Friday, June 11, 2010

consistency vs liveness -- 2 concurrency issues

2 main risks of concurrency: consistency ^ liveness

([[concur programming on windows]] also singles out these 2 "hazards")

Concurrency risk with collections? Race condition (inconsistency problem) is the main risk.

While Consistency is explained in other posts, Liveness is still vague at this stage of my understanding. One way to define it is "every thread should make progress". No waiting forever, no permanent blocking. No deadlock.

Now the upshot. To prevent inconsistency, use locks everywhere, but locks affect liveness.

Atomic variables and thread-safe collections [NO NO NO] seem to be an alternative.

Now I think thread-safe collections are a liveness hazard and can increase chance of deadlocks, because they use locks. Locks are by nature
* exclusive
* u must wait for them, potentially forever.

Single-threaded execution is free from consistency/liveness issues. If you synchronize and serialize too much, you might achieve (but hard to guarantee) consistency but your liveness may be worse than single-threaded due to blocking.

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