In general, higher generations contain objects that live longer and are therefore much less likely to die. Each garbage collection only collects the generations up to some number, and never reclaims the objects in higher generations.
Objects move between generations by being promoted. For most allocation types, this means that the GC copies the objects from a segment in one generation to a segment in a higher generation. For allocation types
, the objects are not actually copied when they are promoted; but instead the whole segment is reattached to the higher generation. The automatic garbage collection promotes objects until they reach the blocking generation.
In the default configuration, there are 8 generations, numbered from 0 to 7. Generation 7 is used to keep objects that survived saving the image. Generations 4, 5 and 6 are not used. Generation 3 is the blocking generation, where long-lived objects accumulate. Generations 0,1, and 2 are ephemeral, and objects that survive a garbage collection in each of these generations are promoted to the next generation.
LispWorks User Guide and Reference Manual - 21 Dec 2011