
! There exists a sequence
! Lets check, if it is indeed a safe sequence?
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Example: P1 Request (1,0,2)
! Check that Request £ Available (that is, (1,0,2) £ (3,3,2) Þ true Allocation Need Available
ABC ABC ABC P0 010 743 230 P1 302 020
P2 302 600
P3 211 011
P4 002 431
! Executing safety algorithm shows that sequence < P1, P3, P4, P2, P0> satisfies safety requirement
! Can request for (3,3,0) by P4 be granted?
! Can request for (0,2,0) by P0 be granted?
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Deadlock Detection
! Allow system to enter deadlock state
! Detection algorithm
! Recovery scheme
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Single Instance of Each Resource Type
! Maintain wait-for graph
! Nodes are processes
! Pi ®Pj ifPi iswaitingforPj
! Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock
! An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph
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Resource-Allocation Graph and Wait-for Graph
Resource-Allocation Graph Corresponding wait-for graph
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Deadlocks
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Several Instances of a Resource Type
! 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.
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Detection Algorithm
1. LetWorkandFinishbevectorsoflengthmandn,respectively Initialize:
(a) Work = Available
(b) For i = 1,2, …, n, if Allocationi 1 0, then
Finish[i] = false; otherwise, Finish[i] = true
2. Findanindexisuchthatboth: (a) Finish[i] == false
(b) Requesti £ Work
If no such i exists, go to step 4
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Detection Algorithm (Cont.)
3. Work=Work+Allocationi Finish[i] = true
go to step 2
4. If Finish[i] == false, for some i, 1 £ i £ n, then the system is in deadlock state. Moreover, if Finish[i] == false, then Pi is deadlocked
Algorithm requires an order of O(m x n2) operations to detect whether the system is in deadlocked state
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Example of Detection Algorithm
! Five processes P0 through P4; three resource types A (7 instances), B (2 instances), and C (6 instances)
! Snapshot at time T0:
Allocation Request Available
ABC ABC ABC P0 010 000 000
P1 200 202 P2 303 000 P3 211 100 P4 002 002
! Sequence
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Example (Cont.)
! P2 requests an additional instance of type C Request
ABC P0 000 P1 202 P2 001 P3 100 P4 002
! State of system?
! Can reclaim resources held by process P0, but insufficient
resources to fulfill other processes; requests
! Deadlock exists, consisting of processes P1, P2, P3, and P4
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Detection-Algorithm Usage
! When, and how often, to invoke depends on:
! How often a deadlock is likely to occur?
! How many processes will need to be rolled back?
4 one for each disjoint cycle
! If detection algorithm is invoked arbitrarily, there may be many cycles in the resource graph and so we would not be able to tell which of the many deadlocked processes “caused” the deadlock.
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Recovery from Deadlock: Process Termination
! Abort all deadlocked processes
! Abort one process at a time until the deadlock cycle is eliminated
! In which order should we choose to abort?
1. Priority of the process
2. How long process has computed, and how much longer to completion
3. Resources the process has used
4. Resources process needs to complete
5. How many processes will need to be terminated
6. Is process interactive or batch?
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Recovery from Deadlock: Resource Preemption
! Selecting a victim – minimize cost
! Rollback – return to some safe state, restart process for that state
! Starvation – same process may always be picked as victim, include number of rollback in cost factor
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ANY QUESTION?
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