Deadlock detection and Lock management pragmatics

Deadlock detection:

One issue the lock manager should deal with is deadlock. Deadlock consists of every member of a set of transactions waiting for some other member of the set to give up a lock. Standard lore has it that one is able to have timeout or deadlock-prevention or deadlock detection.

Timeout causes waits to be denied subsequent to some specified interval. It has the property that as the system turn into more congested, more and more transactions time out (for the reason that time runs slower and because more resources are in use so that one waits more). As well timeout puts an upper limit on the duration of a transaction. Generally the dynamic properties of timeout make it acceptable for a lightly loaded system however inappropriate for a congested system.

Deadlock prevention is achieved by- requesting each locks at once or requesting locks in a specified order or never waiting for a lock or . . . Generally deadlock prevention is a bad deal because one rarely knows what locks are needed in advance (consider looking something up in an index) as well as consequently one locks too much in advance.

Although several situations allow deadlock prevention, general systems be likely to require deadlock detection. IMS for instance started with a deadlock prevention scheme (intent scheduling) however was forced to introduce a deadlock detection scheme to raise concurrency (Program Isolation).

Deadlock detection as well as resolution is no big deal in a data management system environment. The system already has plenty of facilities for transaction backup so that it can deal with other sorts of errors. Deadlock merely becomes another (hopefully infrequent) source of backup. As will be observes the algorithms for detecting and resolving deadlock are not complicated or time consuming.

A deadlock detection-resolution scenario is:

•    Detect a deadlock.
•    Pick a victim (a lock to pre-empt from a process.)
•    Back out a victim which will release lock.
•    Allowance a waiter.
•    (alternatively) Restart victim.

Lock manager is merely responsible for deadlock detection and victim selection, Recovery management implements transaction backup as well as controls restart logic.

How to detect Deadlock:

There are several heuristic ways of detecting deadlock (example linearly order resources or processes as well as declare deadlock if ordering is violated by a wait request.)

Here we limit ourselves to algorithmic solutions.

The detection of deadlock perhaps cast in graph-theoretic terms. We initiate the notion of the wait-for graph.

•    The nodes of the graph are transactions as well as locks.
•    The edges of the graph are directed as well as are constructed as follows:
•    If lock L is granted to transaction T afterwards draw an edge from L to T.
•    If transaction T is waiting for transaction L afterwards draw an edge from T to L.

At any instant there is a deadlock if as well as only if the wait-for graph has a cycle. Therefore deadlock detection becomes an issue of building the wait-for graph and searching it for cycles.

Habitually this ‘transaction waits for lock waits for transaction’ graph can be reduced to a smaller ‘transaction waits for transaction’ graph. The larger graph requirement is maintained only if the identities of the locks in the cycle are relevant. I know of no situation where this is required.

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