CO2101 Concurrency and Threads - Tom Ridge · 2019-10-14 · Threads cansharea process’s resources, including memory and open ﬁles. Many languages (including Java, and Python

CO2101 — Concurrency and Threads

Tom Ridge (tr61)

14th October 2019

tr61 Concurrency 14th October 2019 1 / 31

Recap

Recap

The structure of Operating Systems

Concept of process: Code + Process Control Block

Scheduling algorithms

Reflections on how user interface interacts with OS


Processes and Threads

Heavyweight processes

A process is an operating system abstraction to represent what isneeded to run a program.

It consists of

Sequential Program Execution State (includes state of CPUregisters); plusProtected resources: memory state, I/O state

(If we ignore threads...) NB Strictly sequential — i.e. noconcurrency.





It consists of







It consists of




Processes and Threads PCB, context switching

PCB, address space, context switch

The current state of the process is held in the PCB (ProcessControl Block)

To multiplex several processes we need to allocate CPU time toprocesses using suitable scheduling policies.

preemptive: SRT, RRnon-preemptive: FCFS, SJF

Controlled access to non-CPU resources, e.g. memory, I/O.























Processes and Threads Threads

Threads

A thread is an independent sequence of execution within a process



Concept of Thread (Cont’d)

A thread is similar to a process

Both have a single sequential flow of control with a start and endAt any time a thread has a single point of executionA thread has its own execution stack & program counterSometimes a thread is called a lightweight process

However, a thread differs from a process

A thread cannot exist on its own. It exists within a processUsually created and/or controlled by a processThreads can share a process’s resources, including memory and openfiles.

Many languages (including Java, and Python to some extent) supportthread programming directly.


























Review of sequential programming

What is Sequential Programming?

Traditional activity of constructing a program containing one processusing a (sequential) computer language

The program is supposed to execute on a single processor architecture


Review of sequential programming Single processor architecture

Single Processor Architecture

A CPU is linked to RAM and I/O devices by buses

Both program instructions and data are stored in RAM

The CPU repeatedly executes the cycle of

Fetching, decoding and executing the next instructionReferenced by the current value of program counter (PC)

Can execute just one instruction at any time



























Program

Program Counter

Runtime DataProcessor

(CPU)

I/O

Devices

(RAM)Random Access Memory

Bus


Review of sequential programming Execution of a sequential program

Executing a Sequential Program

The execution sequence is the sequence of values of PC

Deterministic: only one possible sequence of execution

A sequential program gives the system strict instructions on the orderof executing the statements in the program.

For example, the program below

P; Q; R;

tells the system to execute the statements in the order

P => Q => R

i.e, P must precede Q and Q must precede R








P; Q; R;


P => Q => R









P; Q; R;


P => Q => R









P; Q; R;


P => Q => R



Review of sequential programming Sequential example

A Simple Example

A simple program consists of three statements

Java Statements

[ P ] x = 1;

[ Q ] y = x+1;

[ R ] x = y+2;

The final values of x and y depend only on the order of executing thestatements


Review of sequential programming Sequential example

A Simple Example

A simple program consists of three statements

Java Statements

[ P ] x = 1;

[ Q ] y = x+1;

[ R ] x = y+2;

The final values of x and y depend only on the order of executing thestatements


Review of sequential programming The system level view

System Level Execution

Each statement may be compiled into several machine instructions

1 P is treated as a single machine instruction

p: (x = 1) store 1 at the address of x

2 Q (y = x+1 )is broken into 3 machine instructions as follows:

q1: load the value of x into a CPU registerq2: increment the value in this register by 1q3: store the value in this register at the address of y

3 Similarly, R (x = y+2) is broken into 3 machine instructions

r1: load the value of y into a CPU registerr2: increment the value in this register by 2r3: store the result at the address of x






















Total ordering

Total Ordering

What is meant by P must precede Q?

Q can only begin after P finishes

The execution sequence at the program level

P; Q; R;

implies the execution sequence at the system level

p, q1, q2, q3, r1, r2, r3

So the final result is x = 4; y = 2;

Single threaded computation, no overlap in the execution of thestatements — Total Ordering


Total ordering

Total Ordering




P; Q; R;


p, q1, q2, q3, r1, r2, r3




Total ordering

Total Ordering




P; Q; R;


p, q1, q2, q3, r1, r2, r3




Total ordering

Total Ordering




P; Q; R;


p, q1, q2, q3, r1, r2, r3




Total ordering

Nature of Sequential Programming

Total ordering

Deterministic: same input results in same output

Sequential programming⇔ Finding a strict sequence of steps toachieve the desired end

This does not apply for many practical problems


Total ordering


Total ordering





Total ordering


Total ordering





Total ordering


Total ordering





Total ordering


Total ordering





Non-sequential problem solving Calculating factors of a large number

Chinese General Problem

A Chinese princess wanted to marry one of her father’s generals. Eachof them had 1,000,000 soldiers. A general who could win her heartmust be young, handsome, and intelligent enough to:

Find all non-trivial factors of 368,788,194,383 in one month

How would you solve this problem if you were a general with 1,000,000soldiers under your command?



Clever General Bingxing’s Solution

1 Calculated 607280 = d√

368788194383 e

2 Gave each number in [2, 607280] to one of his 607,279 soldiers to checkwhether it is a factor

3 20 minutes later those soldiers who got 7, 17, 23, 119, 161, 257, 391, 1799,2737, 4369, 5911, 30583, 41377, 100487, and 524287 reported that theirnumber were factors

4 Find the remaining factors: 52684027769, 21693423199, 16034269321,3099060457, 2290609903, 1434973519, 943192313, 204996217, 134741759,84410207, 62390153, 12058601, 8912879, 3670009, and 703409

5 General Bingxing won the princess’s hand with these factors in 1 hour

NB: 368788194383 = 7× 17× 23× 257× 524287





368788194383 e2 Gave each number in [2, 607280] to one of his 607,279 soldiers to check

whether it is a factor




NB: 368788194383 = 7× 17× 23× 257× 524287










NB: 368788194383 = 7× 17× 23× 257× 524287










NB: 368788194383 = 7× 17× 23× 257× 524287


Non-sequential problem solving Partial ordering of parallel solution

Parallel Computation and Partial Ordering

The operations carried out by Bingxing’s 607,279 soldiers were NOT ina total order.

They were carried out in parallel

Each individual soldier did his operations in sequence

The operations over the whole computation can be viewed as a partialorder

Many real world problems can be approached in this way


































Concurrent programming

What is Concurrent Programming

Constructing a program containing multiple processes/threads thatexecute in parallel

These processes may run on

A multi-processor systemA single processor system

Needs language support, e.g, Java Threads; POSIX thread support, etc.























Concurrent programming Benefits of concurrency

Why Concurrent Programming?

Improve efficiency in program execution using multi-CPU hardware(Chinese General Problem)

Improve CPU utilisation via multi-tasking on a uni-CPU system(operating systems)

Some applications are inherently non-deterministic and concurrent,e.g, embedded traffic lights controller

The order of program operations is determined by external events, e.g, asensor is triggered by a coming vehicleImpossible to predict the order of these events, e.g, a car from the northcomes first, and then one from the east, and so on























Concurrent programming Representing concurrency in code

How to Represent Concurrent Program

Use COBEGIN/COEND to bracket the processes (this is pseudocode!)

G1; ...; Gk; [ calculate p=607279 ]COBEGINS1; [ is 2 a factor ? ]S2; [ is 3 a factor ? ]...;S(p-1); [ is 607279 a factor ? ]

COEND;R1; ...; Rl; [ report to the princess ]

The program ends only if all processes in COBEGIN/COEND terminate

The statements in COBEGIN/COEND may overlap in execution (butmight not!)


















Concurrent programming Concurrency with multi-processors

Multi-Processor System

A computer with multi-CPUs/cores is a Parallel Computer System

Parallel computation can be implemented on a parallel computersystem

If each task is computed by its own CPU, the computation is calledMaximum Parallel Computation

E.g, if a system has 607279 CPUs, each soldier’s task can be assigned toits own CPU

Maximum parallelism may not be always possible


























Concurrent programming Concurrency with uni-processors

Uni-Processor Multi-Tasking System

A uni-CPU system can support multi-tasking/multi-thread

We can treat each soldier as a process or thread

Each process/thread has its own process counter

The program counter (PC) forks to produce many process/threadcounters, which later re-join

In each CPU cycle, a process is non-deterministically chosen and itsnext command is loaded and executed

There may be many different possible paths

This CPU sharing technique is interleaving.




















































Concurrent programming Interleaving

Example of Interleaving

A concurrent program has two processes p1 and p2:

p1 has three statements A, B, and C; p2 has two S and T

PROCESS p1;BEGIN A; B; C END;

PROCESS p2;BEGIN S; T END;

BEGINCOBEGINp1; p2;

COENDEND



Possible interleavings

1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C




1 A ; B ; C ; S ; T

2 A ; B ; S ; C ; T

3 A ; S ; B ; C ; T

4 S ; A ; B ; C ; T

5 A ; B ; S ; T ; C

6 A ; S ; B ; T ; C

7 A ; S ; T ; B ; C

8 S ; T ; A ; B ; C

9 S ; A ; B ; T ; C

10 S ; A ; T ; B ; C



Back to first example




2 Q (y = x+1) is broken into 3 machine instructions as follows:







(Assume that variables are automatically initialized to zero.)

Already seen that p; q1 ; q2 ; q3 ; r1 ; r2 ; r3results in x=4 ; y=2

What about p ; q1 ; r1 ; q2 ; r2 ; q3 ; r3 ?

Result is x=2; y=2

And q1 ; r1 ; q2 ; r2 ; q3 ; r3 ; p

results in x=1 ; y=1

etc.

The results depend on the order of interleaving.






What about p ; q1 ; r1 ; q2 ; r2 ; q3 ; r3 ?

Result is x=2; y=2

And q1 ; r1 ; q2 ; r2 ; q3 ; r3 ; p


etc.







What about p ; q1 ; r1 ; q2 ; r2 ; q3 ; r3 ?Result is x=2; y=2

And q1 ; r1 ; q2 ; r2 ; q3 ; r3 ; p


etc.








And q1 ; r1 ; q2 ; r2 ; q3 ; r3 ; p


etc.








And q1 ; r1 ; q2 ; r2 ; q3 ; r3 ; presults in x=1 ; y=1

etc.









etc.









etc.




Complexity of Interleaving

Given a concurrent program with processes p and q

PROCESS p;BEGIN S1; ...; Sm END;

PROCESS q;BEGIN T1; ...; Tn END;

BEGINCOBEGIN p; q COEND

END.

The number f(m,n) of interleavings is

the binomial coefficient

Cn+mm = Cn+m

n =(n+m)!

n!m!

The number f(m,n) grows very fast with m and n








END.



Cn+mm = Cn+m

n =(n+m)!

n!m!









END.



Cn+mm = Cn+m

n =(n+m)!

n!m!




n : 2 4 6 8 10 · · ·f(n,n) : 6 70 924 12870 184756 · · ·


Concurrent programming Concurrency critical issues

Issues in Concurrent Programming

The concurrent processes must interact with each other in orderto share resources or exchange data

Synchronisation: when, how, and with what languageabstractions can we synchronise computation to eliminateunacceptable interleavings, and thus unacceptable outputs?

Distribution: how can we distribute processes among a numberof processors, and how can a process on one processor interactwith another process on a different processor.


























Summary

Summary

Processes, threads, context switching

Sequential programming, compared with



Summary

Summary





Summary

Summary





Summary

Next time

Processes sharing resources

Critical regions, mutual exclusion

Algorithms for solving critical region issues


CO2101 Concurrency and Threads - Tom Ridge · 2019-10-14 · Threads cansharea process’s resources, including memory and open ﬁles. Many languages (including Java, and Python

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