CPU Scheduling Scheduling Terminology Scheduling Terminology by liwenting

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CPU Scheduling                     Scheduling Terminology
• Basic Concepts
• Scheduling Criteria
• Scheduling Algorithms
– FCFS, SJF, RR
•   Exponential Averaging
•   Multi-level Queue Scheduling
•   Performance Evaluation
•   Unix Scheduler

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Scheduling Terminology                 Scheduling/Process State Transition

3                                     4

Scheduling/ Queues                    Objectives for Schedulers

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Introduction to Scheduling
CIS 415

Objectives for Schedulers (cont’d)                     CPU burst and I/O Burst
• 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

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Alternating Sequence of CPU And I/O Bursts      Histogram of CPU-burst Times

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Types of Schedulers                         CPU (Short Term) Scheduler
• Selects from among the processes in memory
that are ready to execute, and allocates the
CPU to one of them.
• CPU scheduling decides which process to
switch from ready state to running state.

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Introduction to Scheduling
CIS 415

First-Come, First-Served (FCFS) Scheduling
CPU Scheduling Algorithms
•   First Come First Served (FCFS)
Process Burst Time
•   Shortest Job First (SJF)                                                                           P1          24
•   Round Robin (RR)                                                                                   P2           3
P3           3
•   Multi-level Queues
• Suppose processes arrive in the order P1 , P2 , P3
•   Multi-level with Feedback                                                         The Gantt Chart for the schedule is:
•   (There are many others)                                                                                        P1                       P2         P3
•
0                                         24         27         30

• Waiting time for P1 = 0; P2 = 24; P3 = 27
13
• Average waiting time: (0 + 24 + 27)/3 = 17                                 14

FCFS Scheduling (Cont.)                                                 Shortest-Job-First (SJF) Scheduling

Suppose that the processes arrive in the order                                 • Associate with each process the length of its next CPU burst.
Use these lengths to schedule the process with the shortest time.
P2 , P3 , P1 .
• Two schemes:
• The Gantt chart for the schedule is:                                            – nonpreemptive – once CPU given to the process it cannot be
preempted until completes its CPU burst.
P2        P3                          P1
– preemptive – if a new process arrives with CPU burst length
less than remaining time of current executing process,
0          3            6                                 30              preempt. This scheme is know as the
Shortest-Remaining-Time-First (SRTF).
•   Waiting time for P1 = 6; P2 = 0; P3 = 3                                    • SJF is optimal – gives minimum average waiting time for a
•   Average waiting time: (6 + 0 + 3)/3 = 3                                      given set of processes. It is an idealized algorithm because
difficult to accurately know next CPU burst time.
•   Much better than previous case.
•   Avoided convoy effect short process behind long
process
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Example of Non-Preemptive SJF                                                       Example of Preemptive SJF
Arrival Time Burst Time                                                        Arrival Time Burst Time
P1 0.0        7                                                                P1 0.0        7
P2 2.0       4                                                                 P2 2.0       4
P3 4.0       1                                                                 P3 4.0       1
P4 5.0       4                                                                 P4 5.0       4
• SJF (non-preemptive)                                                         • SJF (preemptive)
P1                  P3       P2             P4                               P1       P2       P3        P2       P4               P1

0        3              7        8        12             16                      0                                                  11               16
2        4         5        7
• Average waiting time = (0 + 6 + 3 + 7)/4 = 4                                 • Average waiting time = (9+1+0+2)/4 = 3

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Introduction to Scheduling
CIS 415

Priority Scheduling                                   Determining Length of Next CPU Burst

• A priority number (integer) is associated with each       • Can only estimate the length.
process
• The CPU is allocated to the process with the
• Can be done by using the length of previous
highest priority (smallest integer ≡ highest priority).     CPU bursts, using exponential averaging.
• SJF is a priority scheduling where priority is the               1. tn = actual lenght of nthCPU burst
predicted next CPU burst time.                                   2. # n +1 = predicted value for the next CPU burst
• Problem !!! Starvation – low priority processes                  3. " , 0 ! " ! 1
4. Define :
may never execute.
τ n+1 = α tn + (1 −α ) n .
τ
• Solution = Aging – as time progresses increase the
priority of the process.

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Predicting Length of Next CPU Burst                   Exponential Averaging and the Value of α
• α =0
– τn+1 = τn
– Recent history does not count.
• α =1
– τn+1 = t n
– Only the actual last CPU burst counts.
• If we expand the formula, we get:
τn+1 = α tn+(1 - α) α tn -1 + …
+(1 - α )j α tn -1 + …
+(1 - α )n=1 tn τ0
• Since both α and (1 - α) are less than or equal to 1,
each successive term has less weight than its
predecessor.
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Exponential Averaging                                              Round Robin (RR)
• Each process gets a small unit of CPU time (time
quantum), usually 10-200 milliseconds. After this
time has elapsed, the process is preempted and
• 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.

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Introduction to Scheduling
CIS 415

Example of RR with Time Quantum = 20                                              Time Quantum and Context Switch Time

Process              Burst Time
P1                    53
P2                    17
P3                    68
P4                    24
• The Gantt chart is:
P1        P2    P3        P4    P1     P3   P4   P1   P3   P3

0        20    37        57        77    97 117 121 134 154 162

• Typically, higher average turnaround than
SJF, but better response.
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Turnaround Time Varies With Time Quantum                                                      Multilevel Queue
• Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
• Each queue has its own scheduling algorithm,
foreground – RR
background – FCFS
• 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
27      – 20% to background in FCFS                               28

Multilevel Queue Scheduling                                                        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

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Introduction to Scheduling
CIS 415

Example of Multilevel Feedback Queue                         Multilevel Feedback Queues
• Three queues:
– Q0 – time quantum 8 milliseconds
– Q1 – time quantum 16 milliseconds
– Q2 – FCFS
• Scheduling
– A new job enters queue Q 0 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 Q 1 job is again served FCFS and receives 16
additional milliseconds. If it still does not complete, it
is preempted and moved to queue Q2.

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Performance for Schedulers                                     Queuing Theory Analysis

33                                   34

Queuing Theory Analysis (cont’d)                                 Queuing Theory Analysis (cont’d)

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Introduction to Scheduling
CIS 415

Queuing Theory Analysis (cont’d)                  Queuing Theory Analysis (cont’d)

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Evaluation of CPU Schedulers by
Simulation Analysis                                   Simulation

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Performance metrics                        Simulation Analysis (cont’d)
• Turnaround time = time from when job is
submitted to the system until when it is
completed
• Normalized timearound time = Turnaround
time / job runtime (also called slowdown)
• Sometime ‘response time’ is used
interchangeably. Strictly speaking, response
time is used in interactive systems to mean
the time from when the user submits a
request to systems’ first reaction

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Introduction to Scheduling
CIS 415

•   Multilevel feedback queues
•   128 priorities possible (-64 to -63)
•   1 Round Robin queue per priority
•   Every scheduling event the scheduler picks
the highest priority (lowest number) non-
empty queue and runs jobs in round-robin

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UNIX Scheduler                                   Process Scheduling in Unix
• Negative numbers reserved for processes waiting in kernel
mode (just woken up by interrupt handlers) (why do they
have a higher priority?)
• Time quantum = 1/10 sec (empirically found to be the
longest quantum that could be used without loss of the
desired response for interactive jobs such as editors)
– short time quantum means better interactive response
– long time quantum means higher overall system throughput since
less context switch overhead and less processor cache flush.
• Priority dynamically adjusted to reflect
– resource requirement (e.g., blocked awaiting an event)
– resource consumption (e.g., CPU time)

Traditional UNIX Scheduling                                     UNIX Priority Calculation
• All processes assigned a baseline priority based on      • Every 4 clock ticks a processes priority is updated:
the type and current execution status (for
convenience here renumber from 0 to 127):                •                          & utilization #
–   swapper 0                                                       P = BASELINE + \$             ! + 2 NiceFactor
%       4     "
–   waiting for disk 20
–   waiting for lock 35                                 • The utilization is incremented by 1 on every clock
–   user-mode execution   50                              tick when the process is running; it is also decayed
every second (next slide)
• At scheduling events, all process’s priorities are
adjusted based on the amount of CPU used, the            • The niceFactor allows user some control of job
current load, and how long the process has been            priority. It can be set by user from –20 to 20.
waiting.                                                 • Jobs using a lot of CPU increase the priority value.
• Most processes are not running, so lots of                 Interactive jobs not using much CPU will return to
computing shortcuts are used when computing new            the baseline.
47                                                                   48
priorities.

Introduction to Scheduling
CIS 415

UNIX Priority Calculation                                         UNIX Priority Decay
• Without decay, long running CPU bound jobs will         •1 job on CPU: load will thus be 1. Assume niceFactor is 0.
get “stuck” at the highest numbered priority.           •Compute utilization at time N:
• Decay function used to weight utilization to recent
CPU usage.                                              •+1 second:         2
U1 = U 0
• A process’s utilization at time t is decayed every                          3
second:
•+2 seconds
• utilt =( 2*load / 2*load+1) * utilt-1 + niceFactor                     2,     2 ) 2          &2#
2

•         U 2 = *U 1 + U 0 ' = U 1 + \$ ! U 0
3+       3     (   3       %3"
• The system load is the system average number of         •+N seconds
runnable jobs during last 1 minute                                        2         '2\$
2

Un = Un ! 1 + % " U n ! 2 ...
3         & 3#
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Summary of Unix Scheduler

• Commonly used implementation with multiple
priority queues
• Priority computed using 3 factors
– PUSER used as a base (changed dynamically)
– CPU utilization (time decayed)
– Value specified at process creation (nice)
• Processes with short CPU bursts are favored
• Processes just woken up from blocked states are
favored even more
• Weighted averaging of CPU utilization
• Details vary in different versions of Unix

Introduction to Scheduling

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