OS Chapter 7

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AI智能生成

操作系统第七章主要探讨了进程和线程的管理。在计算机科学中，进程是正在执行的程序的实例，而线程则是进程中的一个独立执行路径。本章详细介绍了如何创建、调度和终止进程和线程，以及如何进行进程间通信。此外，还讨论了并发编程中的同步和互斥问题，包括死锁、饥饿和优先级反转等现象。为了解决这些问题，引入了各种同步原语，如信号量、互斥锁和条件变量。最后，本章还介绍了一些高级进程和线程管理技术，如线程池、协程和用户级线程。通过学习这一章，读者将掌握进程和线程的基本概念和管理方法，为进一步深入学习操作系统打下坚实的基础。

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The Deadlock Problem

A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set

Example System has 2 disk drives P1 and P2 each hold one disk drive and each needs another one

Example semaphores A and B, initialized to 1 P0 P1 wait (A); wait(B) wait (B); wait(A)

Bridge Crossing Example

Traffic only in one direction Each section of a bridge can be viewed as a resource If a deadlock occurs, it can be resolved if one car backs up Starvation is possible

System Model

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 : the process requests the resource

use: the process can operate on the resource

release: the process releases the resource

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.

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

Basic Facts

If graph contains no cycles => no deadlock

If graph contains a cycle => if only one instance per resource type, then deadlock if several instances per resource type, possibility of deadlock

Methods for Handling Deadlocks

Ensure that the system will never enter a deadlock state提供一个protocol

Allow the system to enter a deadlock state and then recover

Ignore the problem and pretend that deadlocks never occur in the system; used by most operating systems, including UNIX

Deadlock Prevention

Restrain the ways request can be made

Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources

No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task

Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes

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

Safe State

When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state

A system is in safe state only if there exists a safe sequence of ALL the processes in the system

Basic Facts

If a system is in safe state => no deadlocks

If a system is in unsafe state => possibility of deadlock

Avoidance => ensure that a system will never enter an unsafe state.

Avoidance algorithms

Single instance of a resource type Use a resource-allocation graph

Multiple instances of a resource type Use the banker’s algorithm

Resource-Allocation Graph Scheme

Claim edge Pi -> Rj indicated that process Pj 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

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

Banker’s Algorithm

Multiple instances 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

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]

Safety Algorithm

1. Let Work and Finish be vectors of length m and n, respectively. Initialize: Work = Available Finish [i] = false for i = 0, 1, …, n- 1 2. Find an i such that both: (a) Finish [i] = false (b) Needi  Work If no such i exists, go to step 4 3. Work = Work + AllocationiFinish[i] = truego to step 2 4. If Finish [i] == true for all i, then the system is in a safe state

Resource-Request Algorithm for Process Pi

Request = 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 – Request; Allocationi = Allocationi + Requesti; Needi = Needi – Requesti; If safe  the resources are allocated to Pi If unsafe  Pi must wait, and the old resource-allocation state is restored

Example of Banker’s Algorithm

5 processes P0 through P4; 3 resource types: A (10 instances), B (5instances), and C (7 instances) Snapshot at time T0: Allocation Max Available A B C A B C A B C P0 0 1 0 7 5 3 3 3 2 P1 2 0 0 3 2 2 P2 3 0 2 9 0 2 P3 2 1 1 2 2 2 P4 0 0 2 4 3 3

The content of the matrix Need is defined to be Max – Allocation Need A B C P0 7 4 3 P1 1 2 2 P2 6 0 0 P3 0 1 1 P4 4 3 1 The system is in a safe state since the sequence < P1, P3, P4, P0, P2> satisfies safety criteria

Example: P1 Request (1,0,2)

Check that Request  Available that is, (1,0,2)  (3,3,2)  true Allocation Need Available A B C A B C A B C P0 0 1 0 7 4 3 2 3 0 P1 3 0 2 0 2 0 P2 3 0 2 6 0 0 P3 2 1 1 0 1 1 P4 0 0 2 4 3 1 Executing safety algorithm shows that sequence < P1, P3, P4, P0, P2> satisfies safety requirement

Deadlock Detection

Allow system to enter deadlock state Detection algorithm Recovery scheme

Single Instance of Each Resource Type

Maintain wait-for graph Nodes are processes Pi -> Pj if Pi is waiting for Pj

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

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.

Detection Algorithm

Let Work and Finish be vectors of length m and n, respectively Initialize: (a) Work = Available (b) For i = 1,2, …, n, if Allocationi != 0, then Finish[i] = false; otherwise, Finish[i] = true

Find an index i such that both: (a) Finish[i] == false (b) Request i <= Work If no such i exists, go to step 4

Work = Work + Allocationi Finish[i] = true go to step 2

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

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 A B C A B C A B C P0 0 1 0 0 0 0 0 0 0 P1 2 0 0 2 0 2 P2 3 0 3 0 0 0 P3 2 1 1 1 0 0 P4 0 0 2 0 0 2 Sequence will result in Finish[i] = true for all i

P2 requests an additional instance of type C Request A B C P0 0 0 0 P1 2 0 2 P2 0 0 1 P3 1 0 0 P4 0 0 2

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

Recovery from Deadlock

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?

Priority of the process How long process has computed, and how much longer to completion Resources the process has used Resources process needs to complete How many processes will need to be terminated Is process interactive or batch?

Recovery from Deadlock: Resource Preemption

Selecting a victim – minimize cost 选择牺牲者

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