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processor scheduling scheduling
assign system resource (cpu time, io device, etc.) to processes/threads to meet system objectives, such as response time, turnaround time, throughput, or fairness
in practice, these goals often conflict three types of scheduling long-term scheduling (admission scheduler)
decide which jobs/processes to be admitted to the ready queue
admission to the set of currently executing processes
mid-term scheduling (swapper)
remove processes from main memory and place them on secondary memory, or vice versa
swap in/out processes
short-term scheduling (cpu scheduler or dispatcher)
decide which of the ready, in-memory processes to be executed by the processor following a clock interrupt, io interrupt, or os call
execute most frequently scheduling and process state transitions source: pearson nesting of scheduling functions source: pearson queuing diagram source: pearson long-term scheduler
determines which programs are admitted to the system for processing
once admitted a user program becomes a process
controls the degree of multiprogramming
the more processes that are created, the smaller the percentage of time that each process can be executed
may limit the degree of multiprogramming to provide satisfactory service to the current set of processes
or, may increase the degree of multiprogramming if cpu is idle too long
which jobs to admit next can be first come, first served (fcfs), or
priority, expected execution time, i/o requirements
for interactive programs in a time-sharing system
os will accept all authorized comers until the system is saturated
short-term scheduling criteria
the main objective of short-term scheduling is to allocate processor time to optimize system behaviour
can be categorized into two dimensions user-oriented criteria
relate to the behaviour of the system as perceived by the individual user or process (such as response time in an interactive system)
important on virtually all systems system-oriented criteria
focus on efficient utilization of the processor such as throughput
generally of minor importance on single-user systems
performance-related criteria quantitative and can be measured example: response time, throughput not performance-related criteria qualitative and hard to measure example: predictability scheduling criteria source: pearson priority queuing ready queue with the highest priority ready queue with the lowest priority
may suffer from starvation! (ÀÌÇÏ »ý·«)
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