Can anyone help me explain the total meaning of this paragraph?
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.
There are many things I do not understand in this paragraph. My questions are:
- "if the processes are run sequentially, after any number of concurrent exchanges," - as I understand it, the word "sequential" and "concurrent" mean contradictory things. As far as I know "sequential" means one at a time and "concurrent" means occuring at the same time or simultaneous. I translated this as: if the processes are run one at a time, after any number of simultaneous exchanges. How could simultaneous things be happening if things happen one at a time? I am totally confused.
- "the account balances should be $10, $20, and $30 in some order." - Does this mean we should find out all orders of exchanges that lead to three banks having $10, $20 and $30 respectively?
- "the sum of the balances in the accounts will be preserved." - This is a little clearer. Does preserved means "the account balances should be $10, $20, and $30 in some order." just like what is said previously?
Can anyone totally explain things I don't understand. Thanks.