barcode generator excel 2013 ean13 Reserved Resources Deadlock caused by competition for classi ed resources (tape in Software

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units, disk drives, memory) is sometimes mitigated by not allowing any new transactions to be started when the utilization reaches a certain level This technique will not assure deadlock avoidance unless the reserve is kept impractically large, enough to allow all active transactions to complete This technique is used, for instance, by
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in its bu er management Operational experience can be used to balance the amount of reserve against the deadlock frequency to achieve optimum productivity The gathering of su cient deadlock statistics for throughput optimization is an unpleasant task in systems which do not provide rollback 13-2-4 Deadlock Detection Deadlock detection is by de nition too late The only cure when deadlock is detected is to kill one transaction as gracefully as possible Deadlock detection is, however, an important task in the maintenance of system integrity If deadlock avoidance is already part of the system, then deadlock detection provides a backstop which allows the correction of faults in programs or hardware In an environment where the responsibility for deadlock avoidance is placed on the programmers, a deadlock detection facility is vital Anyone who has ever attempted to debug a running on-line system which contains initially a few deadlocked transactions, but which slowly becomes paralyzed as another transactions attempt to access locked objects can recall the horror of working against time, knowing that at some point the debugging transaction itself will be blocked The solution, canceling all transactions and restarting the system, leaves one waiting for the next occurrence There are systems in operation which live with the expectation of occasional deadlocks, owing to the desire to optimize performance In systems which cannot legislate incremental claims out of existence, deadlock is unavoidable and has to be detected and, when detected, resolved Deadlock detection algorithms are similar to deadlock avoidance algorithms A circular chain of dependencies has to be found In Fig 13-6 transactions P1, P2, P4 create the circular deadlock (c, g, k, j, h, f, e, d) However, transactions P3 and P5 are also blocked until the deadlock is removed The deadlock can be broken by application of approach 3: remove owned objects from a transaction The least number of objects in the circle are owned by
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transaction P1 (c, g), but it is in general, di cult to restart a transaction at an arbitrary point (p) unless a checkpoint was made In a transaction-oriented system with recovery facilities an entire transaction can be restarted, so that transaction P2 is the candidate for assassination; it owns the least number of objects (f, e, d) Only objects which have actually been modi ed should be counted when the transaction to be killed is selected Deadlock detection by logical analysis is impossible if the desired object cannot be identi ed A transaction which waits for a request from any of a number of terminals is an example The transaction sleeps until awakened, and then it will check the reason for the alarm and do something appropriate Figure 6-10 showed a le system process which can respond to user service requests, le operation completed signals, and time-exceeded signals It is desirable that these processes do not themselves claim sharable resources so they can remain outside of the domain of deadlock management Time limits placed on transaction execution times provide another means to detect deadlock, blocking, or hibernation The guilty party cannot be positively identi ed when a time limit is exceeded In the example above, it is possible that transaction P5 exceeds its time allotment rst Killing P5, however, will not resolve the deadlock condition The action to be taken when a deadlock is detected will depend on the available recovery facilities One popular multiuser system stops This allows console diagnosis of the problem but leaves remote users quite unhappy
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Preemption If the transaction to be killed has not performed updates, it can be abandoned If automatic restart is not possible, it is at least polite to notify the initiator of the transaction of the situation and request to please enter the request again No satisfactory solution to resolve a deadlock where all transactions have performed updates exists unless rollback facilities are available If updates are infrequent, a requirement to perform a checkpoint before every lock involving an update can be imposed A checkpoint would permit a rollback of transaction P1 to point p in Fig 13-6 In a system which provides general recovery facilities, as described in Sec 11-4, the before-images can be used to roll back any transaction to the point where it entered the database The message which started the transaction can then be replayed from the transaction log Some delay is desirable so that the other transactions have a chance to pass the object which caused the problem
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If automatic rollback from a fault condition is not possible, human intervention is required to check for errors A simple restart will leave the database inconsistent, and the reexecution of the update may introduce further errors While Peter might appreciate a raise of 21% instead of 10%, the department budget totals may will not re ect this fact after an error All transactions blocked during the deadlock have to be traced for veri cation A ected records have to be blocked from further access, listed, and manually veri ed and corrected If blocking is not done, any inconsistencies introduced can further contaminate the database A correction may require real world data, ie, How many widgets do we really have in stock after an inventory le failure Where databases support nancial data, the accountant s tradition of double entry bookkeeping can be helpful Since any transfer of funds from one account
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