Question

Basically what i need is to be able to add items to List (or another collection) constantly, around 3000 times per second in one thread. And to get and remove all items from that list once per 2 seconds.

I don’t like classic ways to do this like using concurrent collections or lock on something every time i need to access collection because it would be slower than i need.

What i’m trying to do is to have 2 collections, one for each thread, and to find a way to make a thread safe switch from one collection to another.

Simplified and not thread-safe example:

var listA = new List<int>();
var listB = new List<int>();

// method is called externally 3000 times per second
void ProducerThread(int a)
{      
    listA.Add(a)      
}
void ConsumerThread()
{
  while(true)
  {
    Thread.Sleep(2000);
    listB = Interlocked.Exchange(ref listA,listB);
    //... processing listB data
    // at this point when i'm done reading data
    // producer stil may add an item because ListA.Add is not atomic
    // correct me if i'm wrong
    listB.Clear();
  }
}

Is there any way to make above code work as intended (to be thread safe) while having producer thread blocked as little as possible? Or maybe another solution?

Answer 1

I would start out by using a BlockingCollection or another IProducerConsomerCollection in System.Collections.Concurrent. That is exactly what you have, a producer/consumer queue that is accessed from multiple threads. Those collections are also heavily optimized for performance. They don’t use use a naive “lock the whole structure anytime anyone does any operation”. They are smart enough to avoid locking wherever possible using lock-free synchronization techniques, and when they do need to use critical sections they can minimize what needs to be locked on such that the structure can often be accessed concurrently despite a certain amount of locking.

Before I move from there to anything else I would use one of those collections and ensure that it is too slow. If, after using that as your solution you have demonstrated that you are spending an unacceptable amount of time adding/removing items from the collection then you could consider investigating other solutions.

If, as I suspect will be the case, they perform quick enough, I’m sure you’ll find that it makes writing the code much easier and clearer to read.