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There are two approaches to deadlock prevention One approach ensures that no cyclic waits can occur by ordering the requests for locks, or requiring all locks to be acquired together The other approach is closer to deadlock recovery, and performs transaction rollback instead of waiting for a lock, whenever the wait could potentially result in a deadlock The simplest scheme under the rst approach requires that each transaction locks all its data items before it begins execution Moreover, either all are locked in one step or none are locked There are two main disadvantages to this protocol: (1) it is often hard to predict, before the transaction begins, what data items need to be locked; (2) data-item utilization may be very low, since many of the data items may be locked but unused for a long time Another approach for preventing deadlocks is to impose an ordering of all data items, and to require that a transaction lock data items only in a sequence consistent with the ordering We have seen one such scheme in the tree protocol, which uses a partial ordering of data items A variation of this approach is to use a total order of data items, in conjunction with two-phase locking Once a transaction has locked a particular item, it cannot
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request locks on items that precede that item in the ordering This scheme is easy to implement, as long as the set of data items accessed by a transaction is known when the transaction starts execution There is no need to change the underlying concurrency-control system if two-phase locking is used: All that is needed it to ensure that locks are requested in the right order The second approach for preventing deadlocks is to use preemption and transaction rollbacks In preemption, when a transaction T2 requests a lock that transaction T1 holds, the lock granted to T1 may be preempted by rolling back of T1 , and granting of the lock to T2 To control the preemption, we assign a unique timestamp to each transaction The system uses these timestamps only to decide whether a transaction should wait or roll back Locking is still used for concurrency control If a transaction is rolled back, it retains its old timestamp when restarted Two different deadlockprevention schemes using timestamps have been proposed: 1 The wait die scheme is a nonpreemptive technique When transaction Ti requests a data item currently held by Tj , Ti is allowed to wait only if it has a timestamp smaller than that of Tj (that is, Ti is older than Tj ) Otherwise, Ti is rolled back (dies) For example, suppose that transactions T22 , T23 , and T24 have timestamps 5, 10, and 15, respectively If T22 requests a data item held by T23 , then T22 will wait If T24 requests a data item held by T23 , then T24 will be rolled back 2 The wound wait scheme is a preemptive technique It is a counterpart to the wait die scheme When transaction Ti requests a data item currently held by Tj , Ti is allowed to wait only if it has a timestamp larger than that of Tj (that is, Ti is younger than Tj ) Otherwise, Tj is rolled back (Tj is wounded by Ti ) Returning to our example, with transactions T22 , T23 , and T24 , if T22 requests a data item held by T23 , then the data item will be preempted from T23 , and T23 will be rolled back If T24 requests a data item held by T23 , then T24 will wait Whenever the system rolls back transactions, it is important to ensure that there is no starvation that is, no transaction gets rolled back repeatedly and is never allowed to make progress Both the wound wait and the wait die schemes avoid starvation: At any time, there is a transaction with the smallest timestamp This transaction cannot be required to roll back in either scheme Since timestamps always increase, and since transactions are not assigned new timestamps when they are rolled back, a transaction that is rolled back repeatedly will eventually have the smallest timestamp, at which point it will not be rolled back again There are, however, signi cant differences in the way that the two schemes operate In the wait die scheme, an older transaction must wait for a younger one to release its data item Thus, the older the transaction gets, the more it tends to wait By contrast, in the wound wait scheme, an older transaction never waits for a younger transaction
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