introduction to cpu scheduling - real-time computing and...

Real-Time Computing and Communications Lab. Hanyang University

어코드 사업단 “리눅스 CFS와 로드밸런서” 교육 January, 2015

Introduction to CPU Scheduling

Hanyang University

Hyunmin Yoon

2

2



Process 1 3 Page

Scheduling 2 11 Page

Q & A 3 25 Page

3

3



This document is based-on linux 3.17.4 version (released at 2014.11.21)

1st revision by Minsoo Ryu, 2014.06. 2nd revision by Hyunmin Yoon, 2014.12.



Process

5

5



Process and Thread

Process Internal programmed activities, such as memory management Program ≠ Process

• Program is a passive entity ex) A file containing a list of instructions stored on disk

• Process is an active entity ex) Program counter, a set of associated resources

Program becomes a process when an executable file is loaded into memory

Thread

A thread is an instruction stream of execution Some modern operating systems support multi-thread for

one process • However, a thread is just a special kind of process in Linux

6

6



Process and Thread in Linux

User’s View

Kernel’s View

Process Thread Thread Process

State

Process State Thread Thread

Process

Process

7

7



Process States

Three key states running: Instructions are being executed. waiting: The process is waiting for some event to occur

(I/O completion, signal reception). ready: The process is waiting to be assigned to a process.

(Wiley, Operating System Concepts 8th Edition)

8

8



State Transitions

(Wiley, Operating System Concepts 8th Edition)

9

9



Linux Process States

(Robert Love, Linux Kernel Development 3rd Edition)

10

10



Linux Process State Flags

Flags Description

TASK_RUNNING The process is runnable; it is either currently running or on a runqueue waiting to run.

TASK_INTERRUPTIBLE

The process is sleeping, waiting for some condition to exist. When this condition exists, the kernel sets the process’s state to TASK_RUNNING. The process also awakes prematurely and becomes runnable if it receives a signal.

TASK_UNINTERRUPTIBLE

This state is identical to TASK_INTERRUPTIBLE except that it does not wake up and become runnable if it receives a signal.

11

11



Linux Process Descriptor

It is an element of the task list It has type struct task_struct

struct task_struct is defined in <linux/shched.h>

It contains all the information that the kernel has and needs about a process

(Robert Love, Linux Kernel Development 3rd Edition)



Scheduling

13

13



Scheduling Criteria

Performance objectives Maximize CPU utilization or throughput Minimize completion time, waiting time, or response time

Real-time constraints Satisfy deadlines

Proportional share constraints Provide CPU cycles proportional to weights

14

14



Classification of Scheduling Policies

Scheduling mechanisms Priority-based scheduling Proportional share scheduling

Target processors Uniprocessor scheduling Multiprocessor scheduling

15

15



Priority-Based 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

Task A (priority 1)

Task B (priority 2) Task C (priority 3) Task D (priority 4) time

16

16



Priority-Based Scheduling Policies

Non-real-time policies FCFS (First-Come, First-Served) SJF (Shortest-Job-First) SRTF (Shortest-Remaining-Time-First)

Real-time policies

RM (Rate Monotonic) EDF (Earliest Deadline First)

17

17



FCFS (First-Come, First-Served)

Process Execution Time P1 24 P2 3 P3 3

Suppose that 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

18

18



Proportional Share Scheduling

Basic concept A weight value is associated with each process The CPU is allocated to the process in proportion to its

weight

Two contexts Fair queueing (in the context of communication networks)

• Packet scheduling Proportional share (in the context of operating systems)

• Process scheduling

time

Task A (weight 25.0%)

Task B (weight 12.5%) Task C (weight 50.0%) Task D (weight 12.5%)

19

19



Proportional Share Scheduling Algorithms

Network scheduling PGPS (= WFQ), Demers et al., 1989 Virtual Clock, Lixia Zhang, 1990 SCFQ, Golestani, 1994 SFQ, Goyal et al., 1996 WF2Q, Bennett et al., 1996

CPU scheduling

Lottery and Stride, Waldspurger, 1995 Hierarchical SFQ, Goyal et al., 1996 BVT, Duda et al., 1999 VTRR, Nieh et al., 2001

20

20



Uniprocessor vs. Multiprocessor Scheduling

Uniprocessor scheduling It is to decide when and which job will run

Multiprocessor scheduling It is to decide not only when but also where a job will run Almost the same goals as those of uniprocessor

scheduling But it raises new issues

• How to assign applications to multiple processors? • How to balance workload among processors? • How to define and exploit affinity? • How to manage processor heterogeneity?

21

21



Multiprocessor Scheduling Policies

The same mechanisms as uniprocessor policies Priority-based scheduling: FCFS, SJF, SRTF, RM, EDF Proportional share scheduling: PGPS, SFQ, WF2Q, Lottery

and Stride, BVT, VTRR

Two approaches Global scheduling

• The system has a single global process queue • Processes are dispatched to any available processors

Partitioned scheduling • Each processor has a separate process queue • Each queue is scheduled by an independent scheduler • Process migration may be allowed or not

22

22



Global vs. Partitioned Scheduling

Global Scheduling Partitioned Scheduling

23

23



Global vs. Partitioned Scheduling

Global scheduling It is generally believed that global scheduling can achieve

better performance However, it can be inefficient due to the contention at the

single queue and increased cache misses

Partitioned scheduling Performance can vary depending on the initial distribution

of processes, i.e., a bin-packing problem Different scheduling policies can be employed across

processors We can use the rich and extensive results from the

uniprocessor scheduling theory

24

24



Global WFQ

Consider the following tasks Process A : weight = 1 Process B : weight = 2 Process C : weight = 2 Process D : weight = 4

CPU #1

CPU #2

25

25



Partitioned WFQ with Load Balancing

Consider the following tasks Process A : weight = 1 Process B : weight = 2 Process C : weight = 2 Process D : weight = 4

CPU #1 (A, D)

CPU #2 (B, C)

migrate migrate migrate

26

26



introduction to cpu scheduling - real-time computing and...

Documents