Editing Scheduling (computing) (section)

===={{Anchor|LONG-TERM}}Long-term scheduling====
The ''long-term scheduler'', or ''admission scheduler'', decides which jobs or processes are to be admitted to the ready queue (in 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, the degree of concurrency to be supported at any one time{{snd}} whether many or few 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 general, most processes can be described as either [[I/O-bound]] or [[CPU-bound]]. An I/O-bound process is one that spends more of its time doing I/O than it spends doing computations. A CPU-bound process, in contrast, generates I/O requests infrequently, using more of its time doing computations. It is important that a long-term scheduler selects a good process mix of I/O-bound and CPU-bound processes. If all processes are I/O-bound, the ready queue will almost always be empty, and the short-term scheduler will have little to do. On the other hand, if all processes are CPU-bound, the I/O waiting queue will almost always be empty, devices will go unused, and again the system will be unbalanced. The system with the best performance will thus have a combination of CPU-bound and I/O-bound processes. In modern operating systems, this is used to make sure that real-time processes get enough CPU time to finish their tasks.<ref name="Galvin">{{cite book
|author1=[[Abraham Silberschatz]] |author2=Peter Baer Galvin |author3=Greg Gagne
| title = Operating System Concepts
| volume = 9
| year= 2013
| publisher= John Wiley & Sons, Inc.
| isbn= 978-1-118-06333-0
}}</ref>

Long-term scheduling is also important in large-scale systems such as [[batch processing]] systems, [[computer cluster]]s, [[supercomputer]]s, and [[render farm]]s. For example, in [[concurrent computing|concurrent systems]], [[coscheduling]] of interacting processes is often required to prevent them from blocking due to waiting on each other. 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.

Some operating systems only allow new tasks to be added if it is sure all real-time deadlines can still be met.
The specific heuristic algorithm used by an operating system to accept or reject new tasks is the ''admission control mechanism''.<ref>
Robert Kroeger (2004).
[https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.71.3977&rep=rep1&type=pdf "Admission Control for Independently-authored Realtime Applications"].
UWSpace.
http://hdl.handle.net/10012/1170 .
Section "2.6 Admission Control".
p. 33.
</ref>