Long-term scheduling: ================= Long term scheduling obviously controls the degree of multiprogramming in multitasking systems, following certain policies to decide whether the system can honour a new job submission or, if more than one job is submitted, which of them should be selected. The need for some form of compromise between degree of multiprogramming and throughput seems evident, especially when one considers interactive systems. The higher the number of processes, in fact, the smaller the time each of them may control CPU for, if a fair share of responsiveness is to be given to all processes. Moreover we have already seen that a too high number of processes causes waste of CPU time for system housekeeping chores (trashing in virtual memory systems is a particularly nasty example of this). However, the number of active processes should be high enough to keep the CPU busy servicing the payload (i.e. the user processes) as much as possible, by ensuring that - on average - there always be a sufficient number of processes not waiting for I/O. Simple policies for long term scheduling are Simple First Come First Served (FCFS): its essentially a FIFO scheme. All job requests (e.g. a submission of a batch program, or an user trying to log in in a time shared system) are honoured up to a fixed system load limit, further requests being refused tout court, or enqueued for later processing. Priority schemes. Note that in the context of long term scheduling ``priority has a different meaning than in dispatching: here it affects the choice of a program to be entered the system as a process, there the choice of which ready process process should be executed. The long-term, or admission scheduler, decides which jobs or processes are to be admitted to the ready queue (in the Main Memory); that is, when an attempt is made to execute a program, its admission to the set of currently executing processes is either authorized or delayed by the long-term scheduler . Thus, this scheduler dictates what processes are to run on a system, and the degree of concurrency to be supported at any one time – whether a high or low amount of processes are to be executed concurrently, and how the split between I/O intensive and CPU intensive processes is to be handled. The long term scheduler is responsible for controlling the degree of multiprogramming . In modern operating systems, this is used to make sure that real time processes get enough CPU time to finish their tasks. Without proper real time scheduling, modern GUIs would seem sluggish. Long-term scheduling is also important in large-scale systems such as batch processing systems, computer clusters, supercomputers and render farms. In these cases, special purpose job scheduler software is typically used to assist these functions, in addition to any underlying admission scheduling support in the operating system.
Posted on: Fri, 01 Aug 2014 14:03:11 +0000