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Assist Ratio Spikes: Behavior During Traffic Surges

Context

A key question when increasing GOGC is: "What happens when allocation suddenly spikes?" The theoretical answer is that assistRatio increases, forcing mutators to do mark work and creating backpressure. But what does this look like in production?

Theoretical Prediction

From GC Pacer and Allocation articles:

trigger = live_heap + (target_heap - live_heap) / assist_ratio

When allocation outpaces GC: 1. Heap grows faster than expected 2. assistRatio increases exponentially 3. Allocators perform mark assist work 4. Backpressure slows down allocation rate 5. Heap growth is throttled

Production Observation

Campaign Start: Traffic Spike

Memory and CPU During Peak

Observations during traffic spike: 1. Transient assistRatio increases: Sharp but brief spikes in assist ratio 2. No runaway heap growth: Memory growth rate flattened, stabilized within bounds 3. Quick stabilization: assistRatio returned to baseline within minutes

After Traffic Normalized

Observations: 1. assistRatio decreased: Returned to baseline levels 2. Background workers handled marking: No extended assist periods 3. No latency degradation: STW and API latency remained stable

Why This Matters

Common Fear: "Spiral of Death"

Concern that increased GOGC could cause: - Allocation spike → higher assistRatio → more GC work → slower allocation → higher assistRatio → ...

Reality: Self-Regulating System

The production data shows: 1. Brief spikes, not sustained elevation: assistRatio spikes are transient 2. Effective throttling: Heap growth rate flattens before hitting limits 3. No latency impact: P99 remains stable despite assist work

Quantitative Evidence

From our 67 services during campaign start:

Metric Observation Interpretation
assistRatio spike duration < 5 minutes Transient, not sustained
Heap growth rate Flattened after spike Effective throttling
API P99 latency Stable (< 5% change) No user-visible impact
STW P99 Stable Assist work didn't prolong pauses

Theoretical Validation

This confirms our understanding from Allocation and GC article:

Assist Mechanism Works as Designed

  1. Feedback loop: assistRatio responds to allocation rate
  2. Backpressure: Mutators are throttled just enough to prevent OOM
  3. Self-correcting: Once GC catches up, assistRatio decreases

Key Insight

The assist ratio spike is transient because: 1. Background workers are always marking, reducing total work needed 2. Mark assist is incremental—small amount per allocation 3. Pacer recalculates each GC cycle, adjusting to new conditions

Practical Implications

For Bursty Workloads

Services with: - Sudden traffic spikes (e.g., campaigns, events) - Batch processing windows - Uneven allocation patterns

Can safely use higher GOGC—the assist mechanism provides natural throttling without user-visible impact.

For Monitoring

Key metrics to track: - assistRatio spike frequency: Should be transient (< 5 minutes) - Heap growth rate during spikes: Should flatten, not accelerate - API latency during spikes: Should remain stable

For Tuning

  • Don't fear assist work: It's the runtime's safety mechanism
  • Do monitor spike duration: Sustained high assistRatio indicates deeper issues
  • Do check latency: If P99 degrades during spikes, reduce GOGC

Edge Cases

When Assist Spikes Indicate Problems

  1. Sustained elevation: assistRatio stays high for > 10 minutes
  2. Latency degradation: P99 increases significantly during spikes
  3. Heap growth acceleration: Rate doesn't flatten during spikes

These indicate: - GOGC is too aggressive for the workload - Memory headroom is insufficient - Allocation pattern is pathological (e.g., massive objects)