1. CPU Scheduling Basic Concepts Scheduling Criteria
Scheduling Algorithms
Multiple-Processor Scheduling
Algorithm Evaluation

2. Basic Concepts Maximum CPU utilization obtained with multiprogramming
CPU–I/O Burst Cycle – Process execution consists of a cycle of CPU execution and I/O wait.
CPU burst distribution

3. CPU And I/O Bursts

4. Histogram of CPU-burst Times

5. CPU Scheduler
Selects from among the processes in memory that are ready to execute, and allocates the CPU to one of them.
CPU scheduling decisions may take place when a process:
Switches from running to waiting state.
Switches from running to ready state.
Switches from waiting to ready.
Terminates.
Scheduling under 1 and 4 is nonpreemptive.
All other scheduling is preemptive.

6. Dispatcher
Dispatcher module gives control of the CPU to the process selected by the short-term scheduler; this involves:
switching context
switching to user mode
jumping to the proper location in the user program to restart that program
Dispatch latency – time it takes for the dispatcher to stop one process and start another running.

7. Scheduling Criteria CPU utilization – keep the CPU as busy as possible
Throughput – # of processes that complete their execution per time unit
Turnaround time – amount of time to execute a particular process

8. Scheduling Criteria
Waiting time – amount of time a process has been waiting in the ready queue
Response time – amount of time it takes from when a request was submitted until the first response is produced, not output (for time- sharing environment)

9. Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time

10. First-Come, First-Served (FCFS)
Process Burst Time
P1 24
P2 3
P3 3
Suppose that the processes arrive in the order: P1 , P2 , P3 . The Gantt Chart for the schedule is:
Waiting time for P1 = 0; P2 = 24; P3 = 27
Average waiting time: ( )/3 = 17 P1 P2 P3 24 27 30

11. FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
P2 , P3 , P1
The Gantt chart for the schedule is:
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: ( )/3 = 3
Much better than previous case.
Convoy effect: short process behind long process P1 P3 P2 6 3 30

12. Shortest-Job-First (SJF) Scheduling
Associate with each process the length of its next CPU burst. Use these lengths to schedule the process with the shortest time.
Two schemes:
nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst.

13. Shortest-Job-First (SJF) Scheduling
Preemptive Version (SRTF)
if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is know as the Shortest- Remaining-Time-First (SRTF).
SJF is optimal – gives minimum average waiting time for a given set of processes.

14. Example of Non-Preemptive SJF
Process Arrival Time Burst Time P P P P
SJF (non-preemptive)
Average waiting time = ( )/4 = 4 P1 P3 P2 7 3 16 P4 8 12

15. Example of Preemptive SJF
Process Arrival Time Burst Time P P P P
SJF (preemptive)
Average waiting time = ( )/4 = 3 P1 P3 P2 4 2 11 P4 5 7 16

16. Predicting Next CPU Burst
The exponential average formula can only estimate the length.
Can be done by using the length of previous CPU bursts, using exponential averaging.
actual length

17. Next CPU Burst

18. Exponential Averaging
 =0
n+1 = n
Recent history does not count.
 =1
n+1 = tn
Only the actual last CPU burst counts.

19. Exponential Averaging
If we expand the formula, we get:
n+1 =  tn+(1 - ) n
n+1 =  tn+(1 - )[  tn-1 +(1 - ) n-1 ] + …
+(1 -  )j  tn-j + …
+(1 -  )n+1 0
Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor.

20. Priority Scheduling
A priority number (integer) is associated with each process
The CPU is allocated to the process with the highest priority (smallest integer  highest priority).
Preemptive
nonpreemptive

21. Priority Scheduling
SJF is a priority scheduling where priority is the predicted next CPU burst time.
Problem  Starvation – low priority processes may never execute.
Solution  Aging – as time progresses increase the priority of the process.

22. Round Robin (RR)
Each process gets a small unit of CPU time (time quantum), usually milliseconds. After this time has elapsed, the process is preempted and added to the end of the ready queue.
If there are n processes in the ready queue and the time quantum is q, then each process gets 1/n of the CPU time in chunks of at most q time units at once. No process waits more than (n-1)q time units.
Performance
q large  FIFO
q small  q must be large with respect to context switch, otherwise overhead is too high.

23. RR with Time Quantum = 20 Process Burst Time P1 53 P2 17 P3 68 P4 24
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response. P1 P2 P3 P4 20 37 57 77 97
117
121
134
154
162

24. Time Quantum vs Context Switch Time

25. Turnaround Time Varies With The Time Quantum

26. Multilevel Queue
Ready queue is partitioned into separate queues: foreground (interactive) background (batch)
Each queue has its own scheduling algorithm:
foreground – RR background – FCFS

27. Multilevel Queue Scheduling must be done between the queues.
Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.
Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR
20% to background in FCFS

28. Multilevel Queue Scheduling

29. Multilevel Feedback Queue
A process can move between the various queues; aging can be implemented this way.
Multilevel-feedback-queue scheduler defined by the following parameters:
number of queues
scheduling algorithms for each queue
method used to determine when to upgrade a process
method used to determine when to demote a process
method used to determine which queue a process will enter when that process needs service

30. Multilevel Feedback Queue
Three queues:
Q0 – time quantum 8 milliseconds
Q1 – time quantum 16 milliseconds
Q2 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

31. Multilevel Feedback Queues

32. Multilevel Feedback Queues
Advantages:
Flexibility – can be used to implement many different policies
Good balance of priority and fairness (and it’s tweakeable)
Disadvantages:
Complexity to code
Run-time efficiency

33. Multiple-Processor Scheduling
CPU scheduling gets more complex when multiple CPUs are available.
Assume homogeneous processors within a multiprocessor.
Note: Future processors may not fit this model!
Schedules can be made either:
Symmetrically – each processor schedules itself from a shared queue
Asymmetrically – one processor only in kernel mode

34. Affinity vs. Balance Processor Affinity Load Balancing:
When a process was running on CPU a, and is migrated to CPU b, there is a penalty for cache/TLB/memory flush/re-load.
Load Balancing:
Processes should be scheduled fairly evenly across multiple processors
Avoids wasting CPU cycles when there is something that could be running
These two are both important, but they tend to fight each other.

35. Processor Affinity
To avoid flushing the caches, each process will “preference” a CPU (or set of CPUs)
“Soft” affinity – can be moved if needed
“Hard” affinity – locked in on the current CPU
Implementation varies – depends on:
System architecture and available resources
Load balancing needs

36. Load Balancing Solutions: Run with a single ready queue Push migration
If a CPU becomes idle, it just pulls the next process
Not used in practice (SMP common)
Push migration
An OS task examines the load every so often, and if needed, pushes some processes from heavily loaded CPUs to lightly loaded ones.
Pull migration
An idle CPU pulls a process from another CPUs queue.

37. Multi-CORE Scheduling
Most multi-core processors today also implement hardware simultaneous multithreading
i.e. Intel i7 (Nehalem)  “Hyperthreading”
CPU is represented to OS as multiple logical CPUs – one for each “hardware thread”
i.e. UltraSPARC T1  8 cores, each with 4 threads  32 logical processors!

38. Multi-CORE Scheduling
OS must schedule the processes into the “logical processors”.
Typical scheduling algorithms (?)
CPU decides which “hardware thread” to run on each core
Coarse-grained parallelism  rotate threads when a long-latency instruction happens, like a load which misses cache
Fine-grained parallelism  “Round Robin” the hardware threads, often every clock cycle.
This is very rare in practice

39. Algorithm Evaluation Deterministic modeling: What you did in 265
takes a particular predetermined workload
defines the performance of each algorithm for that workload.
What you did in 265

40. Algorithm Evaluation N = λ W e.g., if N = 8, λ = 2
Queueing models – Little’s law
N = λ W
N = average queue length
λ = average arrival rate
W = average waiting time in queue
e.g., if N = 8, λ = 2
W = 4

41. Algorithm Evaluation Simulations Clock Random generation of: Processes
Burst times
Arrival & departures rates

42. Algorithm Evaluation Based on probability distributions:
Uniform
Exponential
Poisson
Empirical
Implementation

43. CPU Scheduler Evaluation by Simulation

44. UNIX schedulers

45. UNIX schedulers

46. Bands of Priorities

47. Priority Formula

48. Solaris Scheduling

49. Solaris Dispatch Table

50. Linux pre-2.6 O(n) Scheduler
O(n)  scheduling a task takes O(n) where n = # of tasks
One runqueue for all processors in a symmetric multiprocessor
system
- Task can be scheduled on any CPU
- Good for load balancing
- Bad for memory cache movements, e.g., task previously
on CPU1 is
- run on CPU2  move cache1 to cache2
Single runqueue  CPUs had to contend with shared lock.
No preemption allowed  higher priority process may have
to wait.

51. Linux 0(1) 2.6 Scheduler
- Each CPU has its own runqueue (priority = 1 to 140)
- Tasks are scheduled RR multilevel feedback paradigm.
Tasks whose TQ expires are moved to Expired runqueue (priorities
are recalculated)

52. SMP Tasks Indexed on Priorities

53. Linux 0(1) 2.6 Scheduler Why O(1)?
Bitmap of priorities is read (each priority level points to process)
Since size of bitmap is 140, selection of process does not depend
on the number of processes in the runqueue.
Active runqueue pointer
When active runqueue is empty, pointer is set to expired runqueue,
i.e., expired runqueue now becomes active runqueue and vice
versa
Each CPU (in an SMP system) sets locks on its own runqueues
 all CPUs can schedule without contention from other CPUs.

54. Linux 0(1) 2.6 Scheduler Dynamic Priority Assignment
CPU bound processes are penalized (increased by 5 levels)
I/O bound rewarded (prioirity# decreased by 5 levels)
I/O bound use CPU to set up I/O and is suspended  give other
processes a chance to execute  altruistic
Heuristic for I/O or CPU bound category??
interactivity heuristic
based on: time task executes compared with time it is suspended.
I/O bounds sleep time is large  increase in interactivity metric 
rewarded.
- Priority adjustments only applied to user processes.

55. Windows XP Scheduling Priority-based, pre-emptive.
Dispatcher handles scheduling.
Thread currently running is interrupted when:
(a) it terminates, or
(b) higher priority thread arrives, or
(c) time-quantum expires, or
(d) does I/O
 gives real-time threads priority

56. Windows XP Scheduling Dispatcher uses multi-level priority scheme
Priorities are divided into two classes:
(a) variable (1-15)
(b) real-time (16-31)
thread running at priority 0 is used for memory mgmt.
Processes are rewarded/penalized for sleeping/using up CPU

57. Windows XP Scheduling
Priority Class
Relative Priority