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When synchronizing access to a shared resource, is there ever a reason not to use a read/write lock instead of a vanilla mutex (which is basically just a write lock), besides the philosophical reason of it having more features than I may need?

In other words, if I just default to read/write locks as my preferred synchronization construct of choice, am I shooting myself in the foot?

It seems to me that one good rationale for always choosing a read/write lock, and using the read vs. write lock accordingly, is I can implement some synchronization, then never have to think about it again while gaining the possible benefit of better performance scalability in the future if one day I drop the code into a more high-contention environment. So assuming it has a potential benefit with no actual cost, it would make sense to use it all the time. Does that make sense?

This is on a system that isn't really resource limited, it's probably more of a performance question. Also I've phrased this question generally but I have Qt's QReadWriteLock and QMutex (C++) specifically in mind, if it matters.

adoug
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2 Answers2

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In practice, the write lock in a read/write lock pair is more expensive than a simple mutex. Read/write locks always have some coordination strategy that must be applied when you acquire or release a lock. Depending on the particular implementation, this strategy can be cheap or expensive but it always exists.

In case of QReadWriteLock, there's some logic that gives priority to writers. Even though the implementation of such logic might be efficient, and there're no readers in the waiting queue, it's never totally free.

I'm not familiar with all the details of the QMutex and QReadWriteLock implementation, but the documentation says that QMutex is heavily optimized for non-contended cases. QReadWriteLock has no such remark. Maybe because they just forgot to make such note, but maybe because its behaviour under such circumstances is not as good as of QMutex.

I think, in the best scenario, a penalty for using read/write lock is negligible. But in the worst scenario, when you fight for every nanosecond, it might be noticeable.

Andrew Lygin
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It really boils down on the characteristic of the congestion of your lock. The case when the simple mutex will perform significanly better is heavy congestion with writer preference.

This is a very lengthy and debatable subject. May I recommend Spinlocks and Read-Write Locks and Sleeping Read-Write Locks reading, so you can make an educated decision?

smokku
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