Producer Consumer problem

Notes

• The bounded buffer problem, also called the producer-consumer problem.
• This is a (set of) classic concurrence problem(s).
  • Mostly depending on the number of producers/consumers
  • We will look at the generic case
• This problem was presented by Edsger Dijkstra in 1965.
• In the first problem we want to solve
  • There is a buffer that can hold n entries
  • There are a set of producers who want to write the the buffer.
  • There are a set of consumers who want to read from the buffer.
  • We do not want the buffer to overflow, or a reader to read a non-existant message from the buffer.
  • We need access to the buffer locked down for read/write
• The solution is:
  • A mutex that locks the buffer for read/write
  • A semaphore that only allows a writer to write when there is space in the buffer
  • A semaphore that only allows a reader to read if there is data in the buffer
• Essentially

  • 
    set BufferMutex to unlocked
    set ReadSemaphore to 0 (no data in the buffer)
    set WriteSemaphore to buffer size (there is space to write in the buffer)
    
    To Write:
        block (semaphore) until there is space to write
        acquire the buffer mutex  
        write
        release the buffer mutex 
        mark that there is more data in the buffer
    
    To Read:
        block until there is data available in the buffer
        acquire the buffer mutx
        read
        release the buffer mutex
        mark that there is more space in the buffer
          

• This solution has been designed without deadlock
  • Only the correct number of readers/writers will be allowed to wait for the buffer
  • Others will wait for permission to wait for the buffer
  • Only one entity will be permitted to lock the buffer, and only after they have permission to use the buffer (read/write).
  • SO the buffer will only be locked when an entity is fully prepared to use it, it is waiting on nothing else.
• What would happen if we allowed an entity to lock the buffer first then attempt to get a free space in the buffer?