GC Tuning in .NET

Introduction to GC Tuning

Effective garbage collection (GC) is crucial for the performance and stability of .NET applications. Understanding how the GC works and how to tune its behavior can lead to significant improvements, especially in high-throughput or memory-intensive scenarios. This guide explores common GC tuning techniques and best practices.

Understanding .NET Garbage Collection

The .NET Garbage Collector is an automatic memory management system that reclaims memory occupied by objects that are no longer in use by an application. It operates in generations (0, 1, 2) to efficiently manage the object lifecycle.

Generations:

• Generation 0: Newly allocated objects. Scanned most frequently.
• Generation 1: Objects that survived Gen 0 collections.
• Generation 2: Objects that survived Gen 1 collections. These are long-lived objects.

GC Modes:

• Workstation GC: Optimized for responsiveness, suitable for client applications.
• Server GC: Optimized for throughput, suitable for server applications. It uses multiple GC threads per CPU.

Understanding which GC mode is appropriate for your application is the first step in tuning.

Key GC Tuning Strategies

Tuning the GC often involves adjusting its behavior through configuration settings or by structuring your application's memory usage more effectively.

1. Choosing the Right GC Mode

For server applications, Server GC is generally preferred for better throughput. For client applications, Workstation GC offers better responsiveness.

<runtime>
  <gcServer enabled="true" />
</runtime>

2. Adjusting Heap Size

While the GC automatically manages heap size, in specific scenarios, you might consider influencing it. This is often more about application design than direct GC configuration.

3. Concurrent vs. Non-Concurrent GC

Concurrent GC allows the application threads to run while the GC is performing some of its collection tasks, reducing pauses. Server GC is often concurrent by default.

4. Large Object Heap (LOH) Management

Objects larger than 85,000 bytes are allocated on the LOH. LOH collections can be more expensive. Minimize large object allocations and consider pooling if feasible.

5. Specifying GC Heap Size (Advanced)

In rare cases, you might need to explicitly set the GC heap size for specific generations. This should be done with extreme caution and thorough testing.

<runtime>
  <gcConcurrent enabled="true" />
  <GCSettings>
    <largeObjectHeapCompactionMode value="0" /> 
    <heapVerify enabled="false" />
  </GCSettings>
</runtime>

largeObjectHeapCompactionMode: 0 = Never compact, 1 = Compact LOH only if necessary, 2 = Always compact LOH.

Monitoring GC Performance

Effective tuning relies on accurate monitoring. Use Performance Monitor (PerfMon) to track key GC-related counters.

Key Performance Counters:

• % Time in GC: Percentage of time spent by all processors in GC. A high value indicates potential GC pressure.
• # GC Handles: Number of handles in use.
• # Gen 0 Collections: Number of Gen 0 collections.
• # Gen 1 Collections: Number of Gen 1 collections.
• # Gen 2 Collections: Number of Gen 2 collections.
• Heap size: Current size of the managed heap.
• Large Object Heap size: Current size of the LOH.

Use these counters to identify GC bottlenecks and measure the impact of your tuning efforts.

Best Practices for GC Tuning

Tuning is not just about configuration; it's also about writing efficient code.

• Minimize Allocations: Reduce the number of short-lived objects. Reuse objects where possible.
• Prefer Value Types (Structs): For small, short-lived data structures, value types can avoid heap allocations altogether.
• Use `IDisposable` and `using` Statements: Properly manage unmanaged resources and short-lived managed objects.
• Avoid Excessive Large Object Allocations: Consider object pooling for large, frequently created objects.
• Profile Your Application: Use profiling tools to identify memory allocation hotspots.
• Test Changes Thoroughly: GC tuning can have unintended consequences. Test all changes under realistic load.
• Understand Your Workload: The optimal GC configuration depends heavily on whether your application is CPU-bound, memory-bound, or I/O-bound.

Remember: Premature optimization is the root of all evil. Focus on writing clean, correct code first, then profile and tune based on observed performance issues.