CODEWITHSUNDEEP
javamultithreading
MsA
MsA

Reputation: 2979

Understanding fair ReentrantLock in Java

ReentrantLock API doc says:

  1. The constructor for this class accepts an optional fairness parameter. When set true, under contention, locks favor granting access to the longest-waiting thread.
  2. Note however, that fairness of locks does not guarantee fairness of thread scheduling. Thus, one of many threads using a fair lock may obtain it multiple times in succession while other active threads are not progressing and not currently holding the lock.

I am not able to understand points 2:

If one thread obtain lock multiple times in succession, then as per point 1, other threads will wait for longer and that does mean they will get the lock next time. Then how this does not affect (fairness of) thread scheduling? Thus, I feel fair lock is nothing but longest waiting time first thread scheduling.

Upvotes: 2

Views: 1497

Answers (1)

user176692
user176692

Reputation: 830

I think they're just trying to separate the fairness logic side from the scheduling logic. Threads may be concurrent, but that doesn't mean they try to access Locks simultaneously. Thread priority requests are only 'hints' to the OS, and are never guaranteed the way they may be expected.

So, just because you have threads A and B, which may request a lock, which may even have identical behavior, one thread may execute, acquire the lock, release, re-acquire, before the other locks even request it:

A: Request Lock -> Release Lock -> Request Lock Again (Succeeds) B: Request Lock (Denied)... ----------------------- Time --------------------------------->

Thread scheduling logic is decoupled from the Lock logic.

There are other scheduling issues too, the burden of which often falls on the software designer, see Starvation and Livelock

Upvotes: 1

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