Understanding Garbage Collection - How the dotnet Garbage Collector Works

posted Wed, Feb 5, 2025 by darren horrocks

Garbage collection (GC) is a fundamental component of the .NET runtime, responsible for managing memory automatically and ensuring efficient use of resources. With .NET 8, Microsoft has continued refining the garbage collector, improving performance, reducing latency, and enhancing overall efficiency. This article dives into how the .NET 8 GC works, its key improvements, and how developers can optimise memory usage.

How the .NET 8 Garbage Collector Works

The .NET 8 garbage collector operates as a generational, concurrent, and compacting GC. This means it divides objects into different generations based on their lifespan, collects garbage in a way that minimises application pauses, and compacts memory to reduce fragmentation. Here’s a breakdown of its key components:

Generational Garbage Collection

.NET’s GC follows a generational approach, classifying objects into three generations:

• Generation 0 (Gen 0): Short-lived objects such as temporary variables and method-scoped objects.
• Generation 1 (Gen 1): Objects that survived at least one GC cycle, acting as a buffer between Gen 0 and Gen 2.
• Generation 2 (Gen 2): Long-lived objects such as static data, caches, and application-wide objects.

When the GC runs, it primarily collects Gen 0 objects, since they are short-lived. If memory pressure persists, it progresses to Gen 1 and eventually to a full GC (Gen 2), which is more expensive.

Concurrent and Background Collection

To improve performance, .NET 8’s GC includes background collection:

• Concurrent GC: Allows the application to continue running while the GC reclaims memory.
• Background GC: Used for Gen 2 collections to minimise blocking, keeping latency low for large applications.

These optimisations are particularly beneficial for real-time applications like web servers and gaming engines, where low-latency memory management is critical.

Compacting and Memory Fragmentation Reduction

As objects are allocated and freed, memory fragmentation can occur. The .NET GC mitigates this by compacting memory, moving surviving objects closer together to create larger free memory blocks. This improves cache locality and reduces memory pressure.

Pinned Objects and LOH (Large Object Heap) Handling

• Pinned Objects: Some objects (e.g., those used in interop with native code) cannot be moved, which can lead to fragmentation. The GC optimises handling of pinned objects to reduce their impact on performance.
• Large Object Heap (LOH): Objects larger than 85 KB are allocated in the LOH. .NET 8 includes improved LOH compaction to prevent excessive fragmentation.

What’s New in .NET 8 Garbage Collection?

.NET 8 introduces several key improvements to the GC:

• Better Workload-Based Optimisations: The GC now adapts more effectively to different workloads, making it more efficient in cloud and high-performance applications.
• Improved Memory Compaction Heuristics: Smarter algorithms decide when to compact memory, reducing unnecessary performance overhead.
• Reduced Lock Contention in Multi-Threaded Environments: Enhancements in the GC’s locking mechanisms help prevent contention, particularly in multi-core environments.
• Lower Latency for Interactive Applications: Optimisations ensure that UI and real-time applications experience fewer interruptions due to garbage collection.

How to Optimise for .NET 8’s GC

While the .NET GC is highly optimised, developers can still take steps to improve performance:

1. Reduce Unnecessary Object Allocations

   • Use structs instead of classes for small, short-lived objects.
   • Avoid excessive boxing/unboxing of value types.

2. Use Object Pooling

   • Consider using ObjectPool<T> from Microsoft.Extensions.ObjectPool to reuse objects instead of frequently allocating and deallocating them.

3. Minimise Large Object Heap Usage

   • Keep objects under 85kb when possible to avoid frequent LOH allocations.
   • Reuse large objects instead of allocating new ones.

4. Be Mindful of Pinned Objects

   • Use pinning sparingly, as it can lead to memory fragmentation.

5. Monitor and Profile GC Performance

   • Use dotnet-counters, dotnet-trace, or PerfView to analyse GC performance and identify bottlenecks.

Conclusion

The .NET 8 garbage collector is more efficient than ever, thanks to optimisations in concurrency, fragmentation management, and workload adaptability. By understanding its inner workings and applying best practices, developers can write more performant applications with minimal memory-related bottlenecks. Whether you’re building a high-throughput server, a responsive UI application, or a cloud-native service, optimising for the .NET 8 GC will help ensure smooth and efficient memory management.