Module 6: CPU Scheduling
Silberschatz, Galvin and Gagne ©2009
Operating System Concepts – 8th Edition,
CPU Scheduling
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CPU Scheduling
Basic Concepts
Scheduling Criteria
Scheduling Algorithms
Thread Scheduling
Multiple-Processor Scheduling
Operating Systems Examples
Algorithm Evaluation
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Objectives
To introduce CPU scheduling, which is the basis for multiprogrammed operating systems
To describe various CPU-scheduling algorithms
To discuss evaluation criteria for selecting a CPU-scheduling algorithm for a particular system
To examine the scheduling algorithms of several operating systems
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
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Alternating Sequence of CPU And I/O Bursts
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Histogram of CPU-burst Times
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CPU Scheduler
Short-term scheduler selects from among the processes in ready queue, and allocates the CPU to one of them
Queue may be ordered in various ways
CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
Terminates
Scheduling under 1 and 4 is nonpreemptive
All other scheduling is preemptive
Consider access to shared data
Consider preemption while in kernel mode
Consider interrupts occurring during crucial OS activities
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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
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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
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)
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Scheduling Algorithm Optimization Criteria
Max CPU utilization
Max throughput
Min turnaround time
Min waiting time
Min response time
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First-Come, First-Served (FCFS) Scheduling
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: (0 + 24 + 27)/3 = 17
P1
P2
P3
24
27
30
0
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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: (6 + 0 + 3)/3 = 3
Much better than previous case
Convoy effect short process behind long process
P1
P3
P2
6
3
30
0
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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
SJF is optimal – gives minimum average waiting time for a given set of processes
The difficulty is knowing the length of the next CPU request
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Two schemes:
– nonpreemptive – once CPU given to the process it cannot be preempted until completes its CPU burst.
– preemptive
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.
Example of SJF
Process Burst Arrival Time
P1 6 0
P2 8 0
P3 7 0
P4 3 0
SJF scheduling chart
Average waiting time = (3 + 16 + 9 + 0) / 4 = 7
P4
P3
P1
3
16
0
9
P2
24
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Example of Preemptive SJF
Pid Arrival Burst Time
P1 0 7
P2 2 4
P3 4 1
P4 5 4
SJF (preemptive)
Average waiting time = (9 + 1 + 0 +2)/4 = 3
P1
P3
P2
4
2
11
0
P4
5
7
P2
P1
16
Determining Length of Next CPU Burst
Can only estimate the length
Can be done by using the length of previous CPU bursts, using exponential averaging
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Prediction of the Length of the Next CPU Burst
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Examples of Exponential Averaging
=0
n+1 = n
Recent history does not count
=1
n+1 = tn
Only the actual last CPU burst counts
If we expand the formula, we get:
n+1 = tn+(1 – ) tn -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
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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
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
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Example of Priority Scheduling
ProcessA arri Burst TimeT Priority Arrival
P1 10 3 1
P2 1 1 2
P3 5 4 3
P4 1 2 4
Round Robin (RR)
Each process gets a small unit of CPU time (time quantum), usually 10-100 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
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Example of RR with Time Quantum = 4
Process Burst Time
P1 24
P2 3
P3 3
The Gantt chart is:
Typically, higher average turnaround than SJF, but better response
P1
P2
P3
P1
P1
P1
P1
P1
0
4
7
10
14
18
22
26
30
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Time Quantum and Context Switch Time
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Turnaround Time Varies With The Time Quantum
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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
20% to background in FCFS
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Multilevel Queue Scheduling
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Multilevel Feedback Queue System (MFQS)
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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Example of Multilevel Feedback Queue
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR 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.
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Multilevel Feedback Queues
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Real-Time Scheduling
Can present obvious challenges
Soft real-time systems – no guarantee as to when critical real-time process will be scheduled
Hard real-time systems – task must be serviced by its deadline
Two types of latencies affect performance
Interrupt latency – time from arrival of interrupt to start of routine that services interrupt
Dispatch latency – time for schedule to take current process off CPU and switch to another
Dispatch Latency
How long it takes a real-time (RT) process
to execute in the CPU once it arrives.
RT processes have the highest-priority
Dispatch latency must be small for RT
process to execute fast
Dispatch Latency
Most OS force a wait for:
System call to complete
(may be complex and long)
I/O block transfer to complete (may be too slow)
before a context switch can happen
Dispatch Latency
Dispatch latency is long
Solution: place preemption points and force context switch in
the middle of the other process’s syscall or I/O.
Caveat: Preemption points must be placed at “safe” locations
avoid modifying kernel data structures.
Dispatch Latency
What to do if RT process is waiting for resources heldby one or more low-priority processes?
Priority Inversion (priority inheritance protocol)
All of them inherit the RT process’s high priority
Complete their respective tasks
Release all their resources for the RT process
All low-priority processes will revert
to their original low priorities
Dispatch Latency
Dispatch latency phases:
Conflicts
Dispatch
Dispatch Latency
Conflict phase:
Preempt process running in the kernel
Release low-priority process’s resources
needed by RT process
E.g., in Solaris 2,
with preemption enabled: dispatch latency = 2 ms.
with it disabled: dispatch latency = 100 ms.
Dispatch Latency
How long it takes a RTprocess to execute ?
Real-Time Scheduling
Process Arrival Time Burst Time Deadline
P1 0 5 15
P2 2 5 8
P3 3 1 6
P4 5 4 12
Earliest Deadline First/Minimum Slack (Preemptive)
Is there another valid schedule?
What happens if P3’s burst was 2 instead?
2
P1
3
P2
4
P3
P2
8
12
P4
15
P1
0
Real-Time Scheduling
Process Arrival Time Burst Time Deadline
P1 0 5 15
P2 2 5 8
P3 3 2 6
P4 5 4 12
Earliest Deadline First/Minimum Slack (Preemptive)
2
P1
3
P2
5
P3
P2
P4
P1
0
Priority-based Scheduling
For real-time scheduling, scheduler must support preemptive, priority-based scheduling
But only guarantees soft real-time
For hard real-time must also provide ability to meet deadlines
Processes have new characteristics: periodic ones require CPU at constant intervals
Has processing time t, deadline d, period p
0 ≤ t ≤ d ≤ p
Rate of periodic task is 1/p
Thread Scheduling
Distinction between user-level and kernel-level threads
Many-to-one and many-to-many models, thread library schedules user-level threads to run on LWP
Known as process-contention scope (PCS) since scheduling competition is within the process
Kernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system
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Pthread Scheduling
API allows specifying either PCS or SCS during thread creation
PTHREAD SCOPE PROCESS schedules threads using PCS scheduling
PTHREAD SCOPE SYSTEM schedules threads using SCS scheduling.
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Pthread Scheduling
#include
#define NUM_THREADS 5
int main(int argc, char *argv[])
{
int i;
pthread_t tid[NUM THREADS];
pthread_attr_t attr;
/* get the default attributes */
pthread_attr_init(&attr);
/* set the scheduling algorithm to PROCESS or SYSTEM */
pthread_attr_setscope(&attr, PTHREAD_SCOPE_SYSTEM);
/* set the scheduling policy – FIFO, RT, or OTHER */
pthread_attr_setschedpolicy(&attr, SCHED_OTHER);
/* create the threads */
for (i = 0; i < NUM_THREADS; i++)
pthread_create(&tid[i], &attr, runner, NULL);
/* now join on each thread */
for (i = 0; i < NUM_THREADS; i++)
pthread_join(tid[i], NULL);
}
/* Each thread will begin control in this function */
void *runner(void *param) {
printf("I am a thread\n");
pthread_exit(0); }
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Multiple-Processor Scheduling
CPU scheduling more complex when multiple CPUs are available
Homogeneous processors within a multiprocessor
Asymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharing
Symmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes
Processor affinity – process has affinity for processor on which it is currently running
soft affinity
hard affinity
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NUMA and CPU Scheduling
Non-Uniform Memory Access
parallel architecture where each processor has its own local memory but can also access memory from other processors.
Multiple-Processor Scheduling – Load Balancing
If SMP, need to keep all CPUs loaded for efficiency
Load balancing attempts to keep workload evenly distributed
Push migration – periodic task checks load on each processor, and if found pushes task from overloaded CPU to other CPUs
Pull migration – idle processors pulls waiting task from busy processor
Multicore Processors
Recent trend to place multiple processor cores on same physical chip
Each core has its own register set: seen by OS as a processor
Faster and consume less power
Multiple threads per core also growing
“Hardware” threads: hardware support includes logic for thread switching, thus decreasing the context switch time.
Takes advantage of memory stall to make progress on another thread while memory retrieve happens
Multithreaded Multicore System
2 different levels of scheduling:
Mapping software thread onto hardware thread
- traditional scheduling algorithms like those discussed last time
Which hardware thread a core will run next
- Round Robin (Ultra Sparc1) or dynamic priority-based (Intel Itanium, dual-core processor with two hardware-managed threads per core)
0
1
Operating System Examples
UNIX Scheduling
Linux scheduling
Windows XP scheduling
Solaris scheduling
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UNIX schedulers
UNIX schedulers
Bands of Priorities
Priority Formula
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.
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)
Linux Scheduling
Constant order O(1) scheduling time
Two priority ranges: time-sharing and real-time
Real-time range from 0 to 99 and nice value from 100 to 140
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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 sets locks on its own runqueues
all CPUs can schedule without contention from other CPUs.
Linux 0(1) 2.6 Scheduler
Dynamic Priority Assignment
CPU bound processes are penalized (increased by 5 levels)
I/O bound rewarded (priority# 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 bound sleep time is large
increase in interactivity metric
rewarded.
- Priority adjustments only applied to user processes.
Priorities and Time-slice length
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Linux Scheduling in Version 2.6.23 +
Completely Fair Scheduler (CFS)
Scheduling classes
Each has specific priority
Scheduler picks highest priority task in highest scheduling class
Rather than quantum based on fixed time allotments, based on proportion of CPU time
2 scheduling classes included, others can be added
default
real-time
Linux Scheduling in Version 2.6.23 +
Quantum calculated based on nice value from -20 to +19
Lower value is higher priority
Calculates target latency – interval of time during which task should run at least once
Target latency can increase if say number of active tasks increases
CFS scheduler maintains per task virtual run time in variable vruntime
Associated with decay factor based on priority of task – lower priority is higher decay rate
Normal default priority yields virtual run time = actual run time
To decide next task to run, scheduler picks task with lowest virtual run time
CFS Performance
Linux Scheduling (Cont.)
Real-time scheduling according to POSIX.1b
Real-time tasks have static priorities
Real-time plus normal map into global priority scheme
Nice value of -20 maps to global priority 100
Nice value of +19 maps to priority 139
Windows Scheduling
Windows uses priority-based preemptive scheduling
Highest-priority thread runs next
Dispatcher is scheduler
Thread runs until (1) blocks, (2) uses time slice, (3) preempted by higher-priority thread
Real-time threads can preempt non-real-time
32-level priority scheme
Variable class is 1-15, real-time class is 16-31
Priority 0 is memory-management thread
Queue for each priority
If no run-able thread, runs idle thread
Windows Priority Classes
Win32 API identifies several priority classes to which a process can belong
REALTIME_PRIORITY_CLASS, HIGH_PRIORITY_CLASS, ABOVE_NORMAL_PRIORITY_CLASS,NORMAL_PRIORITY_CLASS, BELOW_NORMAL_PRIORITY_CLASS, IDLE_PRIORITY_CLASS
All are variable except REALTIME
A thread within a given priority class has a relative priority
TIME_CRITICAL, HIGHEST, ABOVE_NORMAL, NORMAL, BELOW_NORMAL, LOWEST, IDLE
Priority class and relative priority combine to give numeric priority
Base priority is NORMAL within the class
If quantum expires, priority lowered, but never below base
Windows Priority Classes (Cont.)
If wait occurs, priority boosted depending on what was waited for
Foreground window given 3x priority boost
Windows 7 added user-mode scheduling (UMS)
Applications create and manage threads independent of kernel
For large number of threads, much more efficient
UMS schedulers come from programming language libraries like C++ Concurrent Runtime (ConcRT) framework
Windows Priorities
Priority-based preemptive scheduler:
- on X axis: classes of priorities
- on Y axis: relative priorities within a class
Base priority for a process: threads cannot go lower
Priority varies based on:
- quantum used: lower priority
- interrupt from keyboard: larger increase than from disk
Quantum varies for foreground vs. background process.
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Solaris
Priority-based scheduling
Six classes available
Time sharing (default) (TS)
Interactive (IA)
Real time (RT)
System (SYS)
Fair Share (FSS)
Fixed priority (FP)
Given thread can be in one class at a time
Each class has its own scheduling algorithm
Time sharing is multi-level feedback queue
Loadable table configurable by sysadmin
Solaris Dispatch Table
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Solaris Scheduling
Solaris scheduling
Algorithm Evaluation
How to select CPU-scheduling algorithm for an OS?
Determine criteria, then evaluate algorithms
Deterministic modeling
Type of analytic evaluation
Takes a particular predetermined workload and defines the performance of each algorithm for that workload
Consider 5 processes arriving at time 0:
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Deterministic Evaluation
For each algorithm, calculate minimum average waiting time
Simple and fast, but requires exact numbers for input, applies only to those inputs
FCS is 28ms:
Non-preemptive SFJ is 13ms:
RR is 23ms:
Queueing Models
Describes the arrival of processes, and CPU and I/O bursts probabilistically
Commonly exponential, and described by mean
Computes average throughput, utilization, waiting time, etc
Computer system described as network of servers, each with queue of waiting processes
Knowing arrival rates and service rates
Computes utilization, average queue length, average wait time, etc
Little’s Formula
n = average queue length
W = average waiting time in queue
λ = average arrival rate into queue
Little’s law – in steady state, processes leaving queue must equal processes arriving, thus:
n = λ x W
Valid for any scheduling algorithm and arrival distribution
For example, if on average 7 processes arrive per second, and normally 14 processes in queue, then average wait time per process = 2 seconds
Simulations
Queueing models limited
Simulations more accurate
Programmed model of computer system
Clock is a variable
Gather statistics indicating algorithm performance
Data to drive simulation gathered via
Random number generator according to probabilities
Distributions defined mathematically or empirically
Trace tapes record sequences of real events in real systems
Evaluation of CPU schedulers by Simulation
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Implementation
Even simulations have limited accuracy
Just implement new scheduler and test in real systems
High cost, high risk
Environments vary
Most flexible schedulers can be modified per-site or per-system
Or APIs to modify priorities
But again environments vary
:
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