程序代写 scheme data structure algorithm EECS 3221:

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
1
Chapter 8: Deadlocks
! System Model
! Deadlock in Multithreaded Applications
! Deadlock Characterization
! Methods for Handling Deadlocks
! Deadlock Prevention
! Deadlock Avoidance
! Deadlock Detection
! Recovery from Deadlock
EECS3221: Operating System Fundamentals 8.2 Deadlocks
2
1

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
EECS3221: Operating System Fundamentals 8.3 Deadlocks
3
Deadlock in Multithreaded Application — Example
! Two mutex locks are created an initialized:
EECS3221: Operating System Fundamentals 8.4 Deadlocks
4
2

Deadlock in Multithreaded Application
EECS3221: Operating System Fundamentals 8.5 Deadlocks
5
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:
EECS3221: Operating System Fundamentals 8.6 Deadlocks
6
3

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.
EECS3221: Operating System Fundamentals 8.7 Deadlocks
7
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
EECS3221: Operating System Fundamentals 8.8 Deadlocks
8
4

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
EECS3221: Operating System Fundamentals 8.9 Deadlocks
9
Resource Allocation Graph With A Deadlock
EECS3221: Operating System Fundamentals 8.10 Deadlocks
10
5

Graph With A Cycle But No Deadlock
EECS3221: Operating System Fundamentals 8.11 Deadlocks
11
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
EECS3221: Operating System Fundamentals 8.12 Deadlocks
12
6

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.
EECS3221: Operating System Fundamentals 8.13 Deadlocks
13
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
EECS3221: Operating System Fundamentals 8.14 Deadlocks
14
7

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
EECS3221: Operating System Fundamentals 8.15 Deadlocks
15
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:
EECS3221: Operating System Fundamentals 8.16 Deadlocks
16
8

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
EECS3221: Operating System Fundamentals 8.17 Deadlocks
17
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 of ALL the processes in the systems such that 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 ! That is: ! If Pi resource needs are not immediately available, then Pi can wait until all Pj have finished ! When Pj is 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 EECS3221: Operating System Fundamentals 8.18 Deadlocks 18 9 Basic Facts ! If a system is in safe state Þ no deadlocks ! If a system is in unsafe state Þ possibility of deadlock ! Avoidance Þ ensure that a system will never enter an unsafe state. EECS3221: Operating System Fundamentals 8.19 Deadlocks 19 Safe, Unsafe, Deadlock State EECS3221: Operating System Fundamentals 8.20 Deadlocks 20 10 Avoidance Algorithms ! Single instance of a resource type ! Use a resource-allocation graph ! Multiple instances of a resource type ! Use the Banker’s Algorithm EECS3221: Operating System Fundamentals 8.21 Deadlocks 21 Resource-Allocation Graph Scheme ! Claim edge Pi ® Rj indicated that process Pi may request resource Rj; represented by a dashed line ! Claim edge converts to request edge when a process requests a resource ! Request edge converted to an assignment edge when the resource is allocated to the process ! When a resource is released by a process, assignment edge reconverts to a claim edge ! Resources must be claimed a priori in the system EECS3221: Operating System Fundamentals 8.22 Deadlocks 22 11 Resource-Allocation Graph Assignment edge Request edge Claim edge Claim edge EECS3221: Operating System Fundamentals 8.23 Deadlocks 23 Unsafe State In Resource-Allocation Graph EECS3221: Operating System Fundamentals 8.24 Deadlocks 24 12 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 graph EECS3221: Operating System Fundamentals 8.25 Deadlocks 25 Banker’s Algorithm ! Conditions: ! Multiple instances of resources ! Each process must a priori claim maximum use ! When a process requests a resource it may have to wait ! When a process gets all its resources it must return them in a finite amount of time EECS3221: Operating System Fundamentals 8.26 Deadlocks 26 13 Data Structures for the Banker’s Algorithm Let n = number of processes, and m = number of resources types. ! Available: Vector of length m. If available [j] = k, there are k instances of resource type Rj available ! Max: n x m matrix. If Max [i,j] = k, then process Pi may request at most k instances of resource type Rj ! Allocation: n x m matrix. If Allocation[i,j] = k then Pi is currently allocated k instances of Rj ! Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rj to complete its task Need [i,j] = Max[i,j] – Allocation [i,j] EECS3221: Operating System Fundamentals 8.27 Deadlocks 27 Safety Algorithm 1. LetWorkandFinishbevectorsoflengthmandn,respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1, ..., n- 1 2. Findanisuchthatboth: (a) Finish [i] = false (b) Needi £ Work If no such i exists, go to step 4 3. Work = Work + Allocationi Finish[i] = true go to step 2 4. IfFinish[i]==trueforalli,thenthesystemisinasafestate otherwise we are in unsafe state. EECS3221: Operating System Fundamentals 8.28 Deadlocks 28 14 Resource-Request Algorithm for Process Pi Requesti = request vector for process Pi If Requesti [j] = k then process Pi wants k instances of resource type Rj 1. If Requesti £ Needi go to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim 2. If Requesti £ Available, go to step 3. Otherwise Pi must wait, since resources are not available 3. Pretend to allocate requested resources to Pi by modifying the state as follows: Available = Available – Requesti; Allocationi = Allocationi + Requesti; Needi = Needi – Requesti; ! If safe Þ the resources are allocated to Pi ! If unsafe Þ Pi must wait, and the old resource-allocation state is restored EECS3221: Operating System Fundamentals 8.29 Deadlocks 29 Example of Banker’s Algorithm ! 5 processes P0 through P4; 3 resource types: A (10 instances), B (5 instances), and C (7 instances) ! Snapshot at time T0: Allocation Max Available ABC ABC ABC P0 010 753 332 P1 200 322 P2 302 902 P3 211 222 P4 002 433 EECS3221: Operating System Fundamentals 8.30 Deadlocks 30 15 Example (Cont.) ! The content of the matrix Need is defined to be Max – Allocation Need ABC P0 743 P1 122 P2 600 P3 011 P4 431 ! The system is in a safe state since the sequence < P1, P3, P4, P2, P0> satisfies safety criteria
! Lets check, if it is indeed a safe sequence?
EECS3221: Operating System Fundamentals 8.31 Deadlocks
31
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?
EECS3221: Operating System Fundamentals 8.32 Deadlocks
32
16

Deadlock Detection
! Allow system to enter deadlock state
! Detection algorithm
! Recovery scheme
EECS3221: Operating System Fundamentals 8.33 Deadlocks
33
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
EECS3221: Operating System Fundamentals 8.34 Deadlocks
34
17

Resource-Allocation Graph and Wait-for Graph
Resource-Allocation Graph Corresponding wait-for graph
EECS3221: Operating System Fundamentals 8.35
Deadlocks
35
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.
EECS3221: Operating System Fundamentals 8.36 Deadlocks
36
18

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
EECS3221: Operating System Fundamentals 8.37 Deadlocks
37
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
EECS3221: Operating System Fundamentals 8.38 Deadlocks
38
19

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 will result in Finish[i] = true for all i
EECS3221: Operating System Fundamentals 8.39 Deadlocks
39
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
EECS3221: Operating System Fundamentals 8.40 Deadlocks
40
20

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.
EECS3221: Operating System Fundamentals 8.41 Deadlocks
41
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?
EECS3221: Operating System Fundamentals 8.42 Deadlocks
42
21

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
EECS3221: Operating System Fundamentals 8.43 Deadlocks
43
ANY QUESTION?
EECS3221: Operating System Fundamentals 8.44 Deadlocks
44
22