CODEWITHSUNDEEP
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pham cuong
pham cuong

Reputation: 871

Will a collection with a total size exceeding 85kB be put on the large-object-heap?

I have a question about about when objects get put into the small- vs large-object-heap.

I have two string with size smaller than 85kB which I add to a collection, such as a List. The combined size of the strings exceeds 85kB.

Will this collection be treated as part of the small-object-heap or as part of the large object-heap? I think it will be SOH but I'm not sure.

Upvotes: 0

Views: 228

Answers (1)

xanatos
xanatos

Reputation: 111940

If the collection is a List<TReference> of any size then it is SOH. If the collection is a TReference[] with a size <= 20k elements at 32 bits, or 10k elements at 64 bits (the size of a reference at 32bits is 4 bytes, at 64bits is 8 bytes), the it is SOH.

The size of an object isn't calculate based on object X + all the objects referenced by object X, but simply by the "direct" size of X.

Why the difference between List<TReference> and TReference[]? Because arrays (technically single dimensional, 0-base indexed arrays) are the only variable length data type directly supported by the IL language. Nearly all the other collection types are simply a reference to a T[] array (or to a bunch of T[] arrays) plus some supporting numeric fields. So for example a List<T> is a reference to a T[] plus a int Length.

The point of SOH vs LOH is "how much fast would be to move it around memory"? Clearly:

public class MySmallClass
{
    public byte[] WithABigObject = new byte[1024*1024];
}

While moving the BigObject (the byte array) would be very slow, moving the MySmallClass (that is simply composed by a reference) is very fast, because it has the size of a reference :-)

Note that in C# creating objects that aren't arrays with a size >= 85k is quite difficult, because, as I've said, between the types that are directly supported by the IL language, only the array has a variable size. Every other type has a fixed size and is quite small. You would need to make a class with 22k int inside to make a LOH without an array :-)

An example of a LOH object without arrays: https://ideone.com/OuVkEs . It is based on multilevel KeyValuePair<,> (a struct) that at the lowest level use Guid (the biggest commonly used struct). The resulting BigGuid128k is a struct of 128k. By including it in your class, your class will become a LOH :-)

Upvotes: 1

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