13) Replacement Policy. First-Come-First-Served, Round-Robin algorithms

In most operating system texts, the treatment of memory management includes a section entitled “replacement policy,” which deals with the selection of a page in main memory to be replaced when a new page must be brought in. This topic

is sometimes difficult to explain because several interrelated concepts are involved:

• How many page frames are to be allocated to each active process

• Whether the set of pages to be considered for replacement should be limited to those of the process that caused the page fault or encompass all the page frames in main memory

• Among the set of pages considered, which particular page should be selected for replacement

We shall refer to the first two concepts as resident set management, which is dealt with in the next subsection, and reserve the term replacement policy for the third concept, which is discussed in this subsection. The area of replacement policy is probably the most studied of any area of memory management.When all of the frames in main memory are occupied and it is necessary to bring in a new page to satisfy a page fault, the replacement policy determines

which page currently in memory is to be replaced. All of the policies have as their objective that the page that is removed should be the page least likely to be referenced in the near future. Because of the principle of locality, there is often a

high correlation between recent referencing history and near-future referencing patterns. Thus, most policies try to predict future behavior on the basis of past behavior. One tradeoff that must be considered is that the more elaborate and sophisticated the replacement policy, the greater the hardware and software overhead to implement it.

2) First-come, first-served (FCFS) – sometimes first-in, first-served and first-come, first choice – is a service policy whereby the requests of customers or clients are attended to in the order that they arrived, without other biases or preferences. The policy can be employed when processing sales orders, in determining restaurant seating, on a taxi stand, etc. In Western society, it is the standard policy for the processing of most queues in which people wait for a service or two.

3) Round-Robin algorithms.

round-robin - A scheduling algorithm in which processes are activated in a fixed cyclic order. Those which cannot proceed because they are waiting for some event (e.g. termination of a child process or an input/output operation) simply return control to the scheduler. The virtue of round-robin scheduling is its simplicity - only the processes themselves need to know what they are waiting for or how to tell if it has happened. However, if a process goes back to sleep just before the event for which it is waiting occurs then the event will not get handled until all the other processes have been activated.

14) Replacement Policy. Shortest Process Next, Shortest Remaining Time algorithms.

In most operating system texts, the treatment of memory management includes a section entitled “replacement policy,” which deals with the selection of a page in main memory to be replaced when a new page must be brought in. This topic

is sometimes difficult to explain because several interrelated concepts are involved:

• How many page frames are to be allocated to each active process

• Whether the set of pages to be considered for replacement should be limited to those of the process that caused the page fault or encompass all the page frames in main memory

• Among the set of pages considered, which particular page should be selected for replacement

We shall refer to the first two concepts as resident set management, which is dealt with in the next subsection, and reserve the term replacement policy for the third concept, which is discussed in this subsection. The area of replacement policy is probably the most studied of any area of memory management.When all of the frames in main memory are occupied and it is necessary to bring in a new page to satisfy a page fault, the replacement policy determines

which page currently in memory is to be replaced. All of the policies have as their objective that the page that is removed should be the page least likely to be referenced in the near future. Because of the principle of locality, there is often a

high correlation between recent referencing history and near-future referencing patterns. Thus, most policies try to predict future behavior on the basis of past behavior. One tradeoff that must be considered is that the more elaborate and sophisticated the replacement policy, the greater the hardware and software overhead to implement it.

2) Shortest Process Next

Nonpreemptive. Assume that the execution time of a program can be accurately estimated in advance. Then a suitable selection criterion is to always pick the shortest process, because in this way throughput is obviously maximised. As obviously, there's a risk of starvation for long processes, and they experience sluggish response times anyway

3) Shortest remaining time, also known as shortest remaining time first (SRTF), is a scheduling method that is a preemptive version of shortest job next scheduling. In this scheduling algorithm, the process with the smallest amount of time remaining until completion is selected to execute. Since the currently executing process is the one with the shortest amount of time remaining by definition, and since that time should only reduce as execution progresses, processes will always run until they complete or a new process is added that requires a smaller amount of time.

Shortest remaining time is advantageous because short processes are handled very quickly. The system also requires very little overhead since it only makes a decision when a process completes or a new process is added, and when a new process is added the algorithm only needs to compare the currently executing process with the new process, ignoring all other processes currently waiting to execute.

Like shortest job first, it has the potential for process starvation; long processes may be held off indefinitely if short processes are continually added. This threat can be minimal when process times follow a heavy-tailed distribution.^[1]

Like shortest job next scheduling, shortest remaining time scheduling is rarely used outside of specialized environments because it requires accurate estimations of the runtime of all processes that are waiting to execute.