How to Identify Garbage Collection Issues in Clockspring

Garbage collection (GC) issues are one of the most common causes of Clockspring performance problems and cluster instability.

When GC pressure increases, the JVM pauses application threads. During those pauses, Clockspring cannot:

• process data

• respond to heartbeats

• keep up with queues

This article explains how to recognize GC problems and how they typically show up in real systems.

Why GC Problems Matter

During a GC pause:

• application threads stop

• cluster coordination stops

• processing throughput drops to zero

If pauses are long or frequent enough, nodes can appear to “fall out of the cluster,” even though nothing is wrong with the network.

Common Signs of GC Pressure

Look for these symptoms together, not in isolation:

• periodic CPU spikes followed by quiet periods

• nodes disconnecting and reconnecting under load

• throughput dropping while queues grow

• slow UI responsiveness

• processors that appear idle even with data queued

These are classic GC pressure indicators.

Where to Look First

1. JVM Logs

GC activity is recorded in the JVM logs.

Signs to look for:

• frequent full GCs

• long pause times

• increasing frequency over time

If GC pauses line up with node disconnects, you’ve found the cause.

2. CPU Patterns

GC pressure often shows as:

• sudden spikes to high CPU

• brief plateaus

• repeating cycles

High CPU alone is not bad. High CPU caused by GC is.

3. Memory Behavior

Watch for:

• heap usage climbing steadily

• heap not returning to a stable baseline after GC

• increased GC frequency as the heap fills

This often indicates objects are being retained longer than expected.

Common Causes of GC Pressure in Clockspring

GC problems are usually caused by flow design or sizing, not bugs.

Typical causes include:

• very large FlowFiles

• excessive splitting of records

• large attributes or embedded content

• unbounded queues

• large in-memory batches

• downstream systems causing retries

Each increases memory churn or retention.

Why GC Issues Get Worse Over Time

GC problems often appear gradually.

Common pattern:

• system starts healthy

• queues slowly grow

• memory churn increases

• GC frequency rises

• pauses get longer

• node instability begins

Nothing “changed,” but pressure accumulated.

What Not to Assume

• High heap usage is not automatically bad

• Increasing heap size is not a fix by itself

• GC issues are rarely network-related

• Restarting hides the symptom, not the cause

GC pressure is a signal, not a failure mode.

What to Do Next

Once GC pressure is identified:

• review flow design

• reduce unnecessary splitting

• limit batch sizes

• address downstream bottlenecks

• confirm heap sizing is appropriate

These fixes reduce pressure instead of masking it.

Summary

Garbage collection issues in Clockspring are a leading cause of:

• node instability

• missed heartbeats

• degraded throughput

When you see cluster flapping or unexplained slowdowns, GC is one of the first places to look.