The charging takes place for every 100ms your code executes and for the number of times the code is triggered. This is a good thing because you no longer pay for an idle compute. Developers can Quit Worrying about the Machinery the Code Runs on
The promise given to developers through IaaS (Infrastructure as a Service)- one of the service models of cloud computing and serverless computing is that they can stop worrying about how many machines are needed at any given point of time, especially during peak hours, whether the machines are working optimally, whether all the security measures are offered and so on.