Linux Process Management Linux Implementation of Threads Threads enable concurrent programming true parallelism Linux implementation of threads PCS vs SCS http:www.cs.uic.edujbellCourseNotesOperatingSystems5CPUScheduling.html
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Presentation Transcript
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Linux Process Management<br>
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Linux Implementation of Threads Threads enable concurrent programming / true parallelism
Linux implementation of threads<br>
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PCS vs SCS http://www.cs.uic.edu/~jbell/CourseNotes/OperatingSystems/5_CPU_Scheduling.html<br>
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Linux Process Scheduling Policy
Policy is the behavior of the scheduler that determines what runs when. A scheduler’s policy often determines the overall feel of a system.
I/O-Bound vs. Processor-Bound Processes
Scheduler policy in Unix systems tends to explicitly favor I/O-bound processes
Linux..<br>
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Process priority 2 ranges
nice value
-20 to +19 with default of 0
Larger nice values correspond to lower priority
ps –el
real-time priority
0 -99
Higher values correspond to higher priority
Used for real-time processes – always higher than normal processes.<br>
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The scheduling policy in action<br>
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Process Scheduling in Unix Based on traditional methods
Timeslice
Priority (Priority exported to user in nice values)
Problematic
Two low priority processes swap to much
Timeslice / priority correlation
Timeslice measurement (timer click)<br>
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The scheduling policy in action Linux: Completely Fair Scheduling
Model process scheduling as if the system had an ideal perfectly multitasking processor.
That is, each process receives 1/n of the processor’s time, where n is the number of runnable processes and we’d schedule them for intimately small durations, so that in any measureable period we’d have urn all n processes for the same time.
Conceptually: we are running 2 processes simultaneously each using 50% of CPU<br>
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Completely Fair Scheduling (CFS) CFS will run each process for some amount of time, round-robin, selecting next the process that has run the least.<br>
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Completely Fair Scheduling (CFS) Determine Time allotted to run CFS calculates how long a process should run as a function of the total number of runnable processes
CFS uses the nice value to weight the proportion of the processor a process should receive.<br>
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Completely Fair Scheduling (CFS) Determine Time allotted to run Each process then runs for a “time-slice” proportional to its weight divided by the total weight of all runnable threads.<br>
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Completely Fair Scheduling (CFS) Picking the next task Red/black trees
Balanced tree
Leftmost leaf contains process with the smallest runtime (time spent running) normalized by the number of runnable processes and by its ‘weight’<br>
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Kernel Preemption Unix kernel is non-preemptive
Linux kernel is pre-emptive<br>
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Real-Time Scheduling Policies SCHED_FIFO and SCHED_RR
Not managed by CFS, but by a separate real-time scheduler.
