Methods and systems for estimating the hypothetical performance of a messaging application are disclosed. A number of pool sizes may be identified, each pool size being a potential size for the memory allocated to the messaging application. An online simulation is running during the execution of the messaging application. The online simulation tracks the requests made by the messaging application and predicts the operation of the messaging application for each pool size. The data predicted includes the number of spill and unspill operations that read and write to disk. In addition, a method for calculating the age of the oldest message in a memory pool is disclosed. The age is used in determining the number of spill and unspill operations.