Wednesday, September 7, 2011

synchronization in hash map -- non-trivial

Q: why bother to synchronize access to a linked list?
%%A: as a rule, mutable shared data between 2 threads need synchronization. Recall java volatile keyword.
%%A: Let's consider a simplified single linked list and removal of a middle node NodeE. Well we just need to reseat pointers -- perhaps in nodeD, right? Just make nodeD point to nodeF. Atomic operation, thread safe right?

But what if another thread is removing NodeF? What if another thread is removing NodeD?

As a necessary but insufficient condition, I feel the composite operation must be atomic -- read the pointer in NodeE, and put that address (address of NodeF) into NodeD's pointer, while entire link is locked down.

What if we are operating on the beginning of the link?
What about insertion?

%%A: Therefore, a linked list remove() needs synchronization.

Q: why bother to synchronize access to a HashMap?
%%A: one reason -- because a hash map uses linked lists, and also the map can be rehashed.

Q: why is CHM read operation wait free??

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