Operating Systems III. Scheduling - Eurecomsoc.eurecom.fr/OS/docs/CourseOS_III_Scheduling.pdfI...
Transcript of Operating Systems III. Scheduling - Eurecomsoc.eurecom.fr/OS/docs/CourseOS_III_Scheduling.pdfI...
Operating Systems
III. Scheduling
Ludovic [email protected], office 470
http://soc.eurecom.fr/OS/
@OS Eurecom
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Outline
Basics of SchedulingDefinitionsSwitching Between Processes
Scheduling algorithms
Scheduling in Windows, Linux and Android
2/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Three-level Scheduling: Admission, CPU andMemory
3/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Three-Level Scheduling: Explanations
I Admission schedulerI Jobs are first stored into an admission queueI Jobs are then admitted in the system
I Memory schedulerI Swap in / swap outI To be avoided: Disk storage is expensive in terms of timeI For example, good to keep in memory 5 processes which
compute 20% of their time, and store all the others processeson disk
I CPU schedulerI See next slides
I In interactive systems, usually only 2 levels of schedulerI Memory and CPU
4/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Scheduling Criteria
All systems
I Fairness: Giving each process a fair share of the CPU
I Policy enforcement: No process should be able to overcomethe rules of the system
I Balance: Keeping all parts of the system busy
Batch/Cloud systems
I CPU utilization: Keep the CPU busy
I Throughput: Maximize the number of processes that arecompleted per time unit (hour)
I Turnaround Time: Minimize time between submission andtermination
5/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Scheduling Criteria (Cont.)
Interactive systems
I Response time: Respond to interactions as fast as possible
I Proportionality: Meet users’ expectations
Real-time systems
I Meeting deadlines: Keep the CPU busy
I Predictability: Know in advance whether time constraintscan always be met, or not
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
When to Schedule?
I At creation of a processI Run parent or child?
I At termination of a process
I When a process blocks on I/O, semaphore, etc.
I When I/O interrupt occursI At hardware clock interrupt
I Non-preemptive schedulingI The same process is given the CPU until it blocks or
voluntarily releases the CPUI Only choice if no timer is availableI Windows 3.1
I Preemptive schedulingI The same process can run only for a pre-defined time-intervalI The OS sets a timer to the time-interval
7/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Non-Preemptive Scheduler
8/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Preemptive Scheduler
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
DefinitionsSwitching Between Processes
Scheduling Implementation
10/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Outline
Basics of Scheduling
Scheduling algorithmsBatch systemsInteractive systemsMultiprocessor systems
Scheduling in Windows, Linux and Android
11/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Presented Scheduling Algorithms
For batch systems
I First-Come First-Served
I Shortest Job First
I Shortest Remaining TimeNext
For interactive systems
I Round-Robin scheduling
I Priority-based scheduling
I Group-based scheduling
I Guaranteed scheduling
I Fair-share scheduling
I Lottery scheduling
For multiprocessors systems
Presentation of three differentapproaches
For RT systems
Attend lectures on RTOS to knowmore!
12/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
First-Come, First-Served (FCFS)
Processes are assigned the CPU in the order they request it
I Single queue of ready processes
I Easy to program, fair
I Non-preemptive scheduling
Example: Average Wait Time for various sets of processes
Process p1 p2 p3
Duration 24 3 3
Arrivaltime (@)
0 0 0
Arrival order AWT=?
p1, p2, p3 0+24+273 = 17
p2, p3, p1
p3, p2, p1
13/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Shortest Job First (SJF)
Scheduler selects the job with the shortest computation time
I Easy to implement
I Non-preemptiveI Optimal when processes are all ready simultaneously
I Minimum AWT
But ... How to predict the next CPU burst time???
1. Specified amount
2. Exponential average
τn+1 = αtn + (1− α)τn
I τ : predicted valueI t: measured computation time
14/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Shortest Job First: Example
Example ]1: Same arrival time
Process p1 p2 p3 p4
Duration 8 4 4 4
@ 0 0 0 0
Algo AWT=?
FCFS (p1,p2, p3, p4)
SJF
Example ]2: Various arrival times
Process p1 p2 p3 p4 p5
Duration 2 4 1 1 1
@ 0 0 3 3 3
Algo AWT=?
FCFS
SJF
Can you find a better scheduling???
15/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Shortest Remaining Time Next (SRTN)
Preemptive version of Shortest Job First
Memo: SJF is non-preemptive
Example: Comparison between SJF and SRTN
Process p1 p2
Duration 10 1
@ 0 1
Algo Scheduling AWT=?
SJF p1 for 10, p2 for 1 0+92 = 4.5
SRTN p1 for 1, p2 for 1, p1 for 9 1+02 = 0.5
16/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Round-Robin (RR)
Each process is assigned a time quantum
I If the process is still running at the end of its quantum, it ispreempted
I Next process is assigned the CPU
I Widely used, easy to implement, fair
How to set the value of the quantum???
I Quantum too short? Quantum too long?I What happens if:
I quantum = ε?I quantum =∞?
I Typical quantum: 10 to 50 ms
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Round-Robin: Explanations
18/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Round-Robin: Example
Process p1 p2 p3
Duration 24 3 3
@ 0 0 0
Quantum 4 4 4
p1 p2 p3 p1 p1 p1 p1 p1
0 4 7 10 14 18 22 26
AWT=???
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Priority-Based Scheduling
Limitations of RRI RR assumes all processes are of equal importance
I For example, sending of an email vs. playing music
Priority-based scheduling: Priorities are assigned to processes
I Static priority or dynamic priorityI The process with the higher priority (may be the lower value)
is chosenI RR between processes of the same priority
20/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Priority-Based Scheduling: Dynamic Priorities
Priorities might be re-evaluated
I Avoids high-priority processes to jeopardize the CPU(starvation of low-priority processes)
I For example, priority can be set to priorityn+1 = quantumtn
where tn is the last computation time
Example: 10 ms quantum
I Process used 1ms → new priority = 10
I Process used 5ms → new priority = 2
I Process used 10ms (i.e., all quantum) → new priority = 1
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Priority-Based Scheduling (Cont.)
22/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Group-based
Scheduling policy is applied accordingto groups of processes
Example: group 1 may have a quantumof 20, and group 2 a quantum of 10
Intra-group scheduling
All algorithms previously presented can be used
Inter-group scheduling
I Fixed-priority preemptive schedulingI Highest priorities for foreground processes (interactivity)
I Time-slice between groupsI e.g., 80% for group 1, 20% for group 2
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Guaranteed Scheduling
I Special case of ”Group-based”
Idea: Give a ratio of the CPU to groups of processes
I For example, users are given a fraction of CPU (e.g., 1n )
I Compute the ratio between actual CPU time consumed andentitled CPU time
I Run the process with the lowest ratio until its ratio has movedabove its closest competitor
24/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Fair-Share Scheduling
Idea: Fairness between users is taken into account firstI For example, in RR, a user with 9 processes gets more CPU
than a user with 2I Allocates a fraction of CPU to users
I One user = one scheduling group
I Other possible notions of fairnessI Resources, etc.
25/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Lottery Scheduling
Idea: Lotto tickets are given to processes
I A random ticket is picked up, the winner gets a quantum oftime
I The number of tickets received by a process is equivalent toits importance
I Processes can exchange tickets for cooperation
I Highly responsive
Example
A video server with three video streams at 10, 20, 25 frames / sec,respectively.→ Each process is given a number of tickets equals to the framerate i.e 10, 20 and 25
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
Batch systemsInteractive systemsMultiprocessor systems
Multiprocessor Scheduling
One scheduling queue for each processor
I Symmetric system
A common ready-queue for all processors
I Asymmetric multi-processing
I Danger: synchronization problems because several processorscould access to the scheduling data structures at the sametime
Only one ready-queue accessed by only one processor
I No synchronization issues
I Drawback?
27/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Outline
Basics of Scheduling
Scheduling algorithms
Scheduling in Windows, Linux and AndroidWindowsSolaris, LinuxAndroid
28/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Windows NT Scheduling
BasicsI Priority-based preemptive round-robin scheduling
I Raises the priority of interactive and I/O bound processes
I CPU cycle-based scheduling (since Vista)
A process runs until ...
I It is preempted by a higher priority process
I It terminates
I Its time quantum expires (12 ms in Windows 2000 - highervalue is used in ”server” versions)
I It calls a blocking system call
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Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Windows NT: Priority Classes
30/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Windows NT: Changing Classes of Processes
31/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Solaris 10: Kernel Architecture
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source = Oracle/Sun Microsystems
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
→ Linux 2.6.22: Priority-Based Scheduler
Scheduling classes
I Real-time processes: SCHED FIFO, SCHED RR
I Interactive and batch processes: SCHED OTHER,SCHED BATCH
I Low-priority processes: SCHED IDLEI One active queue for each of the 140 priorities and for each
processorI Cross-CPU scheduling regularly performed (e.g., every 200 ms)
SCHED OTHER
Round-Robin time-sharing policy with dynamic priorities
I Processes running for a long time are penalized
33/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Linux 2.6.23 → . . . : Completely FairScheduler
I ”Out of balance” tasks are given time to executeI Out of balance task = task has not been given a fair amount
of time relative to others
I Preemption time is variable: it depends upon the time balanceof the task w.r.t. other tasks
I The amount of time provided to a given task is called thevirtual runtime
I Group-based: the virtual runtime can also be computed for awhole group
I Priorities are used as a decay factor for the time a task ispermitted to execute
34/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling
Basics of SchedulingScheduling algorithms
Scheduling in Windows, Linux and Android
WindowsSolaris, LinuxAndroid
Android
I Roughly, the scheduler is based onthe Linux one
I → Fair scheduling approach
I BUT: fairness according to Groupsof processes
I Foreground/Active, visible,service, background, empty
I To reclaim resources, Android maykill processes according to theirrunning priority
Foreground
Visible
Service
Background
Empty
High priority
Low priority
35/35 Fall 2017 Institut Mines-Telecom Operating Systems - Scheduling