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care has to be taken that the reader of the chain sequence is not thrown o the track, as indicated by the stars in the diagram The safe procedure shown on the right of Fig 13-3 unfortunately tends to use more bu ers and more access time to execute, since READ and REWRITE operations for one block are not paired If the entire ring is locked the left choice does become safe, the performance of the speci c transaction increases, but other, simultaneous transactions may be held up
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Sec 13-1 13-1-7 Lock Management
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Many operating systems only provide locks for entire les Setting a lock means that all other updates or even all operations to the le are locked out This can cripple system response time A single lock per le does simplify the referencing mechanism For every open le one lock is de ned When smaller granules are to be locked, the number of required locks is unpredictable, but often large Since shared databases require many locks, the management of the locks frequently becomes a responsibility of the database or the transaction management system The semaphores used for locking have to be dynamically assigned to data items since it is not feasible to preassign a semaphore for every possible data item and its hierarchy Locking schemes for databases, if implemented with care, can support the number of locks associated with a ne granularity A straightforward implementation to handle semaphores dynamically will use a linked list for the locked items The identi cation of an item being locked and its temporary owner is entered into a list of locked items When the lock is released the entry is removed from the list On each reference which might con ict according to Tables 13-2 or 13-4 the list is checked to nd if the object is locked The search for locks requires a linear scan Considerable CPU time will be used if many locks are active and have to be checked A more e ective way to organize semaphores for locks is by using a hash table Only one bit is needed to indicate that an object is locked, so that a modestly sized hash table can accommodate many locks To minimize the number of collisions we choose a generous hash table size, say ms 2 #(transactionsmax ) #(number of locks a transaction may set) A single additional semaphore is used to protect access to the hash table when setting or testing locks The unique object identi cation is the key to be transformed into a bit address in the table, and the setting bit at that address indicates whether the object is occupied or free The important notion in using a hash table for the semaphores is that collisions are not detected or resolved The collisions due to the use of hashing cause some false lock indications, but the rate of false-locks can be set very small by increasing ms A transaction is typically blocked without checking if the lock is true or false; the only check needed is that the occupied signal is not due to another lock request from the same transaction The transaction progress thread de ned in Sec 11-4-2 contains the required data The rate of overall false lock indications will increase in busy times, but the time needed to deal with them remains constant, and this a considerable advantage over the list-processing scheme, which uses more time per check when the demand is already high However, the probability of deadlock (presented in Sec 13-2-3) increases at busy times, and if lexical ordering is used to avoid deadlock the key-toaddress transformation should be of the sequence-maintaining type, as described in Sec 3-5-1 Using the same space to keep object names for collision resolution would greatly reduce ms and increase the number of collisions To regain the low number of
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