How to fix sync loop hang in Kubernetes

KubernetesADVANCEDHIGHUpdated Dec 6, 2025· 149 views

A sync loop hang occurs when the kubelet's main reconciliation loop becomes blocked or unresponsive. The node stops processing pod changes, leaving pods in inconsistent states and blocking cluster operations.

Quick fix

Fixes sync loop hang

ps aux | grep kubelet
sudo systemctl status kubelet
  1. Identify sync loop hang in kubelet logs
  2. Check node resources and I/O state
  3. Test API server connectivity from kubelet
  4. Check container runtime responsiveness
kubernetes — error
ps aux | grep kubelet
sudo systemctl status kubelet
sync loop hang

What this error means

The kubelet runs a sync loop that: 1. Monitors desired pod state from the API server 2. Reconciles actual pod state on the node 3. Creates/updates/deletes containers to match desired state When this loop hangs (blocks indefinitely), the kubelet: - Stops processing new pods - Stops cleaning up terminated pods - Cannot respond to API server requests - Appears to be frozen but may not crash Common causes: deadlocks in kubelet code, I/O operations blocking, network timeouts, or resource exhaustion.

Typical symptoms

  • Pods remain Pending indefinitely without progress
  • kubectl describe pod shows no recent events
  • Node becomes unresponsive to API server (NotReady status after timeout)
  • kubelet logs show no new entries (hung process)
  • Pod deletion requests are not processed
  • Cannot SSH into node (system resources exhausted)
  • kubelet process shows high CPU or is completely idle

Common causes

  1. Deadlock in kubelet pod sync logic
  2. I/O operation blocking (disk, network, storage)
  3. Network timeout to API server during critical operation
  4. Kubelet unable to acquire lock on pod state
  5. Container runtime hanging on container operation
  6. Insufficient system resources (memory, file descriptors)
  7. Bug in kubelet or container runtime version

How to fix "sync loop hang"

1Identify sync loop hang in kubelet logs

SSH into the affected node and check kubelet status:

bash
ps aux | grep kubelet
sudo systemctl status kubelet

Check kubelet logs for hang indicators:

bash
sudo journalctl -u kubelet --no-pager | tail -100

Look for:
- No log entries for extended period (> 30 seconds)
- Repeating error messages (stuck in retry loop)
- "sync pod" or "sync loop" keywords
- "context deadline exceeded"

Check if kubelet process is responsive:

bash
kill -0 <kubelet-pid>  # Exit code 0 = process exists
strace -p <kubelet-pid> 2>&1 | head -10  # See what it's doing

If strace shows I/O wait or network system calls, the process is blocked.

2Check node resources and I/O state

Verify node has sufficient resources:

bash
free -h  # Memory
df -h  # Disk space
blk-iosat  # I/O saturation
top -b -n1 | head -20  # CPU and process info

Check disk I/O specifically:

bash
sudo iostat -x 1 5  # 5 reports, 1 second intervals

Look for util% near 100% (disk saturation).

Check file descriptor limit:

bash
ulimit -n
lsof | wc -l  # Open files count

If disk is full or heavily I/O bound:

bash
sudo lsof | sort -k7 -n | tail -20  # Files by size
sudo du -sh /var/lib/kubelet/* | sort -rh | head

Free space if needed:

bash
sudo rm -rf /var/lib/kubelet/pods/*/volumes/*  # Temporary volumes
3Test API server connectivity from kubelet

The sync loop communicates with the API server. Test connectivity:

bash
curl -k https://<api-server-ip>:6443/api/v1/nodes

If timeout:
- Network partition or firewall issue
- API server overloaded
- kubelet network stuck in TIME_WAIT

Reset network connections:

bash
sudo systemctl restart networking  # On some distros
sudo ip route flush cache

Check kubelet request timeout setting:

bash
ps aux | grep kubelet | grep -o "--request-timeout=[^ ]*"

Default is 70s. If API server is slow, increase it:

bash
# Edit kubelet config
sudo nano /etc/sysconfig/kubelet
# Add: --request-timeout=120s
sudo systemctl restart kubelet
4Check container runtime responsiveness

The sync loop calls container runtime frequently. Verify runtime is responsive:

containerd:

bash
sudo ctr -a /run/containerd/containerd.sock version
sudo ctr -a /run/containerd/containerd.sock tasks list

Docker:

bash
sudo docker ps
sudo docker stats --no-stream

If runtime is slow/hung:
- Check runtime logs (see kubelet-runtime-error article)
- Restart runtime:

bash
sudo systemctl restart containerd  # or docker, crio

- But wait for kubelet to sync first; don't restart both simultaneously

Monitor runtime operations:

bash
sudo strace -p <runtime-pid> -e trace=network,open,openat 2>&1 | head -20
5Review recent kubelet code changes or upgrades

If sync loop hang started after a recent change:

Check current kubelet version:

bash
kubelet --version
kubectl get nodes -o wide  # Shows version

Check release notes for known issues:
- https://github.com/kubernetes/kubernetes/releases
- Search for "sync loop" and "hang"

Common problematic versions have patches. Upgrade if available:

bash
# For kubeadm clusters
sudo kubeadm upgrade plan
sudo kubeadm upgrade node

For managed services (EKS, GKE, AKS), check if a node update is available:

bash
# AWS EKS
aws eks describe-nodegroup --cluster-name <cluster> --nodegroup-name <group>

# GKE
gcloud container node-pools update <pool> --cluster <cluster> --enable-upgrade-on-boot
6Monitor kubelet with verbose logging

Enable debug logging to see what kubelet is doing:

bash
sudo nano /etc/sysconfig/kubelet
# Add: --v=4  # Increase verbosity (1-4)
sudo systemctl restart kubelet

Then monitor logs in real-time:

bash
sudo journalctl -u kubelet -f --output=short-iso

High verbosity will show:
- Pod sync operations
- API server requests
- Container operations
- Lock acquisitions

Watch for:
- Stuck on one operation for > 30 seconds
- "Timed out waiting for..." messages
- Panic or fatal errors

After debugging, reduce verbosity to avoid log spam:

bash
sudo nano /etc/sysconfig/kubelet
# Change: --v=2  # Or remove
sudo systemctl restart kubelet
7Force restart kubelet as emergency recovery

If sync loop is hung and blocking cluster operations:

bash
# Cordon node to prevent new pods
kubectl cordon <node-name>

# Force-restart kubelet (may lose data on tmpfs volumes)
sudo systemctl stop kubelet
sleep 5
sudo systemctl start kubelet

# Monitor recovery
sudo journalctl -u kubelet -f
kubectl get nodes -w

If kubelet won't stop:

bash
sudo killall -9 kubelet

Then restart:

bash
sudo systemctl start kubelet

Pods affected by the restart may show errors. Let the node recover:

bash
kubectl uncordon <node-name>
kubectl get pods -A --field-selector spec.nodeName=<node-name> -w

After recovery, investigate root cause (Steps 1-5).

8Replace node if sync loop hangs repeatedly

If the node exhibits sync loop hangs consistently:

1. Cordon the node:

bash
kubectl cordon <node-name>

2. Drain workloads:

bash
kubectl drain <node-name> --ignore-daemonsets --delete-emptydir-data

3. Remove the node from cluster:

bash
kubectl delete node <node-name>

4. Terminate the actual node:

bash
# AWS
aws ec2 terminate-instances --instance-ids <instance-id>

# GCP
gcloud compute instances delete <instance-name>

# Azure
az vm delete --resource-group <group> --name <vm-name>

5. New node will auto-join (if cluster auto-scaling is enabled):

bash
kubectl get nodes -w

For on-prem, decommission and replace the physical/VM hardware.

Advanced notes

Sync loop hangs are rare in stable Kubernetes versions but more common in bleeding-edge or heavily patched clusters. They indicate either a kubelet bug (file an issue on GitHub) or environmental problems (I/O bottleneck, network issue). For production, establish Node Problem Detector to automatically detect stuck kubelet and trigger node replacement. Kubernetes 1.18+ includes improvements to the sync loop for better responsiveness. Monitor kubelet restart count with Prometheus: rate(kubelet_container_manager_cgroup_manager_duration_seconds_bucket[5m]). For long-running nodes in production, periodic restarts (every 30 days) can prevent accumulation of lock-related issues. Deadlock in the sync loop is usually a golang runtime issue—ensure kubelet is not using too much memory (GC pauses can block). WSL2 may have sync loop issues under heavy I/O; upgrade kernel or consider native Linux deployment.

Frequently asked questions

What does "sync loop hang" mean?
The kubelet runs a sync loop that: 1. Monitors desired pod state from the API server 2. Reconciles actual pod state on the node 3.
How do I fix "sync loop hang"?
A sync loop hang occurs when the kubelet's main reconciliation loop becomes blocked or unresponsive. The node stops processing pod changes, leaving pods in inconsistent states and blocking cluster operations.
What causes "sync loop hang"?
Deadlock in kubelet pod sync logic I/O operation blocking (disk, network, storage) Network timeout to API server during critical operation
Is the "sync loop hang" error serious?
Yes. This is a high-severity error that usually blocks work until it is fixed, but the steps above resolve it.

Official resources & further reading