Exercise 3.43 · 习题

Exercise 3.43: Suppose that the balances in

three accounts start out as $10,$20, and $30, and that multiple processes run,

exchanging the balances in the accounts. Argue that if the processes are run

sequentially, after any number of concurrent exchanges, the account balances

should be $10,$20, and $30 in some order. Draw a timing diagram like the one

in Figure 3.29 to show how this condition can be violated if the

exchanges are implemented using the first version of the account-exchange

program in this section. On the other hand, argue that even with this

exchange program, the sum of the balances in the accounts will be

preserved. Draw a timing diagram to show how even this condition would be

violated if we did not serialize the transactions on individual accounts.

练习 3.43：假设三个账户的余额初始为 $10、$20 和 $30，多个进程并发运行，相互交换这些账户的余额。请论证：若这些进程顺序执行，则在任意次并发交换之后，账户余额仍应以某种顺序为$10、$20 和$30。画出类似图 3.29 的时序图，说明若用本节第一版账户交换程序实现交换操作，上述条件是如何被违反的。另一方面，请论证：即使使用这个交换程序，各账户余额之和仍将被保持。再画一张时序图，说明若我们不对单个账户的事务加以串行化，则连这一条件也会被违反。