8 Storage Management

8.2 Generations and segments

The LispWorks garbage collector 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 i.e. are ephemeral. The LispWorks garbage collector 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.

In memory, a generation consists of a chain of segments. Each segment is a contiguous block of memory, beginning with a header and followed by the allocation area. The generations are numbered from 0 upwards, so that generation 0 is the youngest.

The first generation normally consists of two segments: the first segment is relatively small, and is where most of the allocation takes place. The second segment is called the big-chunk area, and is used for allocating large objects and when overflow occurs (see below for a discussion of overflow).

The second generation (generation 1) is an intermediate generation, for objects that have been promoted from generation 0 (typically for objects that live for some minutes). Long-lived objects created dynamically are normally allocated in the next generation, generation 2.

Note that the division between the generations is a result of the promotion mechanism, and is not a property of a piece of code itself. A piece of system software code that is loaded in the system (for example, a patch) is treated the same as any other code. The garbage collection code is explicitly loaded in the static area using the functionswitch-static-allocation.