CGS 3763 Operating Systems Concepts Spring 2013 - PowerPoint Presentation

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CGS 3763 Operating Systems Concepts Spring 2013

Dan C. Marinescu

Office: HEC 304

Office hours: M-

Wd

11:30 - 12:30

A

MSlide2

Last time: Answers to the midterm

Today:

Answers from week 7 questionsCPU SchedulingNext timeCPU schedulingReading assignmentsChapter 5 and 6 of the textbook

Lecture 22 – Friday, February 29, 2013

2

Lecture 22Slide3

Student Review Form 7 Summary BO KANG

Feb 18

th Monday:Key Points: Threads: single and multithreading processes.Questions:How threads, programs, and processes differ?The classification between user-level threads and kernel-level threads, and the reasons that make each other unique.Among the different threading APIs, is there one that is more effective than the other?What is meant by “race” conditions in relation to thread safeness? How do you detect a race condition, and how are they handled?Can you explain why barrier synchronization is a challenge for multi-threading?

Are there any advantages to single threading over multithreading?Since threads share data, do the other threads have to wait to access the information if one thread is using the data?

How does a programmer know which thread model to use when writing code? 

Lecture 22

3Slide4

Threads versus processes

In the traditional

view a process has two roles:consumer of resources, e.g., address space, open files.code executing on a CPU, in one address space, there is a single line of execution

.The multi-threaded view of processes and threads

 the two roles are distinct:  

A

process is a consumer of resources,

it

owns an address space and a set of open files.

A thread is an instance of code executing sequentially on a CPU.

A thread needs an address space in which to execute, so we think of a thread as belonging to a process. A single process may own many threads, running concurrently within its address space.  Threads are “lightweight processes” less state information must be kept to maintain a thread than a process. 

Lecture 22

4Slide5

Threads versus processes

Any

 applications of threads can be implemented with separate processes.Advantages of using threads instead of processes:Threads share data in memory and can communicate without IPC (Inter-Process Communication) mechanisms, which require expensive system calls.

Thread creation and context switching are much less time-consuming than the same operations for processes.DisadvantagesUse of shared memory results in a need to synchronize access to shared data.

Libraries may need to be rewritten to make them “thread-safe”

Lecture 22

5Slide6

User and kernel threads

User threads – run in user mode, are supported by a user-level thread library.

Kernel threads – run in user mode when executing user functions or library calls; switch to kernel mode when executing system calls.Provide privileged services to applications e.g.,

system calls. The entities handled by the system scheduler

. Used by the kernel to keep track of all processes in the system and

how much

resources

are allocated to

each process.

When multiple user threads are mapped to a single kernel thread then this thread schedules individual user threads.

A kernel-only thread  - executes only in kernel mode environment. Lecture 22

6Slide7

User and kernel threads (cont’d)

Library code decides which user threads the mapping of user to kernel threads.

The use of system calls or other kernel services by an application:If uses heavily system calls, the more user threads per kernel thread, the slower the applications will run, because the kernel thread becomes a bottleneck, all system calls pass through it.If it uses rarely system calls, a large number of user threads can be assigned to a kernel thread without much performance penalty, other than the overhead of the context switch.Increasing the number of kernel threads, adds overhead to the kernel in general, so while individual threads will be more responsive with respect to system calls, the system as a whole will become slower.

It is important to find a good balance between the number of kernel threads and the number of user threads per kernel thread.

Lecture 22

7Slide8

Race condition

A race

condition occurs in concurrent execution of multiple threads/processes when the output is dependent on the timing or other uncontrollable events.Example: two threads T1 and T2 which share a variable x. in T1 there is a statement (SA): x = -2 in T2 there is a statement (SB): x=+17

The result depends the scheduler. If the order of execution is: SA then SB then the final result is x=+17

SB then SA then the final result is x=-2

Lecture 22

8Slide9

Barrier synchronization

A barrier for a group

of n threads /processes means any of them must stop and cannot proceed until all other threads/processes reach this barrier.Barrier synchronization increases the execution time and must be avoided when possible.Example: 10 threads, 9 of them finish after 10 secondsone needs 100 seconds. the first 9 processes have to wait 90 seconds.

Lecture 22

9Slide10

Feb 20th Wednesday:

Key Points: Pthreads, Java threads, CPU Scheduling, CPU burst, I/O burstQuestions: Why would you want to use a thread instead of a pthread? If pthreads take less time, why not just use pthreads instead of threads all the time?Are Pthreads just universally accepted threads used across different OS’s?Pthreads

is a library that allows multiple threads to use the same code. Can multiple threads access the same code simultaneously?Is Pthreads a set of instructions that can be used in any programming language or what is specifically?

What decides what type of scheduling is used, preemptive vs. non-preemptive)? What are the advantages and disadvantages of preemptive vs. non-preemptive modes?CPU scheduling, and the two subtypes of scheduling that it also uses, non-preemptive and preemptive. I don’t know what each of the subcategories

specifically

Lecture 22

10Slide11

Feb 22th Friday:

Key Points:

CPU scheduling metrics, Scheduling Objectives, Scheduling Policies – first-come, first-served (FCFS)Questions:What are batch and interactive systems used for? Please further clarify the difference between interactive and batch processing policies.When to use these different forms of scheduling?Are rules for scheduling polices such as one being faster than another or when to use one over anther?Can a higher priority thread prevent a lower priority from executing?Which example of process scheduling is most efficient? FCFS, SJF, RR or priority?Where and when does Marshaling take place? Is it required for all processes?

Lecture 22

11Slide12

Scheduling policies

First-Come First-Serve (FCFS)

Shortest Job First (SJF)Round Robin (RR)Priority schedulingLecture 22

12Slide13

Desirable properties of scheduling policies

Fairness -

Prevent starvation – ensure that a process/thread is able to run at some point in time.Efficiency – context switching takes some time and if occurs very often the overhead of it lowers the CPU utilization.Lecture 2213Slide14

First-Come First-Served (FCFS)

Thread Burst Time P1

24 P

2 3

P

3

3

Processes arrive in the order: P1  P2 

P3

Gantt Chart for the schedule:

Waiting time for

P

1

= 0;

P

2

= 24;

P

3

= 27

Average waiting time: (0 + 24 + 27)/3 = 17

Convoy effect



short process behind long process

P

1

P

2

P

3

24

27

30

0

14

Lecture 22Slide15

The effect of the release time on FCFS scheduling

Release time

 time when a job is available.Now threads arrive in the order: P2

 P3

 P1

Gantt chart:

Waiting time for

P

1

=

6

; P2

= 0

;

P

3

=

3

Average waiting time: (6 + 0 + 3)/3 = 3

Much better!!

P

1

P

3

P

2

6

3

30

0

15

Lecture 22Slide16

Shortest-Job-First (SJF)

Use the length of the next burst to schedule the thread/process with the shortest time.

SJF is optimal minimum average waiting time for a given set of threads/processesTwo schemes: Non-preemptive  the thread/process cannot be preempted until completes its burst

Preemptive  if a new thread/process arrives with burst length less than remaining time of current executing process, preempt. known as

Shortest-Remaining-Time-First (SRTF)

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Lecture 22Slide17

Example of non-preemptive SJF

Thread Release time Burst Time P1

0.0 7

P2 2.0 4

P

3

4.0 1

P4 5.0 4SJF (non-preemptive)

Average waiting time = (0 + 6 + 3 + 7)/4

= 4

P

1

P

3

P

2

7

3

16

0

P

4

8

12

17

Lecture 22Slide18

Example of Shortest-Remaining-Time-First (SRTF) (Preemptive SJF)

Thread Release time Burst time P1 0.0 7

P

2 2.0 4

P

3

4.0 1

P4 5.0 4Shortest-Remaining-Time-First

Average waiting time = (9 + 1 + 0 +2)/4 = 3

P

1

P

3

P

2

4

2

11

0

P

4

5

7

P

2

P

1

16

18

Lecture 22Slide19

Determining length of next CPU Burst

Needed by the SJF algorithm.

Can be predicted by using:The past history, the length of previous CPU bursts.Exponential averaging based on the past lengths of the CPU bursts

Lecture 22

19Slide20

Prediction of the Length of the Next CPU Burst

Lecture 22

20Slide21

Exponential Averaging

 =0

n+1 = nRecent 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 -

)

t

n

-1

+ …

+(

1 - 

)

j



t

n

-

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

Lecture 22

21Slide22

Round Robin (RR)

Each process gets a small unit of CPU time (

time quantum), usually 10-100 milliseconds. After this time has elapsed, the thread/process is preempted and added to the end of the ready queue.If there are n threads/processes in the ready queue and the time quantum is q, then each thread/process gets 1/n of the processor time in chunks of at most

q time units at once. No thread/process waits more than (n-1)q

time units.Performanceq

large

 FIFO

q

small 

q

must be large with respect to context switch, otherwise overhead is too high22Lecture 22Slide23

RR with time slice q = 20

Thread Burst Time P1 53

P2

17

P

3

68

P

4 24

Typically, higher average turnaround than SJF, but better response

P

1

P

2

P

3

P

4

P

1

P

3

P

4

P

1

P

3

P

3

0

20

37

57

77

97

117

121

134

154

162

23

Lecture 22Slide24

Time slice (quantum) and context switch time

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Lecture 22Slide25

Comparison of FCFS, SJF, and RR

We have three processes A, B, and C.

Each process is characterized byRelease time (R) – the time the process arrives in the systemStart time (S) – the time the scheduler allows it to runWait time (W) – the time till it has to wait until it is started: W = S - RWork (

Wrk) – the amount of time it needs to use the CPUFinish time (F) –

the time the process finishes: F=S+Wrk

Time in system (T) – the time from when the process arrives until it finishes: T = F- R.

We compare

The average time in system

The average waiting time

f

or three scheduling policies: FCFS, SJF, and RR.Lecture 2225Slide26

Proc

Release time

Work

Start time

Finish time

Wait time

till start

Time in system

A

0

3

0

3

0

3

B

1

5

3

3 + 5 = 8

3 – 1 = 2

8 – 1 = 7

C

3

2

8

8 + 2 = 10

8 – 3 = 5

10 – 3 = 7

A

0

3

0

3

0

3

B

1

5

5

5 + 5 = 10

4

10 – 1 = 9

C

3

2

3

3 + 2 = 5

0

5 – 3 = 2

A

0

3

0

6

0

6 – 0 = 6

B

1

5

1

10

1 – 1 = 0

10 – 1 = 9

C

3

2

5

8

5 – 3 = 2

8 – 3 = 5

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Lecture 22Slide27

Scheduling policy

Average waiting time till the job started

Average time in system

FCFS

7/3

17/3

SJF

4/3

14/3

RR

3/3

20/3

27

Lecture 22Slide28

Priority scheduling

Each thread/process has a priority and the one with the highest priority (smallest integer

 highest priority) is scheduled next.PreemptiveNon-preemptiveSJF is a priority scheduling where priority is the predicted next CPU burst timeProblem  Starvation – low priority threads/processes may never execute

Solution to starvation  Aging – as time progresses increase the priority of the thread/process

Priority my be computed dynamically

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Lecture 22Slide29

Priority inversion

A lower priority thread/process prevents a higher priority one from running.

T3 has the highest priority, T1 has the lowest priority; T1 and T3 share a lock.T1

acquires the lock, then it is suspended when T3 starts.Eventually T

3 requests the lock and it is suspended waiting for T1 to release the lock.

T

2

has higher priority than T

1

and runs; neither T

3 nor T1 can run; T1 due to its low priority, T3 because it needs the lock help by T1.Allow a low priority thread holding a lock to run with the higher priority of the thread which requests the lock

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Lecture 22Slide30

Multilevel queues

Ready queue is partitioned into separate queues:

1. foreground (interactive) 2. background (batch)Each queue has its own scheduling algorithmforeground – RRbackground – FCFSThe CPU must be shared among the queuesFixed 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; RR scheduling algorithm

20% to background; FCFS scheduling algorithm

Lecture 22

30Slide31

Lecture 22

31Slide32

Multilevel feedback queue

Multiple queues are defined, each one with its own scheduling strategy and time quantum.

As the process ages it moves amongst the queues.Parameters of the multilevel-feedback-queue scheduler:number of queuesscheduling algorithms for each queuemethod used to determine when to upgrade a processmethod used to determine when to demote a processmethod used to determine which queue a process will enter when that process needs service

Lecture 22

32Slide33

Three queues: Q

0

– RR with time quantum 8 millisecondsQ1 – RR time quantum 16 millisecondsQ2 – FCFSSchedulingA 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 Q

1.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.

Lecture 22

33Slide34

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 LWPKnown as process-contention scope (PCS) since scheduling competition is within the processKernel thread scheduled onto available CPU is system-contention scope (SCS) – competition among all threads in system

Lecture 22

34Slide35

Multiple-Processor Scheduling

CPU scheduling more complex when multiple CPUs are available

Homogeneous processors within a multiprocessorAsymmetric multiprocessing – only one processor accesses the system data structures, alleviating the need for data sharingSymmetric multiprocessing (SMP) – each processor is self-scheduling, all processes in common ready queue, or each has its own private queue of ready processes.NUMA – Non-Uniform Memory Access.Processor affinity – process has affinity for processor on which it is currently running

soft affinityhard affinity

Lecture 22

35Slide36

NUMA and CPU Scheduling

Lecture 22

36Slide37

Multicore Processors

Faster and consume less power

Multiple threads per core; takes advantage of memory stall to make progress on another thread while memory retrieve happens

Lecture 22

37Slide38

Solaris scheduling

The Solaris 10 kernel threads model consists of the

following objects:kernel threads  This is what is scheduled/executed on a processoruser threads  The user-level thread state within a process.

process The object that tracks the execution environment of a program.

lightweight process (lwp) 

Execution context for a user

thread. Associates

a user thread with a kernel thread

.

Fair Share Scheduler (FSS) allows more flexible process priority management. Each project is allocated a certain number of CPU shares via the project.cpu-shares resource control. Each project is allocated CPU time based on its cpu-shares value divided by the sum of the 

cpu-shares values for all active projects.

Anything with a zero 

cpu

-shares

 value will not be granted CPU time until all projects with non-zero 

cpu

-shares

 are done with the CPU.

Lecture 22

38Slide39

Scheduling classes

TS

(timeshare): default class for processes and their associated kernel threads. Priority range 0-59; dynamically adjusted (vary during the lifetime of a process to allocate processor resources evenly.IA (interactive): enhanced version of the TS class that applies to the in-focus window in the GUI. Its intent is to give extra resources to processes associated with that specific window. Like TS, IA's range is 0-59.

FSS (fair-share scheduler): Share-based rather than priority- based. Threads

scheduled based on their associated shares and the processor's utilization. FSS also has a range 0-59.FX (fixed-priority): The priorities for threads associated with this class are

fixed,

do not vary dynamically over the lifetime of the thread

. Range

0-59.

SYS (system):

Used to schedule kernel threads. Threads in this class are "bound" threads, which means that they run until they block or complete. Priorities for SYS threads are in the 60-99 range.RT (real-time): Threads in the RT class are fixed-priority, with a fixed time quantum. Their priorities range 100-159, so an RT thread will preempt a system thread.Lecture 22

39Slide40

Solaris dispatch table

Lecture 22

40Slide41

Solaris scheduling

Lecture 22

41Slide42

Linux scheduling

Two priority ranges:

time-sharing real-timeNice is used  Unix and Unix-like operating systems e.g., as Linux

Invokes a utility or shell script with a particular 

priority and gives a process more or less CPU time than other processes.

−20 is the highest priority and

20 is the lowest priority.

D

efault niceness for processes is inherited from its parent process, usually 0.Real-time range from 0 to 99 and nice value from 100 to 140.

Lecture 22

42Slide43

Evaluation of scheduling algorithms

Deterministic modeling –defines the performance of each scheduling algorithm for a particular type of workload

Simulation – using a set of trace data. Data obtained from past execution of a certain type of workload.Benchmarks –sets of programs to test hardware performance, scheduling algorithms, other types of software.

Lecture 22

43