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Operating Systems and Virtualisation (MOD004890) This docume

Operating Systems and Virtualisation (MOD004890) This document describes two challenges.

Challenge 1. The Producer-Consumer Problem

Explain how the program (challenge1.c) solves the producer-consumer problem by adding comments to the provided source code. Include comments at the top of the code with your SID, a general explanation of the purpose of the code, and instructions on how to compile and run the program.

Challenge 2. Priority Scheduling

Explain how the program (scheduling.c) performs priority scheduling by adding comments to the provided source code. Include comments at the top of the code with your SID, a general explanation of the purpose of the code, and instructions on how to compile and run the program.

Paper For Above Instructions

Introduction

This paper explains how to annotate two C programs supplied for the Operating Systems and Virtualisation (MOD004890) assessment: (1) a producer-consumer implementation using POSIX threads and condition variables (challenge1.c), and (2) a priority-based scheduling example (scheduling.c). For each program I provide the exact header comment to add (including SID placeholder), detailed line-level commentary guidance to add to the source, clear compile-and-run instructions, and a short explanation of correctness, typical pitfalls and suggested improvements. The explanations reference standard OS and POSIX threading literature where appropriate [1][2][3].

Challenge 1 — Producer-Consumer: Top-of-file comment and compile/run

Suggested header comment to prepend to challenge1.c:

/*


* SID: [INSERT YOUR SID HERE]


* Purpose: Demonstrate a single-producer, single-consumer synchronization


* model using a shared buffer, a mutex, and two condition variables.


* How to compile: gcc -pthread -O2 -o challenge1 challenge1.c


* How to run: ./challenge1


*/

Key line-by-line comments to add and rationale:

  • Include statements: Note why each header is required: <pthread.h> for pthread APIs, <stdio.h> and <stdlib.h> for I/O and exit, <unistd.h> for sleep or portability macros.
  • Global synchronization objects: Comment that pthread_mutex_t the_mutex serializes access to the shared buffer, and the two pthread_cond_t variables implement the blocking/wakeup protocol between producer and consumer [3][4].
  • Producer thread: Before locking, explain the loop that produces values 1..MAX. After acquiring the mutex, the producer tests while (buffer != 0) and waits on the producer condition variable. Explain the use of a while-loop (not if) to re-check the condition after spurious wakeups as required by POSIX [3]. After placing a value, the producer signals the consumer with pthread_cond_signal(&condc), then unlocks and prints. Comment that signaling only wakes one waiting thread, suitable here for single consumer.
  • Consumer thread: Mirror comments: lock, while(buffer==0) wait on condc, then consume (set buffer=0), signal producer via condp, then unlock.
  • Cleanup in main: Explain initialization of mutex and cond vars, thread creation ordering, and why pthread_join ensures main waits for both threads. Also note destruction of synchronization objects to avoid resource leaks.

Correctness explanation

The program enforces mutual exclusion with a mutex so producer and consumer cannot concurrently modify the shared buffer. Condition variables allow each thread to block when work cannot proceed: the producer blocks when the buffer is non-empty; the consumer blocks when it is empty. The use of a predicate re-check loop (while) prevents incorrect behavior from spurious wakeups [3]. This classic pattern guarantees no lost wakeups and prevents busy-waiting, satisfying the safety (no two threads access buffer concurrently) and liveness (threads eventually proceed when conditions change) requirements of the problem [1][4].

Common pitfalls and improvements

  • If multiple producers/consumers are used, replace single-item buffer with a bounded queue and use pthread_cond_broadcast or careful signaling; avoid integer overflow on indexing if implementing ring buffers [3].
  • Check return values of pthread APIs to detect errors (omitted in simple examples).
  • Consider using semaphores for a simpler counting-semaphore solution in multi-producer/consumer scenarios [1].

Challenge 2 — Priority Scheduling: Top-of-file comment and compile/run

Suggested header comment to prepend to scheduling.c:

/*


* SID: [INSERT YOUR SID HERE]


* Purpose: Demonstrate non-preemptive priority scheduling: processes with


* higher numeric priority are scheduled before lower ones. Computes wait time


* and turnaround time and prints averages.


* How to compile: gcc -O2 -o scheduling scheduling.c


* How to run: ./scheduling


*/

Key comments to add in the source and rationale:

  • Input parsing: Explain arrays used: pt[] for burst time, pp[] for priority, p[] for process IDs, w[] for wait times, t[] for turnaround times.
  • Sorting loop: Comment that the nested loops perform a descending priority sort so that higher-priority processes are scheduled first. Clarify the swap of corresponding burst times and IDs to maintain tuple integrity.
  • Computation of waiting and turnaround times: Explain recurrence w[i] = t[i-1], t[i] = w[i] + pt[i], and the initial conditions w[0]=0, t[0]=pt[0].
  • Averaging: Note that the code uses integer division for averages; advise casting to double for accurate fractional averages.

Correctness explanation

The program implements a non-preemptive priority scheduler: once a process starts it runs to completion; processes are ordered by priority before execution. This is a valid scheduling policy for many batch systems but can suffer from starvation if low-priority processes are never scheduled. The program uses simple sorting to enforce priority order and computes standard scheduling metrics (waiting time and turnaround time) as described in classic OS texts [1][2].

Improvements and caveats

  • Convert average time computations to floating point to avoid truncation: cast sums to double before dividing by n.
  • Tie-breaking: when priorities are equal, the code retains order as per the stable sort it performs; explicitly documenting FCFS tie-breaking is useful.
  • For preemptive priority scheduling, a different algorithm is required (e.g., using ready queues and preemption points) [1][5].

Conclusion

For both supplied programs, adding the recommended header comments (SID, purpose, compile/run) and the line-level explanations described above will satisfy the assignment requirement to document how the code works. The producer-consumer example demonstrates correct use of mutexes and condition variables to guarantee safe coordination between threads; the scheduling example implements a straightforward non-preemptive priority scheduler and computes standard metrics. For robust production code, add error checking, consider edge cases (multiple producers/consumers, preemption, starvation mitigation), and use floating-point arithmetic for averages [1][3][5].

References

  1. Andrew S. Tanenbaum and Herbert Bos. Modern Operating Systems. 4th ed., Pearson, 2015. [Foundational concepts on synchronization and scheduling]
  2. Abraham Silberschatz, Peter B. Galvin, and Greg Gagne. Operating System Concepts. 10th ed., Wiley, 2018. [Scheduling algorithms and performance metrics]
  3. David R. Butenhof. Programming with POSIX Threads. Addison-Wesley, 1997. [POSIX threads patterns, condition variables and mutex usage]
  4. Edsger W. Dijkstra. "Cooperating Sequential Processes." In Programming Languages, F. Genuys (ed.), Academic Press, 1968. [Seminal discussion of process cooperation]
  5. C. L. Liu and James W. Layland. "Scheduling Algorithms for Multiprogramming in a Hard-Real-Time Environment." Journal of the ACM, 1973. [Classic scheduling theory]
  6. IEEE Std 1003.1. The POSIX.1-2017 Base Specifications. The Open Group. [POSIX thread API specification]
  7. Linux manual pages for pthreads: pthread_cond_wait(3), pthread_mutex_lock(3). man7.org. [API usage examples and notes on spurious wakeups]
  8. W. Richard Stevens. Advanced Programming in the UNIX Environment. 2nd ed., Addison-Wesley, 2005. [Threading and IPC patterns]
  9. Producer–consumer problem. Wikipedia. https://en.wikipedia.org/wiki/Producer–consumer_problem (accessed 2025). [Problem description and common solutions]
  10. Scheduling (computing). Wikipedia. https://en.wikipedia.org/wiki/Scheduling_(computing) (accessed 2025). [Overview of scheduling strategies]

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