Fixing CPU Credit Exhaustion in Azure B-Series VMs

Introduction

If you’re using Azure B-series virtual machines (VMs) and notice a sudden performance degradation—lagging apps, delayed responses, or high CPU wait times—chances are you’ve run into a common but often misunderstood problem: CPU credit exhaustion.

In this blog, we’ll break down:

• What B-series VMs are
• How CPU credits work
• What causes credit exhaustion
• How to identify the problem
• How to fix and prevent it

🔍 What Are B-Series VMs?

Azure B-series VMs are burstable virtual machines designed for workloads that run at low baseline performance but occasionally need to burst to higher performance levels.

You can think of them like a savings account:

• When your VM uses less than its baseline CPU, it accrues credits.
• When it needs more CPU than its baseline allows, it spends credits to burst.
• Once your credits are exhausted, performance gets throttled to baseline limits.

⚠️ What Happens When CPU Credits Are Exhausted?

Once the VM runs out of credits, Azure limits the CPU performance to the baseline percentage—even if there’s a higher load. This leads to:

• Slower application response times
• Longer processing queues
• Poor user experience
• High CPU Ready or Wait in monitoring tools

The system won’t go back to peak performance unless it earns more credits (by running idle) or the VM is restarted/upgraded.

🛠️ How to Identify CPU Credit Exhaustion

You can check credit status using Azure Monitor or Azure CLI.

✅ Azure Portal:

1. Go to your VM > Monitoring > Metrics
2. Select metric: CPU Credits Remaining
3. Set time range (last 24 hours or 7 days)
4. Look for a flat line near zero — that’s your culprit!

✅ Azure CLI:

az monitor metrics list --resource /subscriptions/{subid}/resourceGroups/{rg}/providers/Microsoft.Compute/virtualMachines/{vm-name} --metric "CPU Credits Remaining"

🧯 Fixing the Problem

1. Temporarily Reduce Load

Let the VM idle to build up credits again. This is only a short-term solution.

2. Upgrade to a Larger B-Series Size

Each size has a higher baseline and accrues more credits.

If your workload is consistently exceeding the baseline, move up the size ladder.

3. Switch to a Non-Burstable VM

If your workload is consistently high-CPU, move to D-series or F-series VMs where performance isn’t throttled.

4. Use Auto Scaling

Configure auto-scaling rules to temporarily move to a more powerful VM type during high load, and scale back later.

5. Refactor the Workload

Split background tasks into queues or serverless functions (e.g., Azure Functions) so your VM isn’t always under pressure.

🧪 Real-World Scenario

An e-commerce site running on a B1ms VM experienced slow checkout processes during sale days. Investigation revealed CPU Credits Remaining was at 0. They switched to a B2ms VM temporarily and later migrated to a D2s_v3 VM. Performance normalized instantly and stayed consistent without throttling.

🧩 Pro Tips

• Use Azure Advisor to get VM resizing recommendations.
• Monitor with Alerts: Set up an alert if CPU Credits Remaining < 10
• Regularly review CPU % alongside credit metrics to catch bottlenecks early.

✅ Summary

If you’re running business-critical apps on B-series VMs, monitoring and proactive scaling is key. While they’re cost-effective, they aren’t built for sustained high CPU workloads.

Got questions about optimizing your Azure VM setup? Feel free to reach out or drop a comment below!