Chapter3 OS Essentials Essays

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3.6 What are the differences among short-term, medium term, and long-term scheduling?


The short-term scheduler selects from the ready processes the next process to run and gives it the CPU. The long-term scheduler selects from the pool of processes that are waiting on disk and loads the selected processes into memory. These processes have not yet begun their execution. The medium-term scheduler takes processes that are currently in memory and selects those to be swapped out to disk. These processes will be swapped back in at a later point. This is done to improve process mix or because of memory requirements like overcommitted and needs to free up memory.

A primary difference is in the frequency
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Describe the undesirable circumstances that could arise from not enforcing either the "at most once" or "exactly once" semantics. Describe possible uses for a mechanism that had neither of these guarantees.

Answer: If an RPC mechanism could not support either the “at most once” or “at least once” semantics, then the RPC server cannot guarantee that a remote procedure will not be invoked multiple occurrences. Consider if a remote procedure were withdrawing money from a bank account on a system that did not support these semantics. It is possible that a single invocation of the remote procedure might lead to multiple withdrawals on the server. For a system to support either of these semantics generally requires the server maintain some form of client state such as the timestamp described in the text. If a system were unable to support either of these semantics, then such a system could only safely provide remote procedures that do not alter data or provide time-sensitive results. Using our bank account as an example, we certainly require “at most once” or “at least once” semantics for performing a withdrawal or deposit. However, an inquiry into an account balance or other

3.12 Using the program shown in Figure 3.30, explain what the output will be at Line A.

The output will be Parent: Value = 5. Once the

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