Summary of "Synchronization Hardware: Test & Set, Swap | L 15 | Operating System | GATE 2022 CSE #Vishvadeep Sir"

Summary of "Synchronization Hardware: Test & Set, Swap | L 15 | Operating System | GATE 2022 CSE #Vishvadeep Sir"

This lecture by Vishvadeep Sir focuses on hardware solutions for process synchronization in operating systems, specifically the Test-and-Set and Swap instructions. It builds upon previous discussions of software synchronization solutions and explains why hardware-level instructions are necessary to ensure Atomicity and avoid problems like race conditions and Preemption.

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Main Ideas and Concepts

1. Introduction to Synchronization Hardware

• Hardware solutions involve special CPU instructions designed to provide synchronization support.
• These instructions are privileged and run in kernel mode, ensuring atomic execution without interruption.
• Unlike software solutions, hardware instructions guarantee Atomicity — either the whole instruction executes or none of it does, preventing Preemption during critical operations.

2. Recap of Software Synchronization Solutions

• Previous software-based solutions (Lock variable, turn-based, Peterson’s) had limitations:
  • Lock variable: no mutual exclusion due to Preemption.
  • Turn-based: mutual exclusion but no progress.
  • Peterson’s: satisfied mutual exclusion, progress, and bounded waiting.
• Hardware instructions aim to overcome these issues.

3. Test-and-Set Instruction

• Test-and-Set is an atomic hardware instruction that:
  • Tests the value of a boolean Lock variable.
  • Sets the Lock variable to true.
  • Returns the old value of the Lock.
• Typical usage:
  • If the Lock was false (unLocked), the process sets it to true (Locked) and enters the critical section.
  • If the Lock was true, the process keeps Busy Waiting (spinning) until the Lock becomes false.
• This prevents Preemption between test and set steps, ensuring mutual exclusion.
• Busy Waiting: the process keeps executing on CPU but cannot enter the critical section until the Lock is released.

4. Swap Instruction

• The Swap instruction atomically exchanges values of two variables.
• Used with two boolean variables: Lock and key.
• Procedure:
  • Initialize Lock = false and key = true.
  • A process sets key = true and enters a loop where it Swaps key and Lock.
  • If after Swapping key becomes false, the process enters the critical section.
  • If key remains true, the process keeps Swapping (Busy Waiting).
• Swap-based synchronization is similar to Test-and-Set but uses a different atomic primitive.
• Potential problem: if Preemption occurs at certain points (between statements), mutual exclusion can fail.
• Adding extra assignments to key before entering the critical section can reduce but not eliminate this risk.

5. Atomicity and Preemption

• Atomic instructions prevent Preemption during critical steps, which is essential to avoid race conditions.
• Software solutions fail because the CPU can interrupt between check and set operations.
• Hardware instructions ensure these steps are indivisible.

6. Busy Waiting

• Defined as a process actively executing on the CPU but unable to proceed because it is waiting for a Lock.
• Occurs in Test-and-Set and Swap solutions when a process spins waiting for the Lock to be released.

7. Practical Notes & Assignments

• Understanding Test-and-Set and Swap is critical for solving synchronization problems.
• The instructor hints at a GATE question related to Test-and-Set, suggesting students review deadLock concepts before attempting it.
• Upcoming sessions will cover semaphore (signaled as "sigma 4") and other synchronization topics.

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Methodology / Key Steps for Using Test-and-Set for Synchronization

• Initialize Lock = false.
• Process tries to enter critical section:
  • Call test_and_set(&Lock) which:
    • Returns old value of Lock.
    • Sets Lock = true.
  • If returned value is false, enter critical section.
  • Else, keep calling test_and_set (busy wait).
• On exiting critical section, set Lock = false.

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Methodology / Key Steps for Using Swap for Synchronization

• Initialize Lock = false.
• Before entering critical section, process sets key = true.
• While key == true:
  • Atomically Swap key and Lock.
• If key == false after Swap, enter critical section.
• On exit, set Lock = false.

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Additional Information

• The instructor also promotes Unacademy Plus and Iconic subscriptions for GATE preparation, highlighting features like live classes, quizzes, doubt solving, and personalized coaching.
• Several upcoming sessions and courses are announced, including a new educator launch and machine learning topics.
• Encouragement for viewers to join live sessions for interactive learning.

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Speakers / Sources Featured

• Vishvadeep Sir (main and only)

Category

Educational

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