Computer Science
Deadlock
Deadlock is a situation in which two or more processes are unable to proceed because each is waiting for one of the others to do something. This can occur when two or more processes are competing for the same resources or when a process is unable to release a resource that it has already acquired. Deadlocks can cause a system to become unresponsive and can be difficult to resolve.
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5 Key excerpts on "Deadlock"
eBook - ePubReal-Time Embedded Systems
Open-Source Operating Systems Perspective
- Ivan Cibrario Bertolotti, Gabriele Manduchi(Authors)
- 2017(Publication Date)
- CRC Press(Publisher)
B or vice versa.Unfortunately, this means that the code will be hard to debug, and even the insertion of a debugger or code instrumentation to better understand what is happening may perturb system timings enough to make the Deadlock disappear. This is a compelling reason to address Deadlock problems from a theoretical perspective, during system design , rather than while testing or debugging it.It also depends on a few specific properties of the resources involved and on how the operating system manages them. For example, albeit this technique is not widespread in real-time operating systems, some general-purpose operating systems are indeed able to swap process images in and out of main memory with the assistance of a mass storage device. Doing this, they are able to accommodate processes whose total memory requirements exceed the available memory.In this case, the memory request performed by B in our running example does not necessarily lead to an endless wait because the operating system can temporarily take away—or preempt —some memory from A in order to satisfy B ’s request, so that both process will be eventually able to complete their execution. As a consequence, the same processes may or may not be at risk for what concerns Deadlock, when they are executed by operating systems employing dissimilar memory management or, more generally, resource management techniques.4.2 Formal Definition of Deadlock
In the most general way, a Deadlock can be defined formally as a situation in which a set of processes passively waits for an event that can be triggered only by another process in the same set. More specifically, when dealing with resources, there is a Deadlock when all processes in a set are waiting for some resources previously allocated to other processes in the same set. As discussed in the example of Section 4.1
eBook - ePub- Jie Wu(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
Chapter 5Prevention, Avoidance, and Detection of Deadlock
Distributed systems, in general, exhibit a high degree of resource and data sharing, a situation in which Deadlocks may happen. Deadlocks arise when members of a group of processes which hold resources are blocked indefinitely from access to resources held by other processes within the group. In this chapter methods for prevention, avoidance, and detection of Deadlock in distributed systems are discussed.5.1 The Deadlock Problem
In a computer system Deadlocks arise when members of a group of processes which hold resources are blocked indefinitely from access to resources held by other processes within the group. For example, consider a system with two I/O controllers. The system will immediately allocate an I/O controller to a requesting process upon each request. Suppose two processes P i and P j have the following requests:- P i requests one I/O controller and the system allocates one.
- P j requests one I/O controller and again the system allocates one.
- P i wants another I/O controller but has to wait since the system ran out of I/O controllers.
- P j wants another I/O controller and waits.
In the above example both processes are in the state of permanent blocking and a Deadlock involving these two processes occurs.FIGURE 5.1 Two cities connected by (a) one bridge and by (b) two bridges.5.1.1 Conditions for Deadlocks
Formally, a Deadlock can arise if and only if the following four conditions hold simultaneously [14 ]:- Mutual exclusion . No resource can be shared by more than one process at a time.
- Hold and wait . There must exist a process that is holding at least one resource and is waiting to acquire additional resources that are currently being held by other processes.
- No preemption . A resource cannot be preempted.
- Circular wait . There is a cycle in the wait-for graph.
Figure 5.1 shows a Deadlock example where there are two cities A and B separated by a river. The two cities are connected by a bridge (Figure 5.1 (a) ). Assume that the bridge is so narrow that only one person can walk through at a time. There will be no problem if several persons walk towards the same direction. Deadlock occurs when two persons heading in different directions - one from city A to city B and the other one from city B to city A -
eBook - ePubMastering Concurrency in Python
Create faster programs using concurrency, asynchronous, multithreading, and parallel programming
- Quan Nguyen(Author)
- 2018(Publication Date)
- Packt Publishing(Publisher)
With the example of the Dining Philosophers problem in mind, let us consider the formal concept of Deadlock, and the relevant theories around it. Given a concurrent program with multiple threads or processes, the execution flow enters a situation of Deadlock if a process (or thread) is waiting on a resource that is being held and utilized by another process, which is, in turn, waiting for another resource that is held by a different process. In other words, processes cannot proceed with their execution instructions while waiting for resources that can only be released after the execution is completed; therefore, these processes are unable to change their execution states.Deadlock is also defined by the conditions that a concurrent program needs to have at the same time in order for Deadlock to occur. These conditions were first proposed by the computer scientist Edward G. Coffman, Jr., and are therefore known as the Coffman conditions. These conditions are as follows:- At least one resource has to be in a non-shareable state. This means that that resource is being held by an individual process (or thread), and cannot be accessed by others; the resource can only be accessed and held by a single process (or thread) at any given time. This condition is also known as mutual exclusion.
- There exists one process (or thread) that is simultaneously accessing a resource and waiting for another held by other processes (or threads). In other words, this process (or thread) needs access to two resources in order to execute its instructions, one of which it is already holding, the other of which it is waiting for from other processes (or threads). This condition is called hold and wait.
- Resources can only be released by a process (or a thread) holding them if there are specific instructions for the process (or thread) to do so. This is to say that unless the process (or thread) voluntarily and actively releases the resource, that resource remains in a non-shareable state. This is the no preemption condition.
- The final condition is called circular wait. As suggested by the name, this condition specifies that there exists a set of processes (or threads) such that the first process (or thread) in the set is in a waiting state for a resource to be released by the second process (or thread), which, in turn, needs to be waiting for the third process (or thread); finally, the last process (or thread) in the set is waiting for the first one.
eBook - ePubAdvanced Python Programming
Build high performance, concurrent, and multi-threaded apps with Python using proven design patterns
- Dr. Gabriele Lanaro, Quan Nguyen, Sakis Kasampalis(Authors)
- 2019(Publication Date)
- Packt Publishing(Publisher)
With the example of the Dining Philosophers problem in mind, let us consider the formal concept of Deadlock, and the relevant theories around it. Given a concurrent program with multiple threads or processes, the execution flow enters a situation of Deadlock if a process (or thread) is waiting on a resource that is being held and utilized by another process, which is, in turn, waiting for another resource that is held by a different process. In other words, processes cannot proceed with their execution instructions while waiting for resources that can only be released after the execution is completed; therefore, these processes are unable to change their execution states.Deadlock is also defined by the conditions that a concurrent program needs to have at the same time in order for Deadlock to occur. These conditions were first proposed by the computer scientist Edward G. Coffman, Jr., and are therefore known as the Coffman conditions. These conditions are as follows:- At least one resource has to be in a non-shareable state. This means that that resource is being held by an individual process (or thread), and cannot be accessed by others; the resource can only be accessed and held by a single process (or thread) at any given time. This condition is also known as mutual exclusion.
- There exists one process (or thread) that is simultaneously accessing a resource and waiting for another held by other processes (or threads). In other words, this process (or thread) needs access to two resources in order to execute its instructions, one of which it is already holding, the other of which it is waiting for from other processes (or threads). This condition is called hold and wait.
- Resources can only be released by a process (or a thread) holding them if there are specific instructions for the process (or thread) to do so. This is to say that unless the process (or thread) voluntarily and actively releases the resource, that resource remains in a non-shareable state. This is the no preemption condition.
- The final condition is called circular wait. As suggested by the name, this condition specifies that there exists a set of processes (or threads) such that the first process (or thread) in the set is in a waiting state for a resource to be released by the second process (or thread), which, in turn, needs to be waiting for the third process (or thread); finally, the last process (or thread) in the set is waiting for the first one.
eBook - ePub- Jan Palach(Author)
- 2014(Publication Date)
- Packt Publishing(Publisher)
Erlang , for instance, use this model to implement communication in its parallel architecture. Once data is copied at each message exchange, it is impossible that problems occur in terms of concurrence of access. Although memory use seems to be higher than in shared memory state, there are advantages to the use of this model. They are as follows:- Absence of data access concurrence
- Messages can be exchange locally (various processes) or in distributed environments
- This makes it less likely that scalability issues occur and enables interoperability of different systems
- In general, it is easy to maintain according to programmers
Passage contains an image Identifying parallel programming problems
There are classic problems that brave keyboard warriors can face while battling in the lands where parallel programming ghosts dwell. Many of these problems occur more often when inexperienced programmers make use of workers combined with shared state. Some of these issues will be described in the following sections.Deadlock
Deadlock is a situation in which two or more workers keep indefinitely waiting for the freeing of a resource, which is blocked by a worker of the same group for some reason. For a better understanding, we will use another real-life case. Imagine the bank whose entrance has a rotating door. Customer A heads to the side, which will allow him to enter the bank, while customer B tries to exit the bank by using the entrance side of this rotating door so that both customers would be stuck forcing the door but heading nowhere. This situation would be hilarious in real life but tragic in programming.Note
Deadlock is a phenomenon in which processes wait for a condition to free their tasks, but this condition will never occur.Starvation
This is the issue whose side effects are caused by unfair raking of one or more processes that take much more time to run a task. Imagine a group of processes, A, which runs heavy tasks and has data processor priority. Now, imagine that a process A with high priority constantly consumes the CPU, while a lower priority process B never gets the chance. Hence, one can say that process B is starving
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