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Deadlock Detection and Recovery (Operating System)

If a system does not employ either a deadlock-prevention or a deadlock-avoidance algorithm, then a deadlock situation may occur. In this environment, the system must provide:
An algorithm that examines the state of the system to determine whether a deadlock has occurred.
An algorithm to recover from the deadlock.

Algorithm for detecting deadlock:

For each node, N in the graph, perform the following five steps with N as the starting node.
Initialize L to the empty list, designate all arcs as unmarked.
Add current node to end of L, check to see if node now appears in L two times. If it does, graph contains a cycle (listed in L), algorithm terminates.
From given node, see if any unmarked outgoing arcs. If so, go to step 5; if not, go to step 6.
Pick an unmarked outgoing arc at random and mark it. Then follow it to the new current node and go to step 3.
If this is initial node, graph does not contain any cycles, algorithm terminates. Otherwise, dead end. Remove it, go back to previous node, make that one current node, go to step 3.

Recovery From Deadlock
When a detection algorithm determines that a deadlock exists, there are two options for breaking a deadlock. One solution is simply to abort one or more processes to break the circular wait. The second option is to preempt some resources from one or more of the deadlocked processes.

Process Termination: To eliminate deadlocks by aborting a process, we use one of two methods. In both methods, the system reclaims all resources allocated to the terminated processes.

Abort all deadlocked processes
Abort one process at a time until the deadlock cycle is eliminated –overhead, a deadlock-detection after terminating a process

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. Some issues need to be addressed are:

Selecting a victim: Some process will have to rolled back to break deadlock. Select that process as victim that will incur minimum cost.
Rollback: Determine how far to roll back the process

Total rollback: Abort the process and then restart it. More effective is to roll back the process only as far as necessary to break deadlock

Starvation: Starvation happens if same process is always chosen as victim. To avoid this, we include the number of rollbacks in the cost factor.

Critical Section (Operating System)

One way to avoid race conditions is not to allow two processes to be in their critical sections at the same time.
Critical section is the part of the process that accesses a shared variable.
That is, we need a mechanism of mutual exclusion. Mutual exclusion is the way of ensuring that one process, while using the shared variable, does not allow another process to access that variable.

Race Condition (Operating System)

Race conditions are situations where two or more processes reads or writes some shared data at the same time and the final result is incorrect.
Final result may be different according to order of completion of competing processes.
let us consider a simple but common example: a print spooler. When a process wants to print a file, it enters the file name in a special spooler directory.
Another process periodically checks to see if there are any files to be printed, and if there are, it prints them and then removes their names from the directory.
Consider the situation given in figure below, where in and out are two shared variables.

Process A reads value of in(i.e. 7) but before inserting the name of the document to be printed in index 7, the CPU decides to schedule process B.
Now, process B also reads in, and also gets 7. Process B now continues to run and stores the name of its file in slot 7 and updates in to be an 8.

After some time process A runs again. It looks at value of in stored in its local variable and finds a 7 there, and writes its file name in slot 7.Then it increments value of in and sets in to 8.The printer daemon will not notice anything wrong, but process B will never receive any output. Situations like this are called race conditions.

System Calls (Operating System)

System calls are the interface between the operating system and the user programs. Access to the operating system is done through system calls.
Each system call has a procedure associated with it so that calls to the operating system can be done in a similar way as that of normal procedure call.
When we call one of these procedures it places the parameters into registers and informs the operating system that there is some work to be done. When a system call is made TRAP instruction is executed. This instruction switches the CPU from user mode to kernel (or supervisor) mode.

Eventually, the operating system will have completed the work and will return a result.
Making a system call is like making a special kind of procedure call, only system calls enter the kernel and procedure calls do not.
Example: count = read(file, buffer, nbytes);

To make this concept clearer, let us examine the read call written above. Calling program first pushes the parameters onto stack (steps 1-3) before calling the read library procedure (step 4), which actually makes the read system call.

The library procedure, possibly written in assembly language, typically puts the system call number in specified register (step 5). Then it executes a TRAP instruction to switch from user mode to kernel mode and start execution at a fixed address within the kernel (step 6).
The kernel code examines the system call number and then dispatches to the correct system call handler (step 7). At that point the system call handler runs (step 8).

Once the system call handler has completed its work, control may be returned to the user-space library procedure at the instruction following the TRAP instruction (step 9).
This procedure then returns to the user program in the usual way procedure calls return (step 10).
To finish the job, the user program has to clean up the stack, as it does after any procedure call (step 11).

Process and Thread its Difference in CPU (Operating System)

Process

Process is a program that is ready for execution in CPU. When a program is loaded into memory, it becomes ready for execution and competes with other process to access CPU. Thus when a program is loaded into memory it becomes process. Computers nowadays can do many things at the same time. They can be writing to a printer, reading from a disc and scanning an image. Operating system is responsible for running many processes, usually, on the same CPU. In fact, only one process can be run at a time so the operating system has to share the CPU between the processes that are available to be run, while giving the illusion that the computer is doing many things at the same time. This approach can be directly contrasted with the first computers. They could only run one program at a time.

Now, the main point of this part is to consider how an operating system deals with processes when we allow many to run. To achieve true parallelism we must have multiprocessor system when n processors can execute n programs simultaneously. True parallelism cannot be achieved with singl CPU. In single processor CPU switched from one process to another process rapidly. In 1 sec CPU can serve 100's of processes giving the illusion of parallelism to a user which is called pseudo-parallelism.

Threads

Threads are like mini-processes that operate within a single process. Each thread has its own program counter and stack so that it knows where it is. Apart from this they can be considered the same as process, with the exception that they share the same address space. This means that all threads from the same process have access to the same global variable and the same files. Threads are also called light weight process. The table given below shows the various items that a process has compared to the items that each thread has.

Uses of Thread

Some application need to perform multiple activities at once that share common data and files. Thus reason behind need of multiple threads.
Ability of parallel entities, within a process to all share common variables and data.
Since threads do not have their own address space, it is easier to create and destroy threads than processes. In fact thread creation is around 100 times faster than process creation.
Thread switching has much less overhead than process switching because threads have few information attached with them.
Performance gain when there is a lot of computing and lot of I/O as tasks can overlap. Pure CPU bound thread application no advantage through
Threads are very effective on multiple CPU system for concurrent processing because with multiple CPU's true parallelism can be achieved

**Difference between Process and Threads**
Thread	Process
- Threads are lightweight	- Processes are heavyweight
- Threads are runs in address space of processes	- Processes have their own address space
- Threads shares files and other data	- Processes do not share files and other data
- Thread switching is faster	- Process switching is slower
Threads are easy to create and destroy	Process are difficult to create and destroy

Deadlock, Necessary Conditions of Deadlock/Characterization, Deadlocks Handling, Prevention, Avoidance in OS

In a multiprogramming environment, several processes may compete for a finite number of resources. A process requests resources; if the resources are not available at that time, the process enters into wait state. Waiting processes may never again change state, because the resources they have requested are held by other waiting processes. This situation is called a deadlock. System is deadlocked if there is a set of processes such that every process in the set is waiting for another process in the set.

Example 1
System has 2 disk drives.
P1 and P2 each hold one disk drive and each needs another one.

Example 2
- Porcess A makes a request to use the scanner
- Porcess A given the scanner
- Process B request thee CD writer
- Process B given the CD writer
- Now a requrest the CD writer
- A is denied permission until B releases the CD writer
- Also now B ask for the scanner
- Result is DEADLOCK !!

Necessary conditions for deadlock/Deadlock Characterization

A deadlock can arise if the following four conditions hold simultaneously (Necessary Condition).
1. Mutual exclusion:
Only one process at time can use the resource.
.
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:
A resource can be released only voluntarily by the process holding it, after that process has completed its task.

4. Circular wait:
A set {P0, P1, …, Pn} of waiting processes must exist 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.

Deadlocks Handling

Mainly there are four methods of handling deadlock
1. Deadlock avoidance
To avoid deadlocks the system dynamically considers every request and decides whether it is safe to grant it at this point. The system requires additional prior information regarding the overal potential use of each resource for each process.

2. Deadlock Prevention
It means that we design such a system where there is no chance of having a deadlock. The goal is to ensure that at least one of the Coffman's necessary conditions for deadlock can never hold. Deadlock can be prevented deadlocks by constraining how request for resources can be made in the system and how they are handled.

3. Deadlock Detection and Recovery
We can allow the system to enter a deadlock state, detect it and recover - If a system does not employ either a deadlock prevention or a deadlock avoidance algorithm, then a deadlock situation may occur. In this environment, the system can provide an algorithm that examines the state of the system to determine whether a deadlock has occurred and an algorithm to recover from the deadlock.

4. Ignore the Problem (Ostrich Algorithm) :
We can ignore the deadlock problem altogether. If the deadlock occurs, the system will stop functioning and will need to be restarted manually. This approach is used in systems in which deadlocks occur infrequently and if it is not a life critical system. This method is cheaper than other methods. This solution is used by most operating systems, including UNIX.

Deadlock Prevention

If we simulate deadlock with a table which is standing on its four legs then we can also simulate four legs with the four conditions which when occurs simultaneously, cause the deadlock.

However, if we break one of the legs of the table then the table will fall definitely. The same happens with deadlock, if we can be able to violate one of the four necessary conditions and don't let them occur together then we can prevent the deadlock.

Deadlock prevention methods are given below
1. Attack mutual Exclusion
Must hold only for non-sharable resources. Sharable resources, on the other hand, do not require mutually exclusive access, and thus cannot be involved in a deadlock.

2. Attack Hold and Wait
Ensure 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. There is a drawback of low resource utilization in this method.

3. Attack 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 preempted (or released). If a process requests some resources and if they are not available, we check whether they are allocated to some other process that is waiting for additional resources. If so, we preempt the desired resources from the waiting process and allocate them to the requesting process.

4. Attack Circular Wait
One way to ensure that circular wait never holds is to Impose a total ordering of all resource types, and require that each process requests resources in an increasing or decreasing order of enumeration.

Deadlock Avoidance

Deadlock can be avoided if certain information about processes are available to the operating system before allocation of resources, such as which resources a process will consume in its lifetime. For every resource request, the system sees whether granting the request will cause the system to enter an unsafe state. The system then only grants request that will lead to safe states. In order for the system to be able to determine whether the next state will be safe or unsafe, it must know in advance at any time.

Resources currently available
Resources currently allocated to each process
Resources that will be required and released by these process in the future

It is possible for a process to be in an unsafe state but for this not result in a deadlock. The notion of safe/unsafe state only refers to the ability of the system to enter a deadlock state or not. For example, if a process requests A which would result in an unsafe state but releases B which would prevent circular wait then the state is but the system is not in deadlock. One known algorithm that is used for deadlock avoidance is the Banker's algorithm, which requests resource usage limit to be known in advance. However, for many systems it is impossible to know in advance what every process will request. This means that deadlock avoidance is impossible practically.