When a burstable cloud VM runs out of accrued CPU credits, what happens depends on its provider and credit mode. AWS EC2 in Standard mode is gradually limited to its CPU baseline; in Unlimited mode, it can keep bursting using surplus credits, which may create extra charges. Azure B-series VMs are throttled to base CPU performance until credits build up again.
What CPU credits control
CPU credits are a way to account for burst capacity on certain virtual machines, not a general pool of compute used by every cloud server. A burstable VM earns credits when it runs below a defined CPU baseline and spends them when it runs above that baseline. AWS defines one CPU credit as one vCPU running at 100% utilization for one minute, or an equivalent combination of vCPUs, utilization and time. It is an accounting unit, not a guarantee that a particular amount of application work will finish per credit. AWS explains the credit and baseline concepts.
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If a workload regularly uses less than its baseline, its balance can grow. If it needs more CPU than its baseline for a sustained period, it spends that balance. The consequence of reaching zero depends on the VM’s rules.
What happens on AWS EC2
Standard mode: CPU is limited to baseline
In Standard mode, an EC2 burstable instance uses its accrued balance to burst above baseline. As the balance runs low, AWS gradually lowers CPU utilization toward baseline; when credits are depleted, the instance cannot burst above baseline until it earns more. This means a CPU-bound task that needs more than baseline capacity can take longer. A workload that is mostly waiting on storage or network activity may be less affected, because the documented limit is on CPU bursting. AWS documents Standard-mode behavior.
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Unlimited mode: it can keep bursting, with possible charges
In Unlimited mode, an instance can continue above baseline after its accrued balance reaches zero by using surplus credits. Later, CPU use below baseline can earn credits that pay down the surplus. AWS evaluates this over a rolling 24-hour period, or the instance’s lifetime if it has run for less than 24 hours. If average CPU use is at or below baseline over that period, the hourly instance price covers the spikes. Sustained above-baseline use can leave surplus credits unpaid and result in additional charges per vCPU-hour. AWS also identifies cases in which spent surplus credits are charged, including when they exceed what the instance can earn in a 24-hour period, or when the instance is stopped, terminated or changed to Standard mode. See AWS’s Unlimited-mode rules before changing a running instance’s configuration.
The currently listed AWS Unlimited-mode rates vary by family and operating-system category. AWS lists $0.04 per vCPU-hour for T4g Linux, RHEL and SLES; $0.05 for T2/T3 Linux, RHEL and SLES; and $0.096 for T2/T3 Windows and Windows with SQL Web. AWS says these rates apply across instance sizes, On-Demand, Spot, Reserved Instances and Regions. Prices can change, so verify the target configuration on the AWS EC2 On-Demand pricing page before estimating cost.
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How Azure B-series compares
Azure B-series VMs also earn credits below base CPU performance and spend them above it. When their credits are consumed, they are throttled to base CPU performance until credits accumulate again. That is similar to AWS EC2 Standard-mode depletion; it differs from AWS Unlimited mode, where surplus credits can sustain bursting subject to AWS billing rules. This comparison applies to Azure B-series, not every Azure VM family or cloud provider. Check the selected size’s specifications on Microsoft’s B-family VM page.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to check whether credits caused a slowdown
Confirm the VM is burstable
First verify that the specific VM family uses CPU credits. A fixed-performance VM should not be diagnosed as though it has an EC2 T-family credit balance. For EC2, inspect the instance’s credit configuration rather than inferring it from its family: AWS says T8i, T4g, T3a and T3 generally launch in Unlimited mode, while T2 generally launches in Standard mode, but account-level defaults can change the launch setting.
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Check EC2 mode and CloudWatch metrics
- In the EC2 console, inspect the instance’s credit specification for its current mode, Standard or Unlimited. AWS also documents checking and changing the setting with AWS CLI; use the live Unlimited-mode documentation for the applicable configuration procedure.
- In CloudWatch, review
CPUCreditBalanceandCPUCreditUsage. AWS reports these credit metrics at five-minute frequency. - For an Unlimited instance, also review
CPUSurplusCreditBalanceandCPUSurplusCreditsCharged. Together, these metrics help distinguish a VM that is being limited by its accrued balance from one that continues to burst using surplus credits and may incur charges. See AWS’s CloudWatch monitoring guide.
What to do if CPU demand stays above baseline
- Compare the workload’s average CPU demand with the VM’s baseline and determine whether bursts are occasional or sustained.
- If using Standard mode, consider whether baseline performance is sufficient or whether the workload needs a VM with more consistent CPU capacity.
- If using Unlimited mode, compare the cost of continued surplus-credit use with the cost of a larger fixed-performance instance or another VM class. Do not assume switching modes erases surplus credits already spent; AWS documents circumstances in which they are charged when an instance changes to Standard mode.
- For Azure B-series, compare the selected size’s base CPU performance with the workload’s needs, using the provider’s specifications for that size.
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