1. A method for using observed thread activity to dynamically tune a virtual machine for responsiveness or throughput, comprising: in a computer system, performing the operations of: observing the activity of a thread in a set of threads to determine whether the thread needs at least one of responsiveness or throughput, wherein observing the activity of a thread involves identifying a priority for the thread, wherein if the thread is a high-priority thread, the thread is categorized as needing responsiveness; gathering statistics for the thread from the observations of thread activity to determine a level of responsiveness needed by the thread, wherein gathering the statistics comprises tracking at least one of frequency of context switches of high-priority threads and intervals between the execution of high-priority threads, and maintaining a set of measured values corresponding to the level of responsiveness, the measured values being decayed over time to ensure that the measured values reflect recent behavior; and dynamically tuning a virtual machine upon which the thread will execute based on the gathered statistics and the level of responsiveness.