CSE 457/857 Systems Administration
CSE 457/857Systems AdministrationUNIX Process SchedulingUNIX
Process Scheduling
UNIX Process SchedulingProcess is selected from runnable queue
and begins executionExecution continues until:End of allowed time
(timeslice/quanta)Process blocks syscall (before end of
timeslice)Hardware interrupt occursCurrent process (user table) is
swapped outOS selects another process to execute from the runnable
queueSelection process is called scheduling algorithm
UNIX Process SchedulingSchedulingWhat is the goal?To make the
system more usableEfficiency or Responsiveness?Average wait or
turnaround?What process facilitates process scheduling?I/O bound vs
CPU bound?UNIX Process SchedulingThe easy buttonFirst Come First
Serve (FCFS)First In First Out (FIFO)O(1)AdvantagesFast!Simple,
easy to implementIs this algorithm fair?Is starvation possible? If
so, what kinds of jobs will typically be starved?How could
starvation be mitigated?Does FIFO lead to a responsive system?UNIX
Process SchedulingShortest Job First (SJF)Pick jobs that will need
the least amount of CPU time firstWill this favor interactive jobs
or batch jobs?How can the OS determine CPU need?O(n)Better than
FIFO?If so, how is it better?Is starvation possible?What about
scale?UNIX ProcessesSingle queue O(n)2.2.xList iterated on each
sched()Kernel chooses process to run by goodness metricPriority
considerationsProcess Group (PGID)Interactive vs Batch (SJF)Process
ageAnd other assorted black magic
UNIX Process SchedulingMultiple O(1) queues2.4.x
(2001)DrawKernel space code can now be preempted!What kind of
service does this feature allow Linux to offer?SCHED_FIFOHow to use
SCHED_FIFO? chrtAdvantages?Disadvantage?UNIX Process SchedulingDoes
the system administrator have any control over process
priority?Yes, by use of nice facilityDemoUNIX Processes
SchedulingContinuous Fair Scheduler (CFS)>2.6.23 - ~2007O(1)New
approach rad!Instead of prioritizing process based on predictive
analysis, attempt to give each process a fair-share of CPU (avoid
starvation) and maintain responsivenessBasic algorithmExecute task
with lowest CPU run timeLet run for an arbitrary amount of timeOr
process blocksWhen ready, reinsert into
queuehttp://www.ibm.com/developerworks/library/l-completely-fair-scheduler/UNIX
Process SchedulingContinuous Fair SchedulerQueue is a binary
treeRed-black
treehttps://en.wikipedia.org/wiki/Red-black_treeProcess selected is
left-most (smallest aggregate runtime)No quanta/timesliceTime
allowed to run based on system load ORIf aggregate runtime is
greater than left-most elementnice is still available, implemented
as decayA multiplier to aggregate runtime
UNIX Process SchedulingSCHED_FIFOScheduling policy designed for
special time-critical applications. It uses the First In-First Out
scheduling algorithm. SCHED_BATCHScheduling policy designed for
CPU-intensive tasks. SCHED_IDLEScheduling policy intended for very
low prioritized tasks. SCHED_OTHERDefault Linux time-sharing
scheduling policy used by the majority of processes.
SCHED_RRSimilar to SCHED_FIFO, but uses the Round Robin scheduling
algorithm.
UNIX Process
Schedulinghttps://www.youtube.com/watch?v=d3kfEeMZ65c
