Problem 14: Advanced Synchronization in Multi-Buffered Master-Worker Thread Pools

Grokking Multithreading and Concurrency for Coding Interviews

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Problem 14: Advanced Synchronization in Multi-Buffered Master-Worker Thread Pools

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Overview

In this problem, we are presented with a multi-buffered system where multiple master threads produce data, and multiple worker threads consume this data. The unique aspect of the problem is that while each master thread is dedicated to its own buffer, the worker threads have the flexibility to consume data from any buffer that has data available. This design aims to ensure optimal resource utilization; workers don't remain idle even if a particular buffer is empty. The primary challenge here is to ensure proper synchronization using pthread condition variables to prevent race conditions, especially given the existence of multiple buffers.

Pseudocode

Initialize:
  BUFFER[...]               // Array of buffers.
  BUFFER_LOCK[...]          // Array of locks for each buffer.
  BUFFER_NOT_EMPTY[...]     // Array of condition variables signaling buffer not empty.
  BUFFER_NOT_FULL[...]      // Array of condition variables signaling buffer not full.

Function MASTER_THREAD(thread_id):
  While True:
    data = GenerateData()
    
    AcquireLock(BUFFER_LOCK[thread_id])
    
    While BUFFER[thread_id] is Full:
      WaitForCondition(BUFFER_NOT_FULL[thread_id])
    
    AddToBuffer(BUFFER[thread_id], data)
    
    SignalCondition(BUFFER_NOT_EMPTY[thread_id])
    
    ReleaseLock(BUFFER_LOCK[thread_id])


Function WORKER_THREAD():
  While True:
    chosen_buffer = None
    
    For i from 0 to length(BUFFER) - 1:
      AcquireLock(BUFFER_LOCK[i])
      
      If BUFFER[i] is Not Empty:
        chosen_buffer = i
        Break
      Else:
        ReleaseLock(BUFFER_LOCK[i])
    
    If chosen_buffer is None:
      For i from 0 to length(BUFFER) - 1:
        AcquireLock(BUFFER_LOCK[i])
        
        If BUFFER[i] is Not Empty:
          chosen_buffer = i
          Break
        
        WaitForCondition(BUFFER_NOT_EMPTY[i])
        ReleaseLock(BUFFER_LOCK[i])
    
    data = RemoveFromBuffer(BUFFER[chosen_buffer])
    
    SignalCondition(BUFFER_NOT_FULL[chosen_buffer])
    
    ReleaseLock(BUFFER_LOCK[chosen_buffer])
    
    ProcessData(data)

Algorithm Walkthrough

Let's go through a simple and understandable walkthrough of the algorithm, focusing on the synchronization aspects to ensure clarity:

Initialization

Buffers: Initialize five empty buffers to hold integers.
Mutexes and Condition Variables: Initialize mutexes and condition variables for each buffer.

Execution Starts

Start Master Threads: Launch five master threads, each responsible for one buffer.
Start Worker Threads: Launch two worker threads responsible for consuming data from any buffer.

Master Thread Operation (Repeatedly for each Master)

Generate Data: Each master thread generates a random number.
Lock Buffer: Acquires a lock on the mutex associated with its buffer.
Check if Buffer is Full:
- If the buffer is full, wait on the “buffer not full” condition variable.
- If not, proceed to the next step.
Add Data: Add the generated number to the buffer.
Signal Workers: Signal “buffer not empty” condition variable to wake up worker threads if any are waiting.
Unlock Buffer: Release the lock on the mutex.

Worker Thread Operation (Repeatedly for each Worker)

Choose a Buffer: Each worker thread goes through the buffers to find non-empty ones.
Lock Buffer: Acquires a lock on the current buffer’s mutex.
Check if Buffer is Empty:
- If the buffer is not empty, proceed to the next step.
- If empty, release the lock and check the next buffer.
Consume Data: Remove an item from the buffer.
Signal Master: Signal “buffer not full” condition variable to wake up master threads if any are waiting.
Unlock Buffer: Release the lock on the mutex.
Process Data: (This part is assumed and not explicitly shown in the code)

Synchronization Aspects Highlight

Mutex Locks: Ensuring that only one thread can access a buffer at a time.
Condition Variables:
- Master waits if the buffer is full.
- Worker continues to the next buffer if the current one is empty (does not wait).

Main Thread Sleep and Exit

Main Thread Sleep: The main thread sleeps for 2 seconds, allowing the worker and master threads to do some work.
Exit: After sleeping, the main thread exits, setting stop = true, this makes all other threads exit gracefully.

This walkthrough illustrates the algorithm's flow in a multi-buffer producer-consumer scenario, with a strong focus on synchronization to ensure safe and efficient multi-threaded operation.

Flow Chart

Python3

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