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Priority Scheduling

Idea: Jobs are assigned priorities.Always, the job with the highest priority runs.

Note: All scheduling policies are priority scheduling!

Question: How to assign priorities?

priority 1

priority 2

priority M

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Example for Priorities

Static priorities can lead to starvation!

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Dynamic Priorities

Example: multilevel feedback

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Introduction to Systems Programming Lecture 3

Threads

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ProcessesThe Process Model

• Multiprogramming of four programs• Conceptual model of 4 independent, sequential processes• Only one program active at any instant

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Concepts of a Process

1. Group of resources– Address space– Program image– file handles, child processes, accounting info…

2. Thread of execution– Instruction address (program counter)– Registers and CPU state– Stack of called procedures

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Separating the concepts

• Think of the two concepts separately.

• A thread executes in a process

• … but allow multiple threads in one process.

• All threads share the same address space.

• Every process has at least one thread.

• Also called lightweight process.

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Processes vs. Threads• Computer• Process• Shared resources:

– Physical memory

– Disk

– Printer/keyboard/mouse

• Process• Thread• Shared resources:

– Address space

– I/O Device handles

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The Thread Model (1)

(a) Three processes each with one thread(b) One process with three threads

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Properties of Threads

• Pseudo-parallel: seem to run in parallel, but really take turns.

• No protection between threads:– Share the same address space– Supposed to cooperate

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The Thread Model (2)

• Items shared by all threads in a process

• Items private to each thread

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The Thread Model (3)

Each thread has its own stack

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Example 1: Word Processor

• Edit p.1 need to reformat all pages

• Thread 1: interact with user

• Thread 2: reformat (in background)

• Thread 3: periodic backups

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Word Processor with 3 Threads

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Properties

• Cannot be separate processes: – All threads need to access the same data (book in

memory)

• Better performance: – By the time the user wants to see page 100,

reformatting may be done

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Example 2: Web server

• Gets requests, sends web pages back

• Some pages much more popular than others

• Keep popular pages in memory cache

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A Multi-threaded Web server

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Thread Code

Dispatcher thread Worker thread

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Implementing Threads in User Space

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Properties of User-space Threads

• Thread context switch much faster (x10) without trap to OS

• Can work on OS w/o thread support

• BUT:– How to handle blocked threads? We do NOT want

OS to make whole process blocked.– Threads have to yield CPU voluntarily (no timer

interrupt)

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Implementing Threads in the Kernel

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Scheduling User-space Threads

• 50-msec process quantum• threads run 5 msec/CPU burst

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Scheduling Kernel-space Threads

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Win2000 Processes and Threads

• Win2000 implements kernel-space threads.• OS does not schedule a process – it schedules a thread

inside a process.

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Win2000 Processes and Threads

Kernel

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Win2000 Priorities

Mapping of Win32 priorities to Windows 2000 priorities

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Win2000 Scheduling

A detailed view of the Win2000 Ready-queue

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Win2000 Scheduling Scheme

• Priority scheduling (31 == highest)

• Within each priority: round robin

• Priority of user thread can be – increased (not to exceed 15) when wake up from I/O

wait:• +1 from disk, +6 from kb, +8 from soundcard

– decreased (not below base priority) when thread uses its full quantum

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Inter-Process Communication

Mutual Exclusion

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Terminology• Historically called “Inter-Process Communication” (IPC)

• Really “Inter-Thread Communication” – only the advanced solutions allow communication between separate processes.

• Basic examples assume shared variables – which are a feature of threads

• We mix the use of thread & process in this chapter

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Difficulties with Threads

• Share resources can conflict

• Counter = 0; // global variable• Thread 1 does Counter++ • Thread 2 does Counter–- // “at the same time”• What is the order of values of Counter ?

– 0 : 1 : 0?– 0 : -1 : 0?

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Print Spooler Example

• n slots in a queue (FIFO)

• Two shared (global) control variables:– In : position of first free slot (where next new file

will be inserted)– Out : position of first used slot (next to be printed)

• Threads A & B want to print at “same time”

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Print Spooler State

thread

thread

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Race Conditions

1. NextA = read(In);// NextA == 7

2. Spool(“fA”, NextA);3. NextA++;

//NextA == 84. write(NextA,In);

1. NextB = read(In);// NextB == 7

2. Spool(“fB”, NextB);3. NextB++;

//NextB == 84. write(NextB,In);

Thread A Thread B

Interrupt - context switch

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What went wrong?

• We have a shared variable: In• Context switch in the middle of a “read-update-

write” sequence.

• Result: both wrote their file to slot 7.

• Because of scheduling order – B’s update is lost!!

• This is a Race Condition.

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Mutual Exclusion

• Need to make sure that shared variables are not read and written “at the same time”.

• Mutual exclusion: if one thread is accessing a shared variable, other threads are excluded.

• Not always a shared variable. In general – a critical region.

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Conditions for Mutual Exclusion

1. [Safety] No two threads simultaneously in critical region.

2. [Criticality] No thread running outside its critical region may block another thread.

3. [Liveness] No thread must wait forever to enter its critical region.

No assumptions made about speed of CPU or scheduling.

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Desirable Execution

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Solution 0: Disable Interrupts

• No interrupt no context switch at bad time

But

• User bugs will freeze system

• Only useful for very short intervals

• OS uses this method internally to protect internal data structures.

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int lock; // shared, initially 0

// lock == 1 means “someone in critical region”

// Both threads run this code

while(lock != 0)

/* do nothing */;

lock = 1;

critical_region();

lock = 0;

Software Solution 1: Lock ?

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Does not work!

• If both threads reach while(lock != 0) at (about) the same time, both will find a value 0 both enter critical region.

• Violates the Safety property

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Mutual Exclusion Terminology

• A tight loop like while(lock != 0) is called busy-waiting.

• Busy waiting should usually be avoided wastes CPU.

• A lock using busy waiting is a spin lock.

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Software Solution 2 : Alternation

Thread 0 Thread 1

Busy waiting on spin-lock variable “turn”.Each process sets turn to the other process.

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Properties of Alternation

• Each process lets the other process into critical region.

• Satisfies “Safety”.

• A process cannot go into critical region twice in a row (even if other process is outside critical region).

• Violates Criticality property.

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Concepts for review• Thread• User-space threads• Kernel-space threads• Thread scheduling• Inter-Process-Communication• Mutual Exclusion• Race Condition• Critical Region / Critical Section• Busy-Wait / Spin-lock