Chapter 3 Deadlocks - Αδιέξοδα 3.1. Resource 3.2. Introduction to deadlocks 3.3. The ostrich algorithm 3.4. Deadlock detection and recovery 3.5. Deadlock avoidance 3.6. Deadlock prevention 3.7. Other issues

Resources - Πόροι Examples of computer resources printers tape drives tables Processes need access to resources in reasonable order Suppose a process holds resource A and requests resource B at same time another process holds B and requests A both are blocked and remain so Hardware and software deadlocks

Resources Deadlocks occur when … processes are granted exclusive access to devices we refer to these devices generally as resources Resources may have multiple copies Preemptable (προεκχωρήσιμοι) resources can be taken away from a process with no ill effects (for example memory) Nonpreemptable (μη-προεκχωρήσιμοι) resources will cause the process to fail if taken away (e.g. CDR)

Resources Sequence of events required to use a resource request the resource use the resource release the resource Must wait if request is denied requesting process may be blocked may fail with error code Nature of requesting a resource is highly system dependent (e.g. request system call)

Resource Acquisition t

Introduction to Deadlocks Formal definition : A set of processes is deadlocked if each process in the set is waiting for an event that only another process in the set can cause Only one thread, no interrupts Usually the event is release of a currently held resource None of the processes can … run release resources be awakened Number of processes and resources is unimportant

Four Conditions for Deadlock Mutual exclusion condition each resource assigned to 1 process or is available Hold and wait condition process holding resources can request additional No preemption condition previously granted resources cannot forcibly taken away Circular wait condition must be a circular chain of 2 or more processes each is waiting for resource held by next member of the chain All relate to a policy that a system can or can not have

Deadlock Modeling Modeled with directed graphs resource R assigned to process A process B is requesting/waiting for resource S process C and D are in deadlock over resources T and U

Deadlock Modeling A B C How deadlock occurs

How deadlock can be avoided Deadlock Modeling (o) (p) (q) How deadlock can be avoided

Deadlock Modeling Strategies for dealing with Deadlocks just ignore the problem altogether detection and recovery (ανίχνευση και επανόρθωση) dynamic avoidance (αποφυγή) careful resource allocation prevention (πρόληψη) negating one of the four necessary conditions

The Ostrich Algorithm – Αλγ. στρουθοκαμήλου Pretend there is no problem Reasonable if deadlocks occur very rarely cost of prevention is high UNIX and Windows takes this approach It is a trade off between convenience correctness

Detection with One Resource of Each Type A…G: processes; R…W: resources Note the resource ownership and requests Is this system deadlocked and if yes, which processes are involved? A cycle can be found within the graph, denoting deadlock

Detection with One Resource of Each Type We need a formal algorithm for detecting deadlocks A simple one to detect cycles: Take each node in turn. Do a DFS (depth first search) on it. If it comes to a node it has encountered in this run, then there exists a cycle. Previous graph has a cycle

Detection with Multiple Resources of Each Type Data structures needed by deadlock detection algorithm At all times: Σi=1Cij + Aj = Ej n

Detection with Multiple Resources of Each Type Deadlock detection is based on comparing vectors: Algorithm Look for an unmarked process, Pi for which the i-th row of R is less or equal to A If such a process is found, add the i-th row of C to A, mark the process and go back to step 1 If no such process exists the algorithm terminates

Detection with Multiple Resources of Each Type An example for the deadlock detection algorithm (3/2/1)

Detection with Multiple Resources of Each Type When to look for deadlocks? Every time a resource request is made Detection ASAP Expensive Every k minutes or whenever the CPU utilization drops below a certain threshold

Recovery from Deadlock - Επανόρθωση Recovery through preemption take a resource from some other process (e.g. printer) depends on nature of the resource Recovery through rollback checkpoint a process periodically use this saved state restart the process if it is found deadlocked

Recovery from Deadlock Recovery through killing processes crudest but simplest way to break a deadlock kill one of the processes in the deadlock cycle the other processes get its resources choose process that can be rerun from the beginning (perhaps not in cycle)

Deadlock Avoidance - Αποφυγή So far we assumed that all requests take place at the beginning The system must be able to decide whether granting a resource request is safe or not Is there an algorithm that can always avoid deadlocks? Yes, if certain information is known in advance

Deadlock Avoidance - Resource Trajectories Two process resource trajectories /// and \\\ are impossible to get What scheduler should do at point t ?

Demonstration that the state in (a) is safe – 10 instances Safe and Unsafe States A state is said to be safe if it is not deadlocked and there is some scheduling order in which every process can run to completion even if all of them suddenly request their maximum number of resources immediately (a) (b) (c) (d) (e) Demonstration that the state in (a) is safe – 10 instances

Safe and Unsafe States Demonstration that the state in b is not safe (a) (b) (c) (d) Demonstration that the state in b is not safe An unsafe state is not a deadlocked state

The Banker's Algorithm for a Single Resource (a) (b) (c) Check to see if granting the request leads to unsafe state Three resource allocation states safe unsafe

Banker's Algorithm for Multiple Resources Example of banker's algorithm with multiple resources

Banker's Algorithm for Multiple Resources Look for a row, R, whose unmet resource needs are all smaller than or equal to A. If no such row exists the system will eventually deadlock since no process can run to completion. Assume the process of the row chosen requests all the resources it needs (which is guaranteed to be possible) and finishes. Mark that process as terminated and add all its resources to the A vector Repeat step 1 and 2 until either all processes are marked terminated, in which case the state is safe, or until a deadlock occurs, in which case is not. B requests a printer… (D,A or E, …) E requests a printer… (deadlock)

Deadlock Prevention Attacking the Mutual Exclusion Condition Some devices (such as printer) can be spooled only the printer daemon uses printer resource thus deadlock for printer eliminated Not all devices can be spooled Principle: avoid assigning resource when not absolutely necessary as few processes as possible actually claim the resource

Attacking the Hold and Wait Condition Goal: Prevent processes that hold resources from waiting for more resources Require processes to request resources before starting a process never has to wait for what it needs Problems may not know required resources at start of run also ties up resources other processes could be using Variation: process must give up temporarily all resources before requesting a new one then request all immediately needed

Attacking the No Preemption Condition This is not a viable option Consider a process given the printer halfway through its job now forcibly take away printer !!??

Attacking the Circular Wait Condition (a) (b) Normally ordered resources A resource graph

Attacking the Circular Wait Condition Rule: All requests of a process must be made in numerical order => the resource allocation graph can not have cycles Either i < j or i > j => can’t have deadlocks Same logic with multiple resources: at every instant one assigned resource will be the highest Problem: impossible to find an ordering to satisfy everyone

Attacking the Circular Wait Condition Summary of approaches to deadlock prevention Avoidance and prevention are not widely used in OS, but have special-purpose applications

Other Issues Two-Phase Locking DB systems lock records for update Phase One process tries to lock all records it needs, one at a time if needed record found locked, start over (no real work done in phase one) If phase one succeeds, it starts second phase, performing updates releasing locks Note similarity to requesting all resources at once Algorithm works where programmer can arrange things so that the program can be stopped and restarted

Non-resource Deadlocks Possible for two processes to deadlock each is waiting for the other to do some task Can happen with semaphores each process required to do a down() on two semaphores (mutex and another) if done in wrong order, deadlock results

Starvation Algorithm to allocate a resource may be to give to shortest job first Works great for multiple short jobs in a system May cause long job to be postponed indefinitely even though not blocked Solution: First-come, first-serve policy