What is the Large Object Heap (LOH), and why do objects larger than 85,000 bytes behave differently in the garbage collector?

Objects 85,000 bytes or larger are allocated on the Large Object Heap, a separate heap segment managed differently from the normal small-object heap -- LOH objects are collected only during gen-2 (full) collections and, historically, the LOH was never compacted (leading to potential fragmentation), though modern .NET can optionally compact it. Frequently allocating and discarding large objects (like large arrays or buffers) can cause LOH fragmentation and more frequent expensive full GCs, making techniques like buffer pooling (ArrayPool<T>) particularly valuable for large, frequently-allocated objects.

// This large array allocation goes to the LOH (>= 85,000 bytes)
byte[] largeBuffer = new byte[100_000];

// Better: rent from a pool instead of repeatedly allocating large arrays
byte[] pooledBuffer = ArrayPool<byte>.Shared.Rent(100_000);
try { /* use it */ }
finally { ArrayPool<byte>.Shared.Return(pooledBuffer); }