Module 4
Process scheduling
Ch. 6
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Reading: Chapter 6 (
Silberschatz
)
Module overview
Basic concepts Scheduling criteria Scheduling algorithms Multiprocessor scheduling Algorithm evaluation
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Ch. 6
State transition diagram of a process
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Process queues for scheduling
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Ready queue
We will assume that the first process in a queue is the one that uses the resource: here, proc7 executes
Basic concepts
Multiprogramming is designed to achieve maximum use of resources, especially the CPU
The CPU scheduler is the part of the OS that decides which process in the ready queue gets the CPU when it becomes free
should aim for optimal use of the CPU
CPU is the most valuable resource in a computer, so we are talking about it
However, the principles that we will see also apply to the scheduling of other resources (I / O units, etc.).
Must understand process behavior
To make the right scheduling decision
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The cycles of a process
CPU and I / O cycles (bursts): the execution of a process consists of execution sequences on CPUs and I / O waits
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When to invoke the scheduler
The scheduler must make its decision every time the executing process is interrupted, i.e.
a process presents itself as new or finished an executing process becomes stuck waiting a process changes from running to ready
a process changes from waiting to ready
in conclusion, any event in a system causes an interruption of the CPU and the intervention of the scheduler, who will have to make a decision about which proc. or thread will have the CPU after
Preemption: we have preemption in the last two cases if we remove the CPU from a process that had it and can continue to use it
In the first two cases, there is no preemption
Several problems to resolve in the case of preemption
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Dispatcher
The code of the OS that gives control to the process chosen by the scheduler. It must be concerned with:
change context
change to user mode restart the chosen process
Dispatcher latency
the time required to perform the duties of the dispatcher
it is often overlooked, it must be assumed that it is small compared to the length of a cycle
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Scheduling criteria
There will normally be several processes in the ready queue
When CPU becomes available, which one to choose?
The general idea is to make the choice in the interest of the efficient use of the machine
But the latter can be judged according to different criteria
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Scheduling criteria
Main reason for scheduling
Percentage of use: keep CPUs and I / O modules busy
Time-sharing systems?
Response time (for interactive systems): the time between a request and the response
Servers?
Throughput: number of processes that complete in the unit of time
Batch processing systems?
Turnaround time: the time taken by the process from its arrival to its end.
Loaded systems?
Waiting time: waiting in ready queue (sum of all time spent in ready queue)
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Scheduling criteria:
To maximize
maximize / minimize
CPU use: percentage of use
Throughput: number of processes that complete in the unit of time
To minimize
Response time (for interactive systems): the time between a request and the response
Turnaround time: completion time minus arrival time Waiting time: waiting in ready queue
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Scheduling Criteria Example
P1
P2
P3
P4
P1
P2
Time
P1 P2 P3 P4
0 4 5 7 10,11,12
CPU utilization:
100%
Throughput :
4/24
Turnaround time (P3, P2):
P3: 5
P2: 20
Waiting time (P2):
P2: 13
Response time (P3, P2):
P3: 3 P2: 1
Process arrival
20 22 24
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Now lets take a look at several scheduling methods and see how they behave against these criteria.
we will study specific cases
the study of the general case would require recourse to probabilistic or simulation techniques
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First come, first served
Note, no preemption
(First come, first serve, FCFS)
Example: Process Cycle time P1 24
P2 3
P3 3
If the processes arrive at time 0 in the order: P1, P2, P3 The Gantt chart is:
0 24 27 30
Waiting time for P1 = 0; P2 = 24; P3 = 27 Average waiting time: (0 + 24 + 27) / 3 = 17
P1
P2
P3
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First come, first served
CPU utilisation = 100% Throughput = 3/30 = 0.1
3 processes completed in 30 units of time
Average rotation time: (24 + 27 + 30) / 3 = 27
P1
P2
P3
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0 24 27 30
FCFS scheduling (continued)
If the same processes arrive at 0 but in order P2 , P3 , P1 .
The Gantt chart is:
P2
P3
P1
036 30
Waiting time for P1 = 6 P2 = 0 P3 = 3
Average waiting time: (6 + 0 + 3) / 3 = 3
Much better!
So for this technique, the average wait time can
vary greatly
Exercise: also calculate the average rotation time, throughput, etc.
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Take into account the arrival time!
If the processes arrive at different times, subtract the arrival times
Exercise: repeat the calculations if:
P2 arrives at time 0 P1 arrives at time 2 P3 arrives at time 5
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Convoy Effect with the FCFS
Consider a single CPU bound process and many I/O bound processes (fairly normal situation).
The I/O bound processes wait for the CPU: I/O under utilized (*).
The CPU bound process requests I/O: the other processes rapidly complete CPU bursts and go back to I/O: CPU under utilized.
CPU bound process finishes I/O, and so do the other processes: back to *
Solutions?
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FCFS Scheduling Discussion
Is it simple and easy to program?
Yes!
Can we find other benefits?
It costs little time to make scheduling decision
Does it provide low waiting time?
Not at all, can be quite awful
Does it provide good CPU utilization?
No the convoy effect
OK, forget about it.
Now, lets try to reduce the average waiting time
Idea: It is better to execute ready processes with short CPU bursts
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Shorter First = Shortest Job First (SJF)
The shortest process starts first
Optimal in principle in terms of average waiting time
(see the last example)
But how do we know
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Shortest
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
preemptive if a new process arrives with CPU burst length less than remaining time of current executing process, preempt. This scheme is also know as the Shortest-Remaining-Time-First (SRTF)
Just call it preemptive SJF
Preemptive SJF is optimal gives minimum average waiting time for a given set of processes
Moving a process with short CPU burst in front of a process with longer CPU burst reduces average waiting time
Job
First (SJR) Scheduling
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Example of Non
Process Arrival Time Burst Time P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (non-preemptive)
0 3 78 12 16
P2 arr. P3 arr. P4 arr
Average waiting time = (0 + 6 + 3 + 7)/4 = 4
Preemptive SJF
P1
P3
P2
P4
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Example of Preemptive SJF
Process Arrival Time Burst Time P1 0.0 7
P2 2.0 4
P3 4.0 1
P4 5.0 4
SJF (preemptive)
P1
P2
P3
P2
P4
P1
0 2 4 5 7 11 16
P2 arr.
Average waiting time = (9 + 1 + 0 +2)/4 = 3
P3 arr. P4 arr.
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Small technical detail with SJF
How do we know the length of the next CPU burst?
If you know that, you probably know also tomorrows stock market prices
You will be rich and wont need to waste time in CSI3131 class So, we can only estimate it
Any idea how to estimate the length of the next CPU burst?
Probably similar as the previous bursts from this process
Makes sense to give more weight to the more recent bursts, not just straightforward averaging
Use exponential averaging
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Exponential averaging with SJF
1. t = actual length of nth CPU burst n
2. n+1 = predicted value for the next CPU burst
3. ,01 = relativeweightofrecentvspasthistory 4. Define:
n+1 =tn +(1)n
n+1 = tn +(1)tn1 +(1)2tn2 +
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i
+(1)t ++(1) t
ni n1
The shortest first SJF: review
Difficulty estimating the length in advance
Long processes will suffer from famine when there is a constant supply of short processes
Preemption is required for timeshared environments
A long process can monopolize the CPU if it is the first to enter the system and it does not do I / O
There is an implicit assignment of priorities: preferences for shorter jobs
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SJF Discussion
Does it ensure low average waiting time?
Yes, it was designed that way
As long as our burst-length predictions more-or-less work
Does it provide low response time?
Not necessarily, if there is a steady stream of short CPU bursts, the longer bursts will not be scheduled
This is called starvation
A process is blocked forever, always overtaken by other processes (well, or at least while the system is busy)
Lets see Priority Scheduling.
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Priority Scheduling
A priority number (usually integer) is associated with each process
On some systems (Windows), the higher number has higher priority
On others (Unix) , the smaller number has higher priority
The CPU is allocated to the process with the highest priority
Can be preemptive or non-preemptive
but usually you want to preempt low-priority process when a high priority process becomes ready
Priority can be explicit
Assigned by the sysadmin/programmer, often for political reasons Professor jobs are of higher priority
But also for technical reasons
This device has to be serviced really fast, otherwise the vital data will be lost (real-time processing)
Priority can also be implicit (computed by the OS)
SJF can be seen as priority scheduling where priority is the predicted next CPU burst time
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Priority Scheduling Discussion
Good properties
Professor jobs will be scheduled before student jobs
Allows support of real-time processing
Bad properties
Professor jobs will be scheduled before student jobs OK, give me something else
starvation low priority processes may never execute
How to resolve the starvation problem?
aging keep increasing the priority of a process that has not been scheduled for a long time
What to do with the processes of the same priority level?
FCFS
Might as well add preemption = Round Robin
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Round
Robin (RR) in practice
Most
used
Each process is allocated a quantum of time (e.g. 10-100 millisecs.) To run
(book terminology: time slice)
If it runs for an integer quantum without other interruptions, it is interrupted by the timer and the CPU is given to another process
The interrupted process becomes ready again (at the end of the queue)
Preemptive method
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Ch. 6
P[7]
P[6]
P[0]
P[1]
The ready queue is a circle (including RR)
P[2]
P[3]
P[5]
P[4]
Round
If there is a process in the ready queue and the quantum is q, then each process receives 1 / n of the CPU time in units of max duration. q
If q large FCFS
If q small we will see
Robin performance
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Example: RR Quantum = 20
Process
Cycle
P1 53 P2 17 P3 68 P4 24
P1
P2
P3
P4
P1
P3
P4
P1
P3
P3
0 20 37 57 77 97 117 121134 154162
Normally,
higher turnaround time than SJF but average waiting time better
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A time)
quantum
increases
context
switches
small
(O
S
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Example to see the importance of a good choice
(to be developed as an exercise)
of quantum
Three cycles:
A, B, C, all of 10
Try with:
q =1 q = 10
In this second case, the round-robin works as FCFS and the average rotation time is better
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Ch. 6
Algorithms we have seen so far
First Come First Serve
simple, little overhead, but poor properties
Shortest Job First
needs to know CPU burst times exponential averaging of the past
Priority Scheduling
This is actually a class of algorithms
Round Robin
FCFS with preemption
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Scheduling exercises
Consider three processes P1, P2, P3
Burst times for P1: 14,12,17
Burst times for P2: 2,2,2,3,2,2,2,3,2,2,2,3,2,2,2,3
Burst times for P3: 6,3,8,2,1,3,4,1,2,9,7
All three arrive at time 0, in order P1, P2, P3
Each CPU burst is followed by an I/O operation taking 6 time units
Lets simulate the scheduling algorithms
FCFS
Round Robin (quantum=5)
Non-preemptive SJF or Preemptive SJF (your choice)
Round robin (quantum=5) with Priority scheduling, priorities are P2=P3>P1
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Multilevel Queue
Idea: Partition the ready queue into several queues, and handle each queue separately.
Example:
foreground queue (interactive processes) background (batch processes)
Each queue might have its own scheduling algorithm
foreground RR (for low response time)
background FCFS (for simplicity and low context- switch overhead)
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Multilevel Queue
How to schedule from among the queues?
Fixed priority scheduling
i.e. the processes from foreground queue get the CPU, the background processes get the CPU only if the foreground queue is empty
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 queue, 20% to background queue
not necessarily optimal
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Multilevel Queue Scheduling
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Multilevel Feedback Queue
Idea:
Use multilevel queues
A process can move up and down queue hierarchy
Why would a process move to a lower priority queue?
It is using too much CPU time
Why would a process move to a higher priority queue?
Has been starved of CPU for long time A way to implement aging
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Multilevel Feedback Queue
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
This algorithm is the most general one
It can be adapted to specific systems But it is also the most complex algorithm
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Example of Multilevel Feedback Queue
Scheduler selects processes in Q0 first (highest priority)
If Q0 is empty, the processes from Q1 are selected.
If both Q0 and Q1 are empty, processes from Q2 are selected
If a process arrives in a higher priority queue when another from a lower priority queue is running, the running process will be preempted, to allow the arriving process to run.
When a process exhausts its quantum in either Q0 or Q1, it is preempted and moved to the lower priority queue.
Q0
Q1
Q2
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Example of Multilevel Feedback Queue
Scheduling example
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 preempted and moved to queue Q1 and served again FCFS to receive another 16 additional milliseconds.
If it still does not complete, it is preempted and moved to queue Q2.
Q0 Q1
Q2
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Multilevel Feedback Queue Discussion
Exact properties depend on the parameters
Flexible enough to accommodate most requirements
The convoy example:
One process with long CPU burst time
Several I/O bound processes with short CPU burst time
Even if the all processes start at the same level, the CPU-intensive process will be soon demoted to a low priority queue
The I/O bound processes will remain at high priority and will be swiftly serviced, keeping the I/O devices busy
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In practice
The methods we have seen are all used in practice (except pure SJF which is impossible)
Sophisticated OS provide the system manager with a library of methods, which it can choose and combine as needed after observing the behavior of the system.
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Ch. 6
Also
Our study of scheduling methods is theoretical, does not consider in detail all the problems that arise in CPU scheduling.
Eg. CPU schedulers cannot give the CPU to a process for all the time it needs
Because in practice, the CPU will often be interrupted by some external event before the end of its cycle.
Also, this study does not consider the execution times of the scheduler at all.
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Ch. 6
Summary of scheduling algorithms
First come, first served (FCFS)
simple, low system time (overhead), low quality
Shortest Job First (SJF)
Must know processing times (not practical)
Must predict using the exponential mean of the past
Scheduling with priority
A class of algorithms
Round-Robin
FCFS with preemption
Multilevel Queues
Possible to use different algorithms with each queue
Multilevel Feedback Queues
Combines several techniques
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Ch. 6
Overview of advanced scheduling topics
Scheduling with several identical CPUs Evaluation model
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Ch. 6
Multiple
Good news:
With multiple CPUs, we can share the load We can also share the OS overhead
Bad news:
We are expected to efficiently share the load
Managing the OS structures gets more complicated
The scheduling gets more complex
Processor Scheduling
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Multiple
We assume homogeneous processors
i.e. all processors have the same functionality
Still, the scheduler (and possibly the whole OS) might be run on a single CPU
Asymmetric multiprocessing only one CPU accesses the system data structures and makes scheduling decisions
alleviates the need for data sharing but might become bottleneck
Symmetric multiprocessing (SMP) each CPU is self-scheduling
Either from a common ready queue, or from a private ready queue
Care must be taken when CPUs access common data structures
Virtually all modern operating systems support SMP including Windows XP, Solaris, Linux, Mac OS X.
Processor Scheduling
Approaches
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SMP Scheduling Issues
Processor Affinity
When process runs on a physical CPU, cache memory is updated with content of the process
If the process is moved to another CPU, benefits of caching is lost.
SMP systems try to keep processes running on the same physical CPU know as processor affinity
Load Balancing
Required when each CPU has its own Ready Queue
Push migration: a task is run periodically to redistribute load among CPUs (their ready queue)
Pull migration: An idle CPU (i.e. with an empty ready queue), will pull processes from other CPUs
Linux supports both techniques
Can counteract the benefits of processor affinity, since it moves processes from one CPU to another
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Evaluation algorithms
Deterministic modeling Queuing models Simulation
methods
and
comparison
of
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Ch. 6
Deterministic modeling
Essentially, what we have already done by studying the behavior of several algorithms on several examples
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Use of
Analytical method based on probability theory
Simplified model: in particular, the OS times are ignored
However, it makes estimates possible
queuing
theory
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Ch. 6
Queuing
An important result:
n=W
n: average length of the queue
: process arrival rate in queue
W: average waiting time in the queue
Eg.
if processes arrive 3 per sec.
W and they stay in the queue for 2 seconds n the average queue length will be ???
Exercise: Solve also for and W
Observe that for n to be stable, W must be stable
If n must remain 6 and goes up to 4, what must be W?
theory
:
Littles
formula
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Ch. 6
Simulation
Build a (simplified) model of the sequence of events in the OS
Assign a duration of time to each event
Assume a certain sequence of external events (e.g. arrival of process, etc.)
Run the model for this sequence to get stats
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Ch. 6
Important points in this chapter
Ready Queue for CPU
Scheduling criteria
Scheduling algorithms
FCFS: simple, not optimal
SJF: optimal, difficult implementation exponential averaging
Priorities
Round-Robin: selection of the quantum
Algorithm evaluation, queuing theory,
Littles formula
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Ch. 6
Thank You!
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