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CPU Scheduling
Heechul Yun
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Agenda
- Introduction to CPU scheduling
- Classical CPU scheduling algorithms
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CPU Scheduling Heechul Yun 1 Agenda Introduction to CPU - - PowerPoint PPT Presentation
CPU Scheduling Heechul Yun 1 Agenda Introduction to CPU - - PowerPoint PPT Presentation
CPU Scheduling Heechul Yun 1 Agenda Introduction to CPU scheduling Classical CPU scheduling algorithms 2 CPU Scheduling CPU scheduling is a policy to decide Which thread to run next? When to schedule the next thread? How
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CPU Scheduling
- CPU scheduling is a policy to decide
– Which thread to run next? – When to schedule the next thread? – How long?
- Context switching is a mechanism
– To change the running thread
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Assumption: CPU Bursts
- Execution model
– Program uses the CPU for a while and the does some I/O, back to use CPU, …, keep alternating
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CPU Scheduler
- An OS component that determines which
thread to run, at what time, and how long
– Among threads in the ready queue
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CPU Scheduler
- When the scheduler runs?
– The running thread finishes – The running thread voluntarily gives up the CPU
- yield, block on I/O, …
– The OS preempts the current running thread
- quantum expire (timer interrupt)
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Performance Metrics for CPU Scheduling
- CPU utilization
– % of time the CPU is busy doing something
- Throughput
– #of jobs done / unit time
- Response time (Turn-around time)
– Time to complete a task (ready -> complete)
- Waiting time
– Time spent on waiting in the ready queue
- Scheduling latency
– Time to schedule a task (ready -> first scheduled)
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Example
- Assumption: A, B, C are released at time 0
- The times of Process A
– Response time: 9 – Wait time: 5 – Sched. latency: 0
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A B C A B C A C A C Time
- sched. latency
+ + wait time response time
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Example
- Assumption: A, B, C are released at time 0
- The times of Process B
– Response time: 5 – Wait time: 3 – Latency: 1
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A B C A B C A C A C Time
- sched. latency
+ wait time response time
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Example
- Assumption: A, B, C are released at time 0
- The times of Process C
– Response time: 10 – Wait time: 6 – Latency: 2
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A B C A B C A C A C Time
- sched. latency
+ + + wait time response time
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Administrative
- No office hour today
– If you need to meet me, plz wait until Thursday or schedule a separate appointment via email
- Project 2
– Will be out Thursday – Due: 10/22 (Monday)
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Recap: CPU Scheduling
- CPU scheduling is a policy to decide
– Which thread to run next? – When to schedule the next thread? – How long?
- Context switching is a mechanism
– To change the running thread
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Recap: Example Metrics
- Assumption: A, B, C are released at time 0
- The times of Process C
– Response time: 10 – Wait time: 6 – Latency: 2
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A B C A B C A C A C Time
- sched. latency
+ + + wait time response time
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Workload Model and Gantt Chart
- Workload model
- Gantt chart
– bar chart to illustrate a particular schedule
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Process Arrival Time Burst Time P1 8 P2 1 4 P3 1 10 P4 6 2
P1 P2 P3 P4
8 12 22 24
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Agenda
- Basic scheduling policies
– First-come-first-serve (FCFS) – Shortest-job-first (SJF) – Shortest-remaining-time-first (SRTF) – Round-robin (RR)
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Scheduling Policy Goals
- Maximize throughput (minimize avg. waiting time)
– High throughput (#of jobs done / time) is good
- Minimize scheduling latency
– Important to interactive applications (games, editor, …)
- Fairness
– Make all threads progress equally
- Goals often conflicts
– Frequent context switching may be good for reducing response time, but not so much for maximizing throughput
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First-Come, First-Served (FCFS)
- FCFS
– Assigns the CPU based on the order of the requests. – Implemented using a FIFO queue.
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FCFS
- Example
– Suppose that the processes arrive in the order: P1 , P2 , P3 – Waiting time?
- P1 = 0; P2 = 24; P3 = 27
– Average waiting time
- (0 + 24 + 27)/3 = 17
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Process Arrival Time Burst Time P1 24 P2 3 P3 3 P1 P2 P3 24 27 30
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FCFS
- Example 2
– Suppose that the processes arrive in the order: P2 , P3, P1 – Waiting time?
- P1 = 6; P2 = 0; P3 = 3
– Average waiting time
- (6 + 0 + 3)/3 = 3
– Much better than previous case performance varies greatly depending on the scheduling order
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Process Arrival Time Burst Time P1 24 P2 3 P3 3 P1 P3 P2 6 3 30
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Shortest Job First (SJF)
- Can we always do better than FIFO?
– Yes: if you know the tasks’ CPU burst times
- Shortest Job First (SJF)
– Order jobs based on their burst lengths – Executes the job with the shortest CPU burst first – SJF is optimal
- Achieves minimum average waiting time
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Shortest Job First (SJF)
- Example
– Gantt chart – Average waiting time?
- (3 + 16 + 9 + 0) / 4 = 7
- How to know the CPU burst time in advance?
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Process Arrival Time Burst Time P1 6 P2 8 P3 7 P4 3 P4 P3 P1 3 16 9 P2 24
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Determining CPU Burst Length
- Can only estimate the length
– Next CPU burst similar to previous CPU bursts ? – Predict based on the past history
- Exponential weighted moving average (EWMA)
– of past CPU bursts
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Shortest Job First (SJF)
- What if jobs don’t arrive at the same time?
– Average waiting time
- (0+7+15+9)/4 = 7.5
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Process Arrival Time Burst Time P1 8 P2 1 4 P3 2 9 P4 3 5
P2 P4 P1 P3
8 12 17 26
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Shortest Remaining Time First (SRTF)
- Preemptive version of SJF
- New shorter job preempt longer running job
- Average waiting time
– (9 + 0 + 15 + 2 ) / 4 = 6.5
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Process Arrival Time Burst Time P1 8 P2 1 4 P3 2 9 P4 3 5
P
1
P2 P4 P1 P3
1 5 10 17 26
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Quiz: SRTF
- Average waiting time?
- (9 + 0 + 15 + 2 ) / 4 = 6.5
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Process Arrival Time Burst Time P1 8 P2 1 4 P3 2 9 P4 3 5
P
1
P2 P4 P1 P3
1 5 10 17 26
P1 P2 P3 P4
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So Far…
- FIFO
– In the order of arrival – Non-preemptive
- SJF
– Shortest job first. – Non preemptive
- SRTF
– Preemptive version of SJF
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Issues
- FIFO
– Bad average response time
- SJF/SRTF
– Good average waiting time – IF you know or can predict the future
- Time-sharing systems
– Multiple users share a machine – Need high interactivity low scheduling latency
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Round-Robin (RR)
- FIFO with preemption
- Simple, fair, and easy to implement
- Algorithm
– Each job executes for a fixed time slice: quantum – When quantum expires, the scheduler preempts the task – Schedule the next job and continue...
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Round-Robin (RR)
- Example
– Quantum size = 4 – Gantt chart – Sched. Latency (between ready to first schedule)
- P1: 0, P2: 4, P3: 7. average response time = (0+4+7)/3 = 3.67
– Waiting time
- P1: 6, P2: 4, P3: 7. average waiting time = (6+4+7)/3 = 5.67
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Process Burst Times P1 24 P2 3 P3 3 P1 P2 P3 P1 P1 P1 P1 P1 4 7 10 14 18 22 26 30
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How To Choose Quantum Size?
- Quantum length
– Too short high overhead (why?) – Too long bad scheduling latency
- Very long quantum FIFO
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Round-Robin (RR)
- Example
– Quantum size = 2 – Gantt chart – Scheduling latency
- P1: 0, P2: 2, P3: 4. average response time = (0+2+4)/3 = 2
– Waiting time
- P1: 6, P2: 6, P3: 7. average waiting time = (6+6+7)/3 = 6.33
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Process Burst Times P1 24 P2 3 P3 3 P1 P2 P3 P1 P2 P1 P1 P3 P1 2 4 6 8 9 12 14 10 30
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Discussion
- Comparison between FCFS, SRTF(SJF), and RR
– What to choose for smallest average waiting time?
- SRTF (SFJ) is the optimal
– What to choose for better interactivity?
- RR with small time quantum (or SRTF)
– What to choose to minimize scheduling overhead?
- FCFS
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Example
- Task A and B
– CPU bound, run an hour
- Task C
– I/O bound, repeat(1ms CPU, 9ms disk I/O)
- FCFS?
– If A or B is scheduled first, C can begins an hour later
- RR and SRTF?
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Compute I/O I/O
A or B C
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Example Timeline
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RR with 100ms time quantum RR with 1ms time quantum
C A B
I/O
C
…
A B … A B
I/O
C A B A B … C A B …
I/O
SRTF
C
…
A
I/O
C A C A …
I/O
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Summary
- First-Come, First-Served (FCFS)
– Run to completion in order of arrival – Pros: simple, low overhead, good for batch jobs – Cons: short jobs can stuck behind the long ones
- Round-Robin (RR)
– FCFS with preemption. Cycle after a fixed time quantum – Pros: better interactivity (low average scheduling latency) – Cons: performance is dependent on the quantum size
- Shortest Job First (SJF)/ Shorted Remaining Time First (SRTF)
– Shorted job (or shortest remaining job) first – Pros: optimal average waiting time – Cons: you need to know the future, long jobs can be starved by short jobs
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Recap: Workload Model and Gantt Chart
- Workload model
- Gantt chart
– bar chart to illustrate a particular schedule
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Process Arrival Time Burst Time P1 8 P2 1 4 P3 1 10 P4 6 2
P1 P2 P3 P4
8 12 22 24
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Recap: Scheduling Policies
- First-Come, First-Served (FCFS)
– Run to completion in order of arrival – Pros: simple, low overhead, good for batch jobs – Cons: short jobs can stuck behind the long ones
- Round-Robin (RR)
– FCFS with preemption. Cycle after a fixed time quantum – Pros: better interactivity (low average scheduling latency) – Cons: performance is dependent on the quantum size
- Shortest Job First (SJF)/ Shorted Remaining Time First (SRTF)
– Shorted job (or shortest remaining job) first – Pros: optimal average waiting time – Cons: you need to know the future, long jobs can be starved by short jobs
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Agenda
- Multi-level queue scheduling
- Fair scheduling
- Real-time scheduling
- Multicore scheduling
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Multiple Scheduling Goals
- Optimize for interactive applications
– Round-robin
- Optimize for batch jobs
– FCFS
- Can we do both?
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Multi-level Queue
- Ready queue is partitioned into separate queues
– Foreground: interactive jobs – Background: batch jobs
- Each queue has its own scheduling algorithm
– Foreground : RR – Background: FCFS
- Between the queue?
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Multi-level Queue Scheduling
- Scheduling between the queues
– Fixed priority
- Foreground first; schedule background only when no
tasks in foreground
- Possible starvation
– Time slicing
- Assign fraction of CPU time for each queue
- 80% time for foreground; 20% time for background
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Multi-level Feedback Queue
- Each queue has a priority
- Tasks migrate across queues
– Each job starts at the highest priority queue – If it uses up an entire quantum, drop one-level – If it finishes early, move up one-level (or stay at top)
- Benefits
– Interactive jobs stay at high priority queues – Batch jobs will be at the low priority queue – Automatically!
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 0 Time
A B C
2 5 9
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 1 Time
A B C
3 7
A
1
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 2 Time
A B C
4 3
A
1
B
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 3 Time
A B C
6 3
A
1
B C
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 3 Time
A B C
6 3
A
1
B C
Suppose A is blocked on I/O
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 3 Time
B C
5 2
A B C
Suppose A is blocked on I/O
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 5 Time
A B C
3
A
1
B C
A is returned from I/O
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 6 Time
A B C
3
A B C
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 8 Time
A B C
3
A B C
2
C
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Example of Multilevel Feedback Queues
Priority 0 (time slice = 1): Priority 1 (time slice = 2): Priority 2 (time slice = 4):
time = 9 Time
A B C A B C C
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Completely Fair Scheduler (CFS)
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- Linux default scheduler, focusing on fairness
- Each task owns a fraction of CPU time share
– E.g.,) A=10%, B=30%, C=60%
- Scheduling algorithm
– Each task maintains its virtual runtime
- Virtual runtime = executed time (x weight)
– Pick the task with the smallest virtual runtime
- Tasks are sorted according to their virtual times
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CFS Example
- Tasks are sorted according to their virtual times
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5 6 8 10
Scheduled the “neediest” task
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CFS Example
- Tasks are sorted according to their virtual times
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9 6 8 10
On a next scheduler event re-sort the list But list is inefficient.
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Red-black Tree
– Self-balancing binary search tree – Insert: O(log N), Remove: O(1)
56 Figure source: M. Tim Jones, “Inside the Linux 2.6 Completely Fair Scheduler”, IBM developerWorks
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Weighed Fair Sharing: Example
Weights: gcc = 2/3, bigsim=1/3 X-axis: mcu (tick), Y-axis: virtual time Fair in the long run
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Some Edge Cases
- How to set the virtual time of a new task?
– Can’t set as zero. Why? – System virtual time (SVT)
- The minimum virtual time among all active tasks
- cfs_rq->min_vruntime
– The new task can “catch-up” tasks by setting its virtual time with SVT
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Weighed Fair Sharing: Example 2
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