Two-level scheduling: Difference between revisions

Revision as of 15:57, 26 April 2011

Revision as of 10:38, 6 March 2013

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'''Two-level scheduling''' is a [[computer science]] term to describe a method to more efficiently perform process [[Scheduling (computing)|scheduling]] that involves [[swapped out]] [[process (computing)|processes]].

Consider this problem: A system contains 50 running processes all with equal priority. However, the system's [[computer storage|memory]] can only hold 10 processes in memory simultaneously. Therefore, there will always be 40 processes swapped out written on [[virtual memory]] on the [[hard disk]]. The time taken to swap out and swap in a process is 50 ms respectively.

With straightforward [[Round-robin scheduling]], every time a [[context switch]] occurs, a process would need to be swapped in (because only the 10 least recently used processes are swapped in). Choosing randomly among the processes would diminish the probability to 80% (40/50). If that occurs, then obviously a process also need to be swapped out. Swapping in and out of is costly, and the scheduler would waste much of its time doing unneeded swaps.