C

In a RAG there are two types of edges. Select the two types.

-represents assignment -represents the wait of a process for a resource

In a RAG, a resource is shown as waiting for a resource if __________. Select all that apply.

-the tail of an arrow is attached to the process -the head is pointing towards the resource

In a RAG, a resource is shown as assigned to a process if __________. Select all that apply.

-the tail of the arrow is attached to an instance to the resource -the head is attached to a process

Under the existence of what four conditions can a deadlock arise?

1). Mutual Exclusion - At least one resource must be held in a non-sharable mode. If another process requests that resource, the requesting process must be delayed until the resource has been released. 2). Hold and wait - A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes. 3). No preemption - resources cannot be preempted (i.e. not released with an interrupt). 4). Circular wait - A set of waiting resources must exist such that P0 is waiting for a resource held by P1, P1 is waiting for a resource held by P2, P2 for P3 and so on, and that some resource held by Pn is being waited for by P1 (thus we have a complete circle). Imagine the dining philosophers. *All four conditions must hold for a deadlock to occur. The circular wait condition implies the hold and wait condition so the four conditions are not completely Independent.

What three ways might we approach deadlock problems?

1). We can use a protocol to prevent or avoid deadlocks, ensuring that the system will never enter a deadlocked state 2). We can allow the system to enter a deadlocked state, detect it, and recover it. 3). We can ignore the problem altogether and pretend that deadlocks never occur in the system. The third solution (ignoring deadlocks) is the one used by most operating systems, including Linux and Windows. It is then up to the application developer to write programs that handle deadlocks.

What are the three methods for handling Deadlocks?

1. Deadlock prevention and avoidance 2. Deadlock detection and recovery 3. Ignore the deadlock (this is the most used method)

What are the two protocols used in Hold and Wait?

1. One protocol that we can use requires each process to request and be allocated all its resources before it begins execution. 2. An alternative protocol allows a process to request resources only when it has none.

What is the sequence by which a process requests a resource?

1. Request - the process requests the resource, if the request cannot be granted immediate (in use elsewhere -e.g.) then the requesting process must wait unit it can acquire the resource 2. Use - the process can operate on the resource (e.g. if the resource is a printer, the process can print on the printer) 3. Release - the process release the resource

Bounded Waiting

A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted

What is deadlock characterization?

A deadlock situation can arise if the following four conditions hold simultaneously in a system: 1. Mutual exclusion. At least one resource must be held in a nonsharable mode. 2. Hold and wait. A process must be holding at least one resource and waiting to acquire additional resources that are currently being held by other processes. 3. No preemption. Resources cannot be preempted. 4. Circular wait. No cycles are allowed.

Compare the circular-wait scheme with the deadlock-avoidance schemes (like the banker's algorithm) with respect to the following issues: a. Runtime overheads b. System throughput

A deadlock-avoidance scheme tends to increase the runtime overheads due to the cost of keep track of the current resource allocation. However, a deadlock-avoidance scheme allows for more concurrent use of resources than schemes that statically prevent the formation of deadlock. In that sense, a deadlock-avoidance scheme could increase system throughput.

What is deadlock avoidance?

A method that requires additional information about how resources are to be requested. A deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that a circular-wait condition can never exist. The resource allocation state is defined by the number of available and allocated resources and the maximum demands of the processes.

Hold and Wait

A process waits for some resources while holding another resource at the same time.

Mutual Exclusion (Deadlock)

A resource can only be shared in mutually exclusive manner. It implies, if two process cannot use the same resource at the same time.

What is a safe sequence?

A sequence of processes <P1, P2, ..., Pn> is a safe sequence for the current allocation state if, for each Pi , the resource requests that Pi can still make can be satisfied by the currently available resources plus the resources held by all Pj , with j < i.

What is a deadlock?

A set of processes are deadlocked if each process is waiting for an event that only another process can cause. Operating systems do not typically provide deadlock-prevention facilities, and it remains the responsibility of programmers to ensure that they design deadlock free-programs. Deadlock problems have become more common given current trends towards greater number of threads and processes.

What is a Safe State?

A state is safe if the system can allocate resources to each process (up to its maximum) in some order and still avoid a deadlock. A state is safe if there exists a safe sequence. If no such sequence exists, then the system state is said to be unsafe. An unsafe state may lead to a deadlock.

busy without waiting

A synchronization solution without busy waiting and priority inversion

What is the system model?

A system consists of a finite number of resources to be distributed among a number of competing processes. If a process requests an instance of a resource type, the allocation of any instance of the type should satisfy the request. If it does not, then the instances are not identical, and the resource type classes have not been defined properly.

Circular Wait

All the Processes must be waiting for the resources in a cyclic manner so that the last process is waiting for the resource which is being help by the first process.

What is starvation?

Also known as indefinite blocking. This is a situation in which processes wait indefinitely within the semaphore. Starvation may occur if we remove processes from the list associated with a semaphore in LIFO (last-in, first-out) order.

sleep and wake

Also known as the Producer Consumer problem

Shortest Remaining Time First

At the arrival of every process, the short term scheduler schedules the process with the least remaining burst time among the list of available processes and the running process

Preemptive Priority Scheduling

At the time of arrival of a process in the ready queue, its priority is compared with the priority of the other processes present in the ready queue as well as with the one which is being executed by the CPU at that point of time; the process with the highest priority among all the available processes will be given the CPU next

In a RAG, what shape denotes a process?

Circle

Given the final analysis of deadlock avoidance strategy, which of the four deadlock conditions does deadlock avoidance resolve?

Circular Wait

Given the final analysis of deadlock prevention strategy, which of the four deadlock conditions does deadlock prevention resolve?

Circular Wait

What is deadlock prevention?

Deadlock has four necessary conditions to hold simultaneously true. By ensuring that at least one of these conditions cannot hold, we can prevent the occurrence of a deadlock. Possible side effects of preventing deadlocks by this method, however, are low device utilization and reduced system throughput.

Distinguish deadlock avoidance and deadlock prevention.

Deadlock prevention - provides a set of methods to ensure that at least one of the necessary conditions (1-4) cannot hold, These methods prevent deadlocks by constraining how requests for resources can be made Deadlock avoidance - requires that the operating system be given additional information in advance concerning which resources a process will request and use during its lifetime. With this additional knowledge the OS can decide for each request whether or not the process should wait. If a system does not employ either a deadlock-prevention or a deadlock-avoidance algorithm then a deadlock situation may arise. Given this a system may or may not have the resources to detect a deadlock and act. As said earlier not all OS's include methods for overcoming deadlocks - since they occur infrequently (once a year perhaps) the extra expense of the other methods may not seem worthwhile. Moreover, methods to recover from other problems (e.g. a freeze) might be used to overcome the deadlock.

Round Robin

Every process gets executed in a cyclic way based on a time slice called time quantum

What is a resource allocation graph?

If a resource-allocation graph does not have a cycle, then the system is not in a deadlocked state. If there is a cycle, then the system may or may not be in a deadlocked state. Can be used as an algorithm for deadlock avoidance.

Deadlock detection and recovery

If deadlock occurs the OS applies a recovery method to the system to eliminate the deadlock.

What is deadlock?

In concurrent programming, a deadlock is a situation in which two or more competing actions are each waiting for the other to finish, and thus neither ever does

What are resource allocation graphs?

In order to describe cases of deadlock we can use resource allocation graphs which are simply models of resource allocation/ A directed arc from a resource to a process simply means that the process is currently owning that resource. A directed arc from a process to a resource means that the process is currently blocked waiting for that resource.

How can a process utilize a resource?

In three ways: 1. Request. The process requests the resource. If the request cannot be granted immediately (for example, if the resource is being used by another process), then the requesting process must wait until it can acquire the resource. 2. Use. The process can operate on the resource (for example, if the resource is a printer, the process can print on the printer). 3. Release. The process releases the resource.

interested variable

Makes use of an extra boolean variable to make sure that the progress is provided

Test Set Lock

Mutual exclusion is guaranteed because a process cannot be preempted before setting the lock variable

Is it possible to have a deadlock involving only one single process? Explain your answer.

No. This follows directly from the hold-and-wait condition.

How can we prevent a Circular Wait?

One way to ensure that this condition never holds is to impose a total ordering of all resource types and to require that each process requests resources in an increasing order of enumeration.

In a RAG, what shape denotes a resource?

Rectangle

Highest Response Ratio Next

Scheduling is done on the basis of an extra parameter called response ratio.

Consider a system consisting of four resources of the same type that are shared by three processes, each of which needs at most two resources. Show that the system is deadlock-free.

Suppose the system is deadlocked. This implies that each process is holding one resource and is waiting for one more. Since there are three processes and four resources, one process must be able to obtain two resources. This process requires no more resources and, therefore it will return its resources when done.

What data structures must be maintained to implement the banker's algorithm?

The four data structures needed are: 1. Available 2. Max 3. Allocation 4. Need

How can we prevent Mutual Exclusion?

The mutual exclusion condition must hold. That is, at least one resource must be nonsharable. In general, however, we cannot prevent deadlocks by denying the mutual exclusion condition, because some resources are intrinsically nonsharable.

Process Synchronization

The procedure used to preserve the order of execution of cooperative processes is known as __________.

No preemption

The process which once scheduled will be executed till the completion. No other process can be scheduled by the scheduler meanwhile.

Turn Variable

The processes share a variable called turn variable, which is a lock

What is deadlock recovery?

There are two ways to break deadlock. One is simply to abort one or more processes to break the circular wait (Process Termination). The other is to preempt some resources from one or more of the deadlocked processes (Resource Preemption).

What is the Resource-Request Algorithm?

This algorithm is used to determine whether or not requests can be safely granted.

lock variable

This is a busy waiting solution which can be used for more than two processes

What is the Safety Algorithm?

This is an algorithm used to find out whether or not a system is in a safe state.

What is Resource Preemption?

To eliminate deadlocks using resource preemption, we successively preempt some resources from processes and give these resources to other processes until the deadlock cycle is broken. If preemption is requires then three issues must be addressed: They are: selecting a victim, rollback, and starvation.

How can we prevent a Hold and Wait?

To ensure that the hold-and-wait condition never occurs in the system, we must guarantee that, whenever a process requests a resource, it does not hold any other resources. Two protocols are used.

How can we prevent No Preemption?

To ensure that this condition does not hold, we can use the following protocol. If a process is holding some resources and requests another resource that cannot be immediately allocated to it (that is, the process must wait), then all resources the process is currently holding are preempted.

A cooperative process one that can affect the execution of other process or can be affected by the execution of other process

True

A deadlock is a situation where each of the computer processes wait for a resource which is assigned to another process

True

Semaphore is a variable on which read, modify and update happens automatically in kernel mode

True

Semaphore is the variable(s) which stores the entire wake-up calls that are being transferred from producer to consumer

True

Semaphores are based on Dijkstra's Shortest Path First algorithm.

True

The resource allocation graph (RAG) is information about all processes which are holding some resources or waiting for some resources.

True

When two or more processes cooperate with each other, their order of execution must be preserved

True

What is deadlock detection?

Uses detection algorithms in two instances. They are: 1. Single Instance of Each Resource Type. Used if all resources have only a single instance. This uses a wait-for graph. Deadlock exists if and only if the wait-for graph contains a cycle. 2. Several Instances of a Resource Type. Used for a resource-allocation system with multiple instances of each resource type. The algorithm employs several time-varying data structures that are similar to those used in the banker's algorithm. The date structures used are: available, allocation, and request.

What is Process Termination?

Uses two methods: • Abort all deadlocked processes. This method clearly will break the deadlock cycle, but at great expense. The deadlocked processes may have computed for a long time, and the results of these partial computations must be discarded and probably will have to be recomputed later. • Abort one process at a time until the deadlock cycle is eliminated. This method incurs considerable overhead, since after each process is aborted, a deadlock-detection algorithm must be invoked to determine whether any processes are still deadlocked.

Paterson solution

Uses two variables that are turn variable and interested variable

What is the Bankers Algorithm?

When a new process enters the system, it must declare the maximum number of instances of each resource type that it may need. This number may not exceed the total number of resources in the system. When a user requests a set of resources, the system must determine whether the allocation of these resources will leave the system in a safe state. If it will, the resources are allocated; otherwise, the process must wait until some other process releases enough resources.

Deadlock Ignorance

Which deadlock strategy is mainly used by operating systems Windows and Linux?

Does a process request resources?

Yes, a process must request a resource before using it and must release the resource after using it. A process may request as many resources as it requires to carry out its designated task.

4) For a deadlock to arise, which of the following conditions must hold simultaneously? (choose all that apply ) a) Mutual exclusion b) Starvation c) Hold and wait d) No preemption e) Circular wait

a) Mutual exclusion c) Hold and wait d) No preemption e) Circular wait

For Mutual exclusion to prevail in the system : a) at least one resource must be held in a non sharable mode b) the processor must be a uniprocessor rather than a multiprocessor c) there must be at least one resource in a sharable mode d) All of these

a) at least one resource must be held in a non sharable mode Explanation: If another process requests that resource (non - shareable resource), the requesting process must be delayed until the resource has been released.

5. Which one of the following is the deadlock avoidance algorithm? a) banker's algorithm b) round-robin algorithm c) elevator algorithm d) karn's algorithm

a) banker's algorithm

4. The circular wait condition can be prevented by a) defining a linear ordering of resource types b) using thread c) using pipes d) all of the mentioned

a) defining a linear ordering of resource types

) One way to ensure that the circular wait condition never holds is to : a) impose a total ordering of all resource types and to determine whether one precedes another in the ordering b) to never let a process acquire resources that are held by other processes c) to let a process wait for only one resource at a time d) All of these

a) impose a total ordering of all resource types and to determine whether one precedes

For non sharable resources like a printer, mutual exclusion : a) must exist b) must not exist c) may exist d) None of these

a) must exists Explanation: A printer cannot be simultaneously shared by several processes.

9. Which one of the following is a visual ( mathematical ) way to determine the deadlock occurrence? a) resource allocation graph b) starvation graph c) inversion graph d) none of the mentioned

a) resource allocation graph

A deadlock avoidance algorithm dynamically examines the __________, to ensure that a circular wait condition can never exist. a) resource allocation state b) system storage state c) operating system d) resources

a) resource allocation state Explanation: Resource allocation states are used to maintain the availability of the already and current available resources.

Each request requires that the system consider the __________, _____________, ____________ to decide whether the current request can be satisfied or must wait to avoid a future possible deadlock. (choose three) a) resources currently available b) processes that have previously been in the system c) resources currently allocated to each process d) future requests and releases of each process

a) resources currently available c) resources currently allocated to each process d) future requests and releases of each process

1. What is the reusable resource? a) that can be used by one process at a time and is not depleted by that use b) that can be used by more than one process at a time c) that can be shared between various threads d) none of the mentioned

a) that can be used by one process at a time and is not depleted by that use

0. To avoid deadlock a) there must be a fixed number of resources to allocate b) resource allocation must be done only once c) all deadlocked processes must be aborted d) inversion technique can be used

a) there must be a fixed number of resources to allocate

Deadlock prevention is a set of methods : a) to ensure that at least one of the necessary conditions cannot hold b) to ensure that all of the necessary conditions do not hold c) to decide if the requested resources for a process have to be given or not d) to recover from a deadlock

a) to ensure that at least one of the necessary conditions cannot hold

To ensure that the hold and wait condition never occurs in the system, it must be ensured that : a) whenever a resource is requested by a process, it is not holding any other resources b) each process must request and be allocated all its resources before it begins its execution c) a process can request resources only when it has none d) All of these

all of these Explanation: c - A process may request some resources and use them. Before it can can request any additional resources, however it must release all the resources that it is currently allocated.

For a Hold and wait condition to prevail : a) A process must be not be holding a resource, but waiting for one to be freed, and then request to acquire it b) A process must be holding at least one resource and waiting to acquire additional resources that are being held by other processes c) A process must hold at least one resource and not be waiting to acquire additional resources d) None of these

b) A process must be holding at least one resource and waiting to acquire additional

For sharable resources, mutual exclusion : a) is required b) is not required c) None of these

b) is not required Explanation: They do not require mutually exclusive access, and hence cannot be involved in a deadlock.

3) Multithreaded programs are : a) lesser prone to deadlocks b) more prone to deadlocks c) not at all prone to deadlocks d) None of these

b) more prone to deadlocks

8. A problem encountered in multitasking when a process is perpetually denied necessary resources is called a) deadlock b) starvation c) inversion d) aging

b) starvation

3. A system is in the safe state if a) the system can allocate resources to each process in some order and still avoid a deadlock b) there exist a safe sequence c) both (a) and (b) d) none of the mentioned

c) both (a) and (b)

7. For effective operating system, when to check for deadlock? a) every time a resource request is made b) at fixed time intervals c) both (a) and (b) d) none of the mentioned

c) both (a) and (b)

Given a priori information about the ________ number of resources of each type that maybe requested for each process, it is possible to construct an algorithm that ensures that the system will never enter a deadlock state. a) minimum b) average c) maximum d) approximate

c) maximum

1) The number of resources requested by a process : a) must always be less than the total number of resources available in the system b) must always be equal to the total number of resources available in the system c) must not exceed the total number of resources available in the system d) must exceed the total number of resources available in the system

c) must not exceed the total number of resources available in the system

2) The request and release of resources are ___________. a) command line statements b) interrupts c) system calls d) special programs

c) system calls

2. Which of the following condition is required for deadlock to be possible? a) mutual exclusion b) a process may hold allocated resources while awaiting assignment of other resources c) no resource can be forcibly removed from a process holding it d) all of the mentioned

d) all of the mentioned

6. What is the drawback of banker's algorithm? a) in advance processes rarely know that how much resource they will need b) the number of processes changes as time progresses c) resource once available can disappear d) all of the mentioned

d) all of the mentioned

To ensure no preemption, if a process is holding some resources and requests another resource that cannot be immediately allocated to it : a) then the process waits for the resources be allocated to it b) the process keeps sending requests until the resource is allocated to it c) the process resumes execution without the resource being allocated to it d) then all resources currently being held are preempted

d) then all resources currently being held are preempted

Portability

dependency on the hardware platform

Mutual Exclusion

if one process is executing inside critical section then the other process must not enter in the critical section

) The disadvantage of a process being allocated all its resources before beginning its execution is : a) Low CPU utilization b) Low resource utilization c) Very high resource utilization d) None of these

low resource utilization

Progress

make sure some process is in the critical section and doing work.

Which of the following best describes critical section?

regions of a program that try to access shared resources

First Come First Serve

the job which comes first in the ready queue will get the CPU first

Shortest Job First

the process with the lowest burst time relative to the list of available processes in the ready queue is going to be scheduled next

Which of the following best describes race condition?

when two or more threads try to read, write and possibly make the decisions based on the memory that they are accessing concurrently