The default value is 2000000. The default value is T . It takes one argument, the amount of dynamic space in use before the GC measured in bytes. The function must take three arguments, the amount of dynamic spaced retained by the GC, the amount of dynamic space freed, and the new threshold which is the minimum amount of space in use before the next GC will occur.
All values are byte quantities.
The system periodically checks whether this threshold has been exceeded, and only then does a garbage collection. This variable's value is either a function of one argument or nil . The default value of this variable is nil . These variables' values are lists of functions to call before or after any GC occurs. The system provides these purely for side-effect, and the functions take no arguments.