CODEWITHSUNDEEP
c#multithreadingmutex
Luka Rahne
Luka Rahne

Reputation: 10447

Mutex Mechanisms

What is most apropriate mutex alg in C#/.NET for this kind of task.

I was thinking about simple lock or ReaderWriterLockSlim , but I am not sure which one to choose and if there is something better for this task.

Thanks.

Upvotes: 0

Views: 475

Answers (3)

We B Martians
We B Martians

Reputation: 375

Although this is an old thread (post, whatever), I came across a unique approach to lock versus MutEx that is beneficial under certain circumstances.

If your collisions by multiple entities desiring access are infrequent, consider the following approach:

  • Acquire, with lock(s) (MutEx(s) ... whatever), copies of relevant data
    • Ensure that at least one of the copied items is sufficient to compare against the original in order to detect whether or not a change has occurred
    • Release the lock(s)
  • Perform the manipulations against the copies, keeping stable your comparison item
  • Acquire again the lock(s)
  • Compare your reference item with its original to determine whether or not a change has occurred
  • If not, apply your results if so, abandon your results, re-acquire the copies and start over
    • Release the lock(s)

For example:

  1. Lock checking account and savings account; acquire copies of both balances and the times of latest transactions; unlock
  2. Calculate the changes ... for example, transfer $5 from savings to checking
  3. Lock again the accounts; compare the actual versus your copies of the transaction times- if they match, apply the calculated values and unlock, if not, re-acquire, unlock, and restart

The idea is that if there are any activities that need to lock also but do not affect your code's outcome (admittedly, the example is not a good one for this case), you have not prevented that other code's execution.

There exists some hardware for this but it is no longer "mainstream:" the IBM System 370 had an atomic compare and update instruction for just this situation.

Upvotes: 1

Brian Gideon
Brian Gideon

Reputation: 48949

You are going to need to perform your own benchmarks. I think you will find that in most cases a plain old lock will be faster than a ReaderWriterLockSlim even if most of the accesses qualify as read-only. The reason being that the overhead of servicing the lock is a lot higher. It has been awhile since I did the benchmarks, but I believe the ReadWriterLockSlim was about 5x slower than a lock. Obviously, holding the lock longer will reduce the overall impact of the overhead. At some point it stops being the dominating factor. Mileage will vary from one situation to another so benchmarking on your own is about the best advice I can give here.

Upvotes: 1

dmeister
dmeister

Reputation: 35604

I don't know about ReaderWriterLockSlim especially, but a reader writer mutex can be used if multiple reads are allowed to access the critical section in parallel. If that assumption is true, depends on our use case. This is often, but not always the case.

If the assumption is meet, a reader write mutex should be a good fit.

What do you mean by "collision probability" in that context? The probability that two threads try to access the critical section concurrently?

Upvotes: 0

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