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Scheduling

Scheduling. Unix Scheduling. Allocating CPU to a Process The amount of time that a process is allocated the CPU is called a time quantum Processes are assigned a priority CPU is allocated based on the priority of the process. Process Scheduling.

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Scheduling

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  1. Scheduling

  2. Unix Scheduling • Allocating CPU to a Process • The amount of time that a process is allocated the CPU is called a time quantum • Processes are assigned a priority • CPU is allocated based on the priority of the process

  3. Process Scheduling • Unix uses the round robin with multilevel feedback • In short, CPU is allocated to a process for a time quantum, when the time quantum is exceeded, the process is preempted and is added to one of several priority queues.

  4. RRMF CPU quantum = 24

  5. Scheduling Algorithm

  6. Scheduling Parameters • Priority Field • Two ranges for priorities • User priorities • Kernel priorities • High • Low

  7. Range of Priorities • Ranges of process priorities • 0 thru 127 (system dependent) • Threshold Priority (50) • PUSER 50 – user mode execution • Priorities less then 50 are general kernel level processes • PSWP 0 - kernel mode while swapping

  8. Priority Ranges 0 50

  9. Priorities • Some Priorities are hard coded • Process that about to go to sleep receive a priority based on reason for sleeping • Some are calculated • Decay function CPU ~ CPU/2 • Priority ~ (recent CPU usage/2) + base priority

  10. When Are Priorities Changed • When a process is about to sleep • Process returning from kernel mode to user mode • After the CPU usage is recalculated (caused by the clock)

  11. Priority Example • 3 process • Initial priority 60 • Clock interrupts 60 times a second • No syscalls • No other processes are ready to run

  12. Priority Example

  13. Priority Example 2

  14. User Influence on Priorities • Users can have a nominal impact on the priority assigned to their process • Nice systcall • nice(value) • Where value is added to the priority calculation • P = (recent CPU usage/2) +(base P) + nice

  15. Fair Share Scheduling • Similar in concept to RRML, but allows block of time to be allocated to a set or groups of processes • Divide user community into set of fare share groups

  16. Fair Share Scheduling • Processes within a group are subject to the same scheduling and priority constraints • Each group or set of processes are allocated a portion of the CPU

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