The main interfaces are those which control the blocking generation.
For generations lower than the blocking generation, objects that survive are promoted, and the system does not automatically promote objects to higher generations. Thus if the application generates long-lived objects, they will accumulate in the blocking generation.
The behavior when the blocking generation grows is controlled by set-blocking-gen-num and set-gen-num-gc-threshold. It may also be useful to set the maximum segment size with set-maximum-segment-size.
Explicit garbage collection can be done by calling gc-generation and marking-gc. Since repeated use of marking-gc will cause a lot of fragmentation, the arguments what-to-copy and max-size-to-copy can be used to specify that part of the data should be collected by copying.
gc-generation can also be used to promote objects to a higher generation than the blocking generation.
It is normally less important to tune the ephemeral segments, that is the segments below the blocking generation. Functions that may be useful include set-default-segment-size, set-spare-keeping-policy and set-delay-promotion.
LispWorks User Guide and Reference Manual - 21 Dec 2011