Forcing Garbage Collection

Forcing Garbage Collection (Rarely Needed)

GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced, blocking: true, compacting: true);
GC.WaitForPendingFinalizers();
GC.Collect();

• Forces collection of all generations and waits for finalizers. Use only for diagnostic tools or when transitioning between phases (e.g., benchmark setup vs measurement).
• In production code, let the GC run heuristically—manual collection can hurt throughput and latency.

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Questions & Answers

Q: When is it acceptable to call GC.Collect()?

A: During benchmarking (to start from a clean slate) or tooling scenarios (e.g., before capturing a memory snapshot). Avoid in normal application flow.

Q: What’s the impact of forcing a full GC in production?

A: It pauses all managed threads, potentially causing latency spikes and throughput loss, negating the GC’s heuristics.

Q: How do you compact the LOH manually?

A: Set GCSettings.LargeObjectHeapCompactionMode = CompactOnce, call GC.Collect() with compacting enabled, typically during maintenance windows.

Q: Why call GC.WaitForPendingFinalizers() between collections?

A: To ensure finalizers from the first collection run before initiating another GC pass, guaranteeing cleanup of finalizable objects.

Q: How do you trigger GC in benchmarks without skewing results?

A: Force GC during setup/cleanup phases, not inside the measured benchmark method, so the measurement represents steady-state behavior.

Q: Can GC.Collect() free native resources?

A: Only indirectly—finalizers may release native handles. For deterministic cleanup, implement IDisposable instead of relying on GC.

Q: How do you monitor if someone accidentally added GC.Collect() in production?

A: Use ETW/EventPipe or dotnet-trace to capture GC start reasons. Forced GCs show up with reason Induced.

Q: What alternatives exist for managing memory spikes?

A: Reduce allocation rates, pool objects, and fix leaks rather than forcing collections. Use GC.TryStartNoGCRegion for temporary low-latency windows instead.

Q: How does forcing GC affect NoGCRegion?

A: Calling GC.Collect() invalidates NoGCRegion. Instead, exit the region properly or avoid entering it if you plan to induce GC.

Q: Can you request Gen0-only collections?

A: Yes via GC.Collect(0), but even that incurs overhead. Rely on GC heuristics unless you have a proven diagnostic need.