AWS Burstable Instances
In this modern cloud computing era, enterprises have to close day books with balance of gigs in bytes and Hz.
Innovation in cloud pricing models leads to various terminologies like baseline utilization, accrued CPU credits, burstable performance, unlimited mode and standard mode.
Let us see explore little more on these.
Baseline utilization is expressed as a percentage of vCPU utilization that is allowed for an hour for the type of EC2 instance when the accrued CPU credit is zero.
Caution: Resulting CPU utilization percentage from Linux top command may differ from the result of AWS CloudWatch CPU utilization metrics. We need to consider some details here.
- CloudWatch may include time stolen by hypervisor
Accrued CPU Credits:
Accrued Credit is applicable for burstable types of instances. At a millisecond-level resolution, these instance types continuously earn a specific rate of CPU credits depending on the type of instance. For example, for T2.micro, CPU credits that can be earned per hour is 6, maximum credits that can be earned is 144 and baseline is 10%. The unspent CPU resource is accrued as Accrued CPU Credits. If the Accrued CPU Credits go beyond 144, it is not added further.
CPU credits measure the amount of time that the virtual machine is allowed to run at its maximum capacity. One CPU credit is equal to 1 virtual CPU running at 100% utilization for 1 minute. If the machine has 60 credits, it can utilize 1 vCPU for 1 hour. If the machine’s CPU credit balance is zero, it will run at baseline performance.
Expiry of Accrued CPU Credits:
Accrued CPU credits on a running instance do not expire. If a T2 instance is stopped, the instance loses all its accrued credits. T3 and T4g won’t lose the CPU credit balance for seven days after a stop and the credits are lost thereafter; if started within seven days, no credits are lost.
T2, T3, T3a, and T4g instances have the ability to burst to a higher level than baseline when required by workload.
Configured as unlimited, a burstable performance instance can spend surplus credits to burst beyond the baseline. If CPU utilization falls below the baseline, it uses the CPU credits that it earns to pay down the surplus credits that it spent earlier. The ability to earn CPU credits to pay down surplus credits enables Amazon EC2 to average the CPU utilization of an instance over a 24-hour period. If the average CPU usage over a 24-hour period exceeds the baseline, the instance is billed for the additional usage at a flat additional rate per vCPU-hour.
For example, if T2.small (baseline 20%) runs at 30% CPU utilization on average over a 24-hour period without Accrued Credits, the instance will be billed for the additional 10% CPU usage at a flat additional rate per vCPU-hour.
When these instances have Accrued CPU credits, they can give performance more than baseline performance up to the Accrued CPU credits; without Accrued CPU credits, they can do baseline performance.
Types of Credits:
There are two types of credits
T2 Standard instances get 30 launch credits per vCPU at launch. Launch credits are spent first, before earned credits. Unspent launch credits are accrued in the CPU credit balance, but do not count towards the CPU credit balance limit.
Each instance continuously earns (at a millisecond-level resolution) a set rate of CPU credits per hour, depending on the instance size.