TTS Traffic Analysis

• Recall that unlock is always a simple store
• In the worst case everyone will try to enter the CS at the same time
  • First time P transactions for ts and one succeeds; every other processor suffers a miss on the load in Test loop; then loops from cache
• The lock-holder when unlocking generates an upgrade (why?) and invalidates all others
• All other processors suffer read miss and get value zero now; so they break Test loop and try ts and the process continues until everyone has visited the CS

(P+(P-1)+1+(P-1))+((P-1)+(P-2)+1+(P-2))+… = (3P-1) +
(3P-4) + (3P-7) + … ~ 1.5P 2 asymptotically

• For distributed shared memory the situation is worse because each invalidation becomes a separate message (more later)

Goals of a Lock Algorithm

• Low latency: If no contender the lock should be acquired fast
• Low traffic: Worst case lock acquire traffic should be low; otherwise it may affect unrelated transactions
• Scalability: Traffic and latency should scale slowly with the number of processors
• Low storage cost: Maintaining lock states should not impose unrealistic memory overhead
• Fairness: Ideally processors should enter CS according to the order of lock request (TS or TTS does not guarantee this)