.NET - Reducing GC contention on array creation

Question

I have some code that deal with a lot of copying of arrays. Basically my class is a collection that uses arrays as backing fields, and since I don't want to run the risk of anyone modifying an existing collection, most operations involves creating copies of the collection before modifying it, hence also copying the backing arrays.

I have noticed that the copying can be slow sometimes, within acceptable limits but I am worried that it might be a problem when the application is scaled up and starts using more data.

Some performance analysis testing suggests that while barely consuming CPU resources at all, my array copy code spends a lot of time blocked. There are few contentions, but a lot of time blocked. Since the testing application is single threaded, I assume there is some GC contention magic going on. I'm not confident enough in how the GC works in these scenarios, so I'm asking here.

My question - is there a way to create new arrays that reduces the strain on the GC? Or is there some other way I can speed this up (simplified for testing and readability purposes):

public MyCollection(MyCollection copyFrom)
{

    _items = new KeyValuePair<T, double>[copyFrom._items.Length]; //this line is reported to have a lot of contention time

    Array.Copy(copyFrom._items, _items, copyFrom._items.Length);

    _numItems = copyFrom._numItems;

}

Answer 1

There's a concept of persistent immutable data structure. This is one of the possible solutions that basically let's you create immutable objects, while still modifying them, in a memory efficient way.

For example,

Roslyn has a SyntaxTree object, that is immutable. You can modify the immutable object, and get back modified immutable object. Note that the "modified immutable object" has possibly no memory allocations, because it can build on the "first immutable object".

The same concept is also used in Visual Studio text editor itself. The TextBuffer is immutable object, but each time you press a keyboard button, new immutable TextBuffer is created, however, they do not allocate memory(as that would be slow).

Also, if it's true that you're facing the issues with LOH, it can help sometimes when you allocate the big memory block yourself, and use that as "reusable" memory pool, thus avoiding GC completely. It's worth considering.

Answer 2

Not so sure what's going on here, but contention is a threading problem, not an array copying problem. And yes, a concurrency analyzer is liable to point at a new statement since memory allocation requires acquiring a lock that protects the heap.

That lock is held for a very short time when allocations come from the gen #0 heap. So having threads fighting over the lock and losing a great deal of time being locked out is a very unlikely mishap. It is not so fast when the allocation comes from the Large Object Heap. Happens when the allocation is 85,000 bytes or more. But then a thread would of course be pretty busy with copying the array elements as well.

Do watch out for what the tool tells you, a very large number of total contentions does not automatically mean you have a problem. It only gets ugly when threads end up getting blocked for a substantial amount of time. If that is a real problem then you next need to look at how much time is spent on garbage collection. There is a basic perf counter for that, you can see it in Perfmon.exe. Category ".NET CLR Memory", counter "% Time in GC", instance = yourapp. Which is liable to be high, considering the amount of copying you do. A knob you can tweak if that is the real problem is to enable server GC.

Answer 3

No. You can wait for the new runtime in 2015 though that will use SIMD instructions for the Array.Copy operation. This will be quite a lot faster. The current implementation is very sub-optimal.

At the end, the trick is in avoiding memory operations - which sometime just is not possible.