Reading note: 7 concurrency models in 7 weeks - Threads and Locks

Day 1

• Problem: race condition when accessing shared memory, JVM/compiler/hardware optimize the code, causing change in logic for multiple-threads-program
• Solution: instrinsic lock - synchronized, volatile keyword
• Problem with instrinsic lock: dead lock
• Solution: Acquire locks in a fixed, global order, do not call alien methods, hold lock for short amount of time.
• Read more: Java memory model

Day 2

• Problem: the need of thread interruption, timeout when acquiring lock, control acquiring and releasing lock
• Solution: Reentrant Lock, Atomic variables
• Read more:
  • Fairness in Reentrant Lock
  • ReentrantReadWriteLock
    • The lock is separated into read_lock and write_lock. This is for optimizing for workload with a lot of read and sporadic write.
    • Read and Write lock can also have fairness and timeout
    • Read -> Write is upgraded, and Write -> Read is downgraded
  • Spurious Wakup: the event when a call to wait returns even if the condition variable does not update, which forces developers to check the condition variable in a while loop. A good design software does not need to care about this because it uses better locking mechanisim instead of condition variable.
  • AtomicReferenceFieldUpdater