G1GC vs ZGC Pause Behavior

Speaker Notes

Walk-through: Two side-by-side panels compare GC pause behavior. G1GC (left, red) shows pauses that scale with heap size — from 10ms on small heaps to 10+ seconds on large heaps. ZGC (right, blue) shows constant sub-1ms pauses regardless of heap size, at the cost of 5-15% throughput overhead from its load barrier on every object reference.

Key decision point: If your P99 GC pause exceeds 500ms, don’t try to tune G1GC parameters — switch the algorithm to ZGC or Shenandoah. Algorithm selection beats parameter tuning every time. ZGC’s throughput cost is real but predictable; G1GC’s worst-case pauses are unpredictable and grow with your data.