B3CC: Concurrency 01: Introduction Trevor L. McDonell Utrecht - - PowerPoint PPT Presentation

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B3CC: Concurrency 01: Introduction Trevor L. McDonell Utrecht - - PowerPoint PPT Presentation

May 08, 2023 2.69k likes •3.26k views

B3CC: Concurrency 01: Introduction Trevor L. McDonell Utrecht University, B2 2020-2021 Today 1. Introduction 2. What is concurrency? 2 Course structure 3 henlo Trevor L. McDonell - BBG 5.68 MS Teams - t.l.mcdonell@uu.nl mention

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B3CC: Concurrency 01: Introduction

Trevor L. McDonell Utrecht University, B2 2020-2021

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Today

  • 1. Introduction
  • 2. What is concurrency?

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Course structure

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henlo

  • Trevor L. McDonell
  • BBG 5.68 MS Teams
  • t.l.mcdonell@uu.nl


mention [INFOB3CC] in the subject

  • Ivo Gabe de Wolff
  • BBG 5.70 MS Teams
  • i.g.dewolff@uu.nl

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ig: jasper.samoyed

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SLIDE 5

Teaching assistants

  • Bart Wijgers
  • Oscar Fickel
  • Erwin Glazenburg
  • Kevin Quach
  • Karel Kubat
  • Hugo Peters
  • Frans Haak

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SLIDE 6
  • Online teaching
  • Lectures (online, recorded)
  • Werkcollege (online & in person)
  • Online exam
  • Send us feedback!
  • Anonymous survey(s) on blackboard

Human malware

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https://imgur.com/gallery/ooZcL1b

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SLIDE 7

Coffeebot

  • Add your name to the coffee bot for some social

interaction!

  • You’ll get an email once per week with the details
  • f somebody else to have a virtual coffee with

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ig: sarahandersencomics

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SLIDE 8

Topics

  • Program a multithreaded application
  • Managing threads
  • Synchronise with locks, etc.
  • Software transactional memory
  • Parallelism
  • Work & span
  • Concurrent algorithms / data structures

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Goals

  • By the end of the course you should be able to:
  • Design and implement a multithreaded application
  • Understand the difference between concurrency and parallelism
  • Reason about the properties/complexity of parallel algorithms

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SLIDE 10

Haskell

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https://xkcd.com/1312

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SLIDE 11

Homepage

  • http://www.cs.uu.nl/docs/vakken/b3cc/
  • Feel free to let me know if there are broken links, missing slides, etc.

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SLIDE 12

Resources

  • Parallel and Concurrent Programming in Haskell


https://simonmar.github.io/pages/pcph.html

  • Many more on the website


http://www.cs.uu.nl/docs/vakken/b3cc/ resources.html

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SLIDE 13

Sessions

  • Lectures:
  • Mon: 15:15 - 17:00 (online)
  • Thu: 11:00 - 12:45 (online)
  • Werkcollege:
  • Tue: 15:15 - 17:00 (in person & online)
  • Thu 09:00 - 10:45 (online)
  • Participation in lectures is expected (please ask questions!)

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Course components

  • Exam (50%)
  • Mid session exam: 17-12-2020 (50%) (Online)
  • Final exam: 04-02-2021 (50%) (Online)
  • Practicals (50%) (exact dates may change!)
  • Assignment 1: 28-11-2020 (individual) (20%)
  • Assignment 2: 19-12-2020 (in pairs) (40%)
  • Assignment 3: 06-02-2021 (in pairs) (40%)

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If you wait until the last minute, it only takes a minute to do. — Key Skills for Professionals, 2013, pp65

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Software installation

  • A recent version of GHC
  • I recommend using stack
  • https://docs.haskellstack.org/en/stable/README/
  • The starting frameworks will use stack

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Concurrency control

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What is concurrency?

  • Consider multiple tasks being executed by the computer…
  • Tasks are concurrent with respect to each other if:
  • They may be executed out-of-order
  • Implies they can be executed at the same time, but this is not required
  • Concurrency: deal with lots of things at once

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What is parallelism?

  • Consider multiple tasks being executed by the computer…
  • Tasks are parallel if they are executed simultaneously:
  • Requires multiple processing elements
  • The primary motivation for parallel programming is to reduce the overall

running time (wall clock) of the program: parallel execution

  • Parallelism: do lots of things at once

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Question

  • What does it mean for an application to be concurrent but not parallel?
  • Give an example
  • What does it mean for an application to be parallel but not concurrent?
  • Give an example

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Concurrency vs. Parallelism

  • Concurrency: composition of independently executing processes
  • Parallelism: simultaneous execution of (possibly related) computations

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Concurrency

  • Programming with multiple threads of control
  • A tool for structuring programs with multiple interactions
  • Examples: GUI, web server, different tasks in a game engine loop, …
  • There is no single right answer. In this course we will discuss several

approaches: it is up to you to pick which is right for your application

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Concurrency control

  • Concurrency appears in many contexts:
  • Multithreading: concurrent threads share an address space
  • Multiprogramming: concurrent processes execute on a uniprocessor
  • Multiprocessing: concurrent processes on a multiprocessor
  • Distributed processing: concurrent processes executing on multiple nodes

connected by a network

  • Concurrency is also used in different forms:
  • Multiple applications (multiprogramming)
  • Structured applications (applications is a set of threads/processes)
  • Operating system structure (OS is a set of threads/processes)

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Concurrency control

  • Concurrent processes (threads) need special support
  • Communication among processes
  • Allocation of processor time
  • Sharing of resources
  • Synchronisation of multiple processes
  • Concurrency can be dangerous to the unwary programmer:
  • Sharing global resources (order of read & write operations)
  • Management of allocation of resources (danger of deadlock)
  • Programming errors are difficult to locate (Heisenbugs)

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Example: access to a global queue

  • Inserting:

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head last

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Example: access to a global queue

  • Inserting:
  • Create new object

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head last

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Example: access to a global queue

  • Inserting:
  • Create new object
  • Set last->.next to &new

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head last

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Example: access to a global queue

  • Inserting:
  • Create new object
  • Set last->.next to &new
  • Set last to &new

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Thread B:

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Thread B:

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Set last->.next to &new
  • Thread B:

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Set last->.next to &new
  • Thread B:
  • Create new object

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Set last->.next to &new
  • Thread B:
  • Create new object
  • Set last->.next to &new

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Set last->.next to &new
  • Thread B:
  • Create new object
  • Set last->.next to &new
  • Set last to &new

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head last

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Example: concurrent access to a global queue

  • Thread A:
  • Create new object
  • Set last->.next to &new
  • Set last to &new
  • Thread B:
  • Create new object
  • Set last->.next to &new
  • Set last to &new

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head last

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Example: concurrent access to a global queue

  • Lessons learned
  • We have to control access to shared resources (such as shared variables)
  • We can do this by controlling access to the code utilising those shared

resources: critical sections

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head last

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Example: concurrent access to a global queue

  • Only one thread at a time should have access to the queue:
  • Thread A creates a new object, sets last->.next pointer
  • Thread A is suspended
  • Thread B is scheduled: since Thread A is currently in insert, has to wait
  • Thread A is resumed, the data structure is in the same state as it was when

it was suspended

  • Thread A completes operation
  • Thread B is allowed to execute insert

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Concurrency control

  • Processes can
  • Compete for resources
  • Processes may not be aware of each other
  • Execute must not be affected by each other
  • OS is responsible for controlling access
  • Cooperate by sharing a common resource
  • Programmer responsible for controlling access
  • Hardware / OS / programming language may provide support
  • Threads of a process usually do not compete, but cooperate

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Concurrency control

  • We face three control problems:
  • Mutual exclusion: critical resources =>> critical sections
  • Only one process at a time is allowed in a critical section
  • e.g. only one process at a time is allowed to send commands to the GPU
  • Deadlock: e.g. two processes and two resources
  • Starvation: e.g. three processes compete for a single resource

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Requirements for mutual exclusion

  • Implementation:
  • Only one thread at a time is allowed in the critical section for a resource
  • No deadlock or starvation
  • A thread must not be delayed access to a critical section when there is no
  • ther thread using it
  • A thread that halts in its non-critical section must do so without interfering

with other threads

  • No assumptions made about relative thread speed or number of processes
  • Usage
  • A thread remains inside its critical section for a finite time only
  • No potentially blocking operations should be executed inside the critical

section

  • No deadlock or starvation

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Mutual exclusion

  • Three ways to satisfy the implementation requirements:
  • Software approach: put responsibility on the processes themselves
  • Systems approach: provide support within the OS or programming language
  • Hardware approach: special-purpose machine instructions

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Therac-25

  • Computer controlled medical radiation device (1982)
  • Two operating modes: a low-current electron beam; or high-energy x-rays
  • Involved in at least six incidents, resulting in serious injury or death
  • A race condition could cause the high-power electron beam to be

administered directly to the patient

  • Resulted in radiation doses 100x higher than normal
  • Additional problems related to poor software development practices

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https://en.wikipedia.org/wiki/Therac-25

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Northeast blackout (2003)

  • Widespread power outage throughout USA and Canada
  • Second most widespread blackout in history (at the time)
  • Affected an estimated 10M people in Ontario and 45M in 8 US states
  • A race condition prevented an alarm from going off
  • Operators were unaware of the need to redistribute power—a minor

problem—which cascaded into complete collapse of the electrical grid

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https://en.wikipedia.org/wiki/Northeast_blackout_of_2003

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Mars Pathfinder

  • Launched by NASA in 1996
  • Sojourner became the first rover to operate outside the Earth-Moon system
  • Control computer contained a priority inversion bug
  • Triggered under certain high loads causing a system reset
  • Successfully patched remotely

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https://en.wikipedia.org/wiki/Mars_Pathfinder

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Next time…

  • Haskell refresh
  • Make sure you have access to a modern Haskell installation

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Photo by Jf Brou

tot ziens

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Extra slides

  • Jonathan Blow (Braid, The Witness)


Techniques for dealing with lack of motivation, malaise, depression

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