Ticket Lock

• Similar to Bakery algorithm but simpler
• A nice application of fetch & inc
• Basic idea is to come and hold a unique ticket and wait until your turn comes
  • Bakery algorithm failed to offer this uniqueness thereby increasing complexity

  Shared: ticket = 0, release_count = 0;
  Lock: fetch & inc reg1, ticket_addr
  Wait: lw reg2, release_count_addr /* while ( release_count != ticket); */
  sub reg3, reg2, reg1
  bnez reg3, Wait
  
  Unlock: addi reg2, reg2, 0x1 /* release_count ++ */
  sw reg2, release_count_addr

• Initial fetch & inc generates O(P) traffic on bus-based machines (may be worse in DSM depending on implementation of fetch & inc)
• But the waiting algorithm still suffers from 0.5P 2 messages asymptotically
  • Researchers have proposed proportional backoff i.e. in the wait loop put a delay proportional to the difference between ticket value and last read release_count
• Latency and storage-wise better than Bakery
• Traffic-wise better than TTS and Bakery (I leave it to you to analyze the traffic of Bakery)
• Guaranteed fairness: the ticket value induces a FIFO queue