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Computer System > Week 7 - Concurrency > Flashcards

Week 7 - Concurrency Flashcards

1
Q

What does the view of an OS imply?

A

It implies that

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2
Q

What are the differences between historic computer and computer this year?

A

Historical, computer were seen expensive, due to maximise utilisation

Now: Users expect concurrent execution of multiple tasks
Stream musics, edit file, update software etc.

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3
Q

What challenges arise when processes do not work independently in a system?

A

Competition for resources - processes may compete for access to shared resources like devices, files, or data, whether local or remote.

Cooperation: Processes can cooperate using mechanisms such as messages, shared memory or files

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4
Q

Why do distributed applications face unique challenges?

A
  1. They support millions of users simultaneously, who may be reading or updating information.
  2. Examples,e include special networking sites (SNS), bank accounts, and online shopping platforms
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5
Q

Why is it important to interleave multiple independent task in applications?

A
  1. Avoid freezing - Apps with GUIs should remain responsive during time-consuming tasks.
  2. Improve user experience - For system like banking databases,users should not wait in long queues for transactions.
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6
Q

What are the key considerations for programmers and resources efficiency when building applications?

A
  • Programmer Efficiency: Developers should not need to create an OS for every app they build
  • Resource Efficiency: Applications should maximise the use of computing resources to ensure optimal performance.
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7
Q

How can we managed concurrency and ensure correctness?

A
  1. Concurrency Management:
    Ensure multiple tasks can progress simultaneously.
    Avoid conflicts when accessing shared resources.
  2. Correctness:
    Prevent issues like race conditions, deadlocks, and starvation.
    Maintain program logic and data integrity.
  3. Key Questions:
    How can we do this conveniently?
    How can we do this efficiently?
    How can we maintain responsiveness?
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8
Q

How can we provide programming mechanisms to allow concurrency?

A
  1. Threads: Lightweight processes enabling parallel execution within a program
  2. Programming Mechanisms:
    - Thread management libraries (e.g Java’s, Thread class
    - Synchronisation tools (e.g. mutexes, semaphores)
    - High-level abstraction (e.g thread pools,asynchrony/await)
  3. Goals: Efficient task execution and it is to improve responsiveness and resource utilisation
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9
Q

What is the primary concern of OS design regarding processes?

A

OS design focuses on:

  1. Management of Processes: Efficiently handling multiple processes to maximise CPU utilisation.
  2. Management of Threads: Enabling lightweight, concurrent execution within a process
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10
Q

What is the 3 main types of processing in OS design?

A
  1. Multi-programming (multitasking)
  2. Multiprocessing
  3. Distributed Processing
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11
Q

Explain the function(s) of the 3 main types of processing OS design

A
  1. Multiprogramming: Execute multiple tasks by sharing CPU time. Also, ensure efficient resources use and the system responsiveness
  2. Multiprocessing:
    - Involves multiple processors working in parallel
    - Enhances performance and fault torelance
  3. Distributed processing:
    - Distribute tasks across different systems
    - Enables scalability, fault tolerance, and resource sharing
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12
Q

What are the 3 different contexts in concurrency?

A
  1. Multiple applications: this was invented to allow processing time to be shared among active applications
  2. Structured applications: extension of modular design and structured programming
  3. Operating system structure: OS themselves implemented as a set of processes or threads.
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13
Q

Define some of the concurrency key terms

A
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14
Q

What are the 2 main principles of concurrency?

A
  1. Interleaving and Overlapping
  2. Uni processor -
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15
Q

What were the difficulties of concurrency?

A
  1. Sharing of global resources
  2. Difficult for the OS to manage the allocation of resources optimally
  3. Coordination of ‘ parallel’ and asynchronous ‘ processes’
    • Ensuring correct behaviour
    • Debugging of programming errors: behaviour can be non-deterministic, making it hard to identify and reproduce
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16
Q

What design and management issues does concurrency raise for an operating system?

A

The OS concerns are such as:

  • Tracking processes: Keep track of all active and suspended processes
  • Resoruces allocation and de allocation: Manage resources such as CPU, I/O etc efficiently for each process.
  • Data and resource protection: Ensure processes cannot interfere with each other’s data or resources
  • Output independence: Ensure the process outcomes remain consistent, regardless of execution speed or order.
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17
Q

What are responsibilities of the OS in managing concurrent processes?

A

The OS must:

  1. Process Management: Maintain control over process states and transitions
  2. Resource Management: Allocate CPU,memory, I/O efficiently
  3. Protection mechanisms: Adding isolation to prevent data corruption and resource misuse
  4. Consistency and independence: Ensure that process execution remains independent of variations in speed or scheduling
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18
Q

Process interaction

A
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19
Q

What causes conflict among concurrent processes, and what are some examples of resources they compete for?

A

There’s conflict when multiple processes compete for the same resource.

E.g: I/O devices: Printer, disk devices

Memory: Shaed or limited memory

Processor time: CPU cycles for excution

Clock: Timers and counters

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20
Q

What are the 3 key control problems faced in resource completion, and what do they ensure?

A
  1. Mutual exclusion: Ensures that only one process can access a critical section or resource at a time.
  2. Deadlocks: It happens when processes wait indefinitely for resources held by each other.
  3. Starvation: A process waits indefinitely due to resource allocation favouring others.
21
Q

Example of synchronisation process:

A
22
Q

What is a race condition?

A

A race condition occurs when:

  1. Multiple processes or threads read and write shared data terms concurrently.
  2. The final result depends on the order of execution of theses processes
23
Q

What are the key characteristics of the ‘Race condition’ in concurrent programming?

A
  • Processes “race” to complete their operations
  • The process that performs the last update determines the final value of the shared data
  • Can lead to unpredictable and incorrect results.
24
Q

What are the risk of a race condition?

A
  1. Unpredictable behaviour: Results may vary depending on the execution order
  2. Data inconsistency: Shared data may end up in an invalid state
  3. Program failures: Critical operations may produce incorrect outcomes, leading to bugs or crashes
25
Q

How to prevent the risk of race condition?

A

Use synchronisation mechanisms like locks,semaphores, or atomic operations to prevent race conditions

26
Q

Why do Race Conditions occur?

A
  1. Order execution:
  2. Scheduling:
  3. Outdated information:
27
Q

Critical section

A
28
Q

What are the two protocols in the critical section?

A
  1. Entry Protocol:
    • Process requests permission for entering a critical section: may have to wait/suspended until entry is granted
    • Process has to communicate that it entered critical section
  2. Exit Protocol: Process communicates to other processes that it has left the critical section
29
Q

What happens when the process is pre-emptied or interrupted in the critical sections?

A

The process can finish the execution of its critical section, even if it is pre-emptied or interruption.

30
Q

What problems can arise when enforcing mutual exclusion?

A

There are two major problems:

  1. Deadlocks:
    - Occurs when 2 or more processes wait indefinitely for resources held by each other.
    - None of the processes can proceed, resulting in a standstill.
  2. Starvation:
    - it happens when a process waits indefinitely to enter its critical section and if the the resource allocation consistently favours other processes
31
Q

What are the 3 requirements to solve the critical section problem?

A
  1. Serialisation of Access
  2. Bounded Waiting (No Starvation)
  3. Progress (likeness,No deadlocks)
32
Q

What is serialisation of access in the critical section problem?

A

Serialisation of access ensures:

  • Only one process can be enter the critical section at any given time
  • Prevents conflicts by avoiding simultaneous access to shared resources.
  • This ensures mutual exclusion maintain data consistency and system stability.
33
Q

What is bounded waiting in the critical section problem?

A

Bounded waiting ensures:

  1. Fairness: A process waiting to enter its critical section will be granted access within a defined waiting time period
  2. No starvation: So every process gets a turn to execute its critical section and prevent indefinitely delays
34
Q

What is progress (likeness, No deadlock) in critical section?

A

Progress ensures:

  • Non-interference: A process halted in its non-critical section does not block others from entering their critical section
  • Finite execution: The progress remains inside its critical section for a finite time only
  • Fair selection: Only processes actively waiting to enter their critical section participate in deciding which process enters next
35
Q

What are the 3 solutions to the critical section problem?

A
  1. Software solutions: Shared lock variables and busy waiting (polling/spinning/strict alternation)
  2. Hardware Solutions: Disabling interrupts and special H/W instructions (like compare&swap)
  3. Higher Level OS Constructs: Semaphores, Monitors, Message Passing
36
Q

What is a lock variable, and how is it used in concurrent programming?

A

A lock variable is shared data item used to protect critical sections. It is used to ensure only one process can enter the critical section at a time.

37
Q

What are the steps to use the lock variable?

A

Acquire the lock: Before entering the critical section.

Release the lock: After exiting the critical section

38
Q

What is a mutex?

A

A mutex (short for mutual exclusion) is a type of lock used to prevent multiple processes from accessing a critical section simultaneously.

It ensures mutual exclusion where only one process can hold the mutex at a time.

39
Q

How does Mutex relates to locks in critical section?

A

It acquired before critical section and released after. This is to prevent race conditions and ensures data integrity.

40
Q

Share lock

A
41
Q

What are the problems that shared lock have?

A
42
Q

What is busy waiting (aka polling or spinning) in concurrency?

A

Busy waiting occurs when a process continuously checks if a lock has become available, consuming CPU cycles without making progress.

Busy waiting presents shared memory, where the lock status is held in shared memory,allowing processes to check it repeatedly.

43
Q

What is a spin lock, and where is it commonly used?

A

A spin lock is a type of busy waiting mechanism.

Spin lock is use to hold a lock while waiting for it to become available.

It is commonly used at the kernel level in operating system with special hardware instructions.

44
Q

What is the drawbacks in busy waiting?

A
  • Wastes CPU resources
  • May prevent the lock-holding process from completing its critical section and releasing the lock.
45
Q

What is the drawbacks in busy waiting?

A
  • Wastes CPU resources
  • May prevent the lock-holding process from completing its critical section and releasing the lock.
46
Q

What is strict alternation in busy waiting?

A

Strict alternation enforces turn-taking between two processes for accessing the critical section.

  1. Shared Variable (Token): Represents the process ID of the next process allowed to enter the critical section
  2. Entry to critical section: Process Pi waits (busy-waits) until token == i (its process ID)
  3. Exit the Critical Section: Process Pi sets token to the next process ID, allowing the other processes to proceed.
47
Q

What are the limitation of strict alternation in concurrency?

A
  1. Inefficiency: Processes may wait unnecessarily, even if the other process isn’t ready to enter its critical section.
  2. Deadlocks Potential: If a process halts or fails outside its critical section, the other process can be stuck waiting indefinitely
  3. Not Scalable: Only works efficiently for two processes; complex to implement for more.
48
Q

What is the main benefit of strict alternation in busy waiting?

A

Mutual exclusion is guaranteed!

  • Only one process can enter its critical section at a time, as controlled by the turn variable.
  • The process with the current token has exclusive access to the critical section.
49
Q

What is the likeness progression problem in strict alternation?

A
  1. Dependence on turn variable: Both processes rely on the turn variable to switch
  2. Problem:
    - If one process is held up in its non-critical section,it won’t’ update the turn variable.
    - This causes other process to block, leading to a lack of progress or responsiveness

Computer System (5 decks)

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