Automatic memory management is one of the most significant features of a Lisp system. Whenever an object, such as a cons cell, is required to hold an aggregate of values, the system calls the appropriate function to create a new object and fill it with the intended values. The programmer need not be concerned with the low level allocation and management of memory as the Lisp system provides this functionality automatically.
When an object is no longer required (that is, it has become "garbage"), the system must automatically reclaim ("collect") the space it occupies and reallocate the space to a new object. Whenever the space for new objects is exhausted, a "garbage collector" (GC) is run to determine (by a process of elimination) all the existing objects that are still required by the running program. Any other objects still in the image are necessarily garbage, and the space they occupy can be reclaimed.
For a description of how LispWorks uses the address space of different Operating Systems, and factors affecting the maximum image size, see Address Space and Image Size.
Garbage collection with a naive algorithm is extremely inefficient.
The LispWorks GC works in unison with the storage allocator to arrange allocated objects in a series of "generations". Each generation contains objects of a particular age. In practice most Lisp data objects are only required for a very short period of time. That is, they are ephemeral. The LispWorks GC concentrates its efforts on repeatedly scanning the most recent generation. Such a scan requires only a fraction of a second and reclaims most of the space allocated since the last collection. Any object in the most recent generation that survives a number of such collections is promoted to the next youngest generation. Eventually this older generation becomes full, and only then is it collected. The generations are numbered from 0 upwards, so that generation 0 is the youngest.
The remainder of this chapter describes the LispWorks GC in more detail. The implementation and the programmatic interface differ between 32-bit and 64-bit LispWorks.
LispWorks User Guide and Reference Manual - 21 Dec 2011