1. A computer program product to tune garbage collection of virtual machines, the computer program product comprising: a tangible computer readable storage medium having computer readable program code embodied therewith, the computer readable program code comprising: computer readable program code configured to update a load variable to reflect that a new garbage collection process has started, the load variable having a value that is based at least in part on a number of garbage collection processes that are currently running on a physical computing device; computer readable program code configured to determine a number of garbage collection threads to participate in garbage collection based at least in part on the current value of the load variable, where the garbage collection threads are used to perform at least a portion of at least one phase of the new garbage collection process; and computer readable program code configured to re-update the load variable to reflect completion of the new garbage collection process.