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When a burstable cloud VM runs out of accrued CPU credits, it can no longer sustain CPU performance above its baseline using those credits. What happens next depends on the provider and instance mode: AWS EC2 Standard instances are brought back to baseline, AWS Unlimited instances can keep bursting using surplus credits and may incur charges, and Azure B-series VMs are throttled to base CPU performance.
What CPU credits control
CPU credits are a burst-capacity mechanism, not a general pool of compute shared by all cloud servers. On a burstable VM, credits accrue while CPU use is below a defined baseline and are spent when the VM runs above it. AWS describes this model for its EC2 burstable-performance instance types; Azure documents a similar model for its B-series VMs.
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 for burst capacity, not a guarantee that a particular amount of application work will finish per credit. AWS explains the credit and baseline concepts.
What happens on AWS EC2
The outcome depends on whether the instance is in Standard or Unlimited mode. Check the mode on the actual instance: defaults vary by family and can also be configured at the account level.
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Standard mode: CPU is limited to baseline
As an EC2 Standard instance’s accrued credits run low, AWS gradually reduces CPU utilization to the baseline rather than imposing a sudden drop. Once the balance is depleted, the instance cannot burst above baseline again until it accrues credits. A CPU-bound job that needs more than baseline capacity may take longer; work limited mainly by storage, network, or other bottlenecks may be less affected. AWS Standard mode details.
Unlimited mode: bursting can continue, with possible charges
An EC2 Unlimited instance can keep running above baseline after its accrued balance reaches zero by using surplus credits. Later operation below baseline earns credits that can pay down that surplus. AWS evaluates whether surplus is paid down over a rolling 24-hour period, or over the instance’s lifetime if shorter. If the surplus is not paid down under AWS’s rules, additional charges may apply for surplus credits.
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AWS identifies chargeable cases including surplus that exceeds what can be earned in a 24-hour period, and surplus remaining when an instance is stopped, terminated, or changed to Standard mode. Changing modes does not erase surplus already spent. See AWS Unlimited mode concepts for the rules.
What happens on Azure B-series
Azure B-series VMs accumulate credits below their base CPU performance threshold and spend them when operating above it. After the credits are consumed, the VM is throttled to base CPU performance until credits accrue again. This describes B-series specifically; it should not be generalized to every Azure VM family or other cloud providers. See Microsoft’s B-family size series documentation.
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How to check whether credits explain a slowdown
- Confirm the VM is burstable. Check the exact instance or size family. A fixed-performance VM should not be diagnosed as though it has an EC2 T-family credit balance.
- For EC2, check the credit mode. Inspect the instance in the EC2 console or use AWS CLI to check whether it is Standard or Unlimited. Do not infer the setting from the family name: T8i, T4g, T3a, and T3 generally launch Unlimited by default, while T2 generally launches Standard, but account-level settings can change defaults.
- Review CloudWatch credit metrics. Examine
CPUCreditBalanceandCPUCreditUsage. For Unlimited instances, also examineCPUSurplusCreditBalanceandCPUSurplusCreditsCharged. AWS provides these metrics at five-minute frequency. The AWS monitoring guide describes them. - Compare demand with baseline. A low balance alongside sustained CPU demand above baseline is consistent with a credit-related limit on a Standard instance. In Unlimited mode, surplus-credit metrics help distinguish continued paid bursting from a baseline slowdown.
- For Azure, check the selected B-series size and its base performance. Use the current Azure specifications for that size rather than assuming every B-series VM has the same baseline.
Can running out of credits cost extra?
Not necessarily. On AWS Standard mode, depletion limits bursting to baseline; the cited documentation does not describe an additional surplus-credit charge for Standard mode. On AWS Unlimited mode, the instance can continue bursting, and charges can apply when surplus credits are not paid down according to AWS’s rules.
AWS’s EC2 On-Demand pricing page currently lists Unlimited-mode credit rates of $0.04 per vCPU-hour for T4g Linux, RHEL, and SLES; $0.05 per vCPU-hour for T2/T3 Linux, RHEL, and SLES; and $0.096 per vCPU-hour for T2/T3 Windows and Windows with SQL Web. AWS states these rates apply across instance sizes, On-Demand, Spot, Reserved Instances, and Regions. Prices can change, so check the live EC2 pricing page for the exact configuration before estimating cost.
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What to do if the limit is hurting performance
- If bursts are occasional, confirm the configured mode and monitor credit balance, usage, surplus balance, and charges against the workload’s CPU pattern.
- If CPU demand regularly stays above baseline, compare the cost and performance of a larger fixed-performance instance or a different VM class with continued bursting and any resulting surplus charges.
- For Azure B-series, compare the workload’s demand with the selected size’s base CPU performance and choose a size or VM family suited to sustained demand if baseline throttling is unacceptable.
Do not assume every cloud uses the same credit rules
The documented AWS and Azure examples differ in what happens after accrued credits run out: EC2 Unlimited can continue bursting with surplus credits subject to billing rules, while Azure B-series returns to base CPU performance. Those rules do not establish how other providers or VM families behave. Check the documentation for the specific provider, family, and configuration rather than treating “CPU credits” as a universal cloud-server feature.
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