set-gc-parameters &key maximum-buffer-size minimum-buffer-size big-object promote-min-buffer promote-max-buffer new-generation-size minimum-overflow maximum-overflow minimum-for-sweep minimum-for-promote enlarge-by-segments => <no values>
An object that is bigger than this value is "big". That is, it is not allocated from the small objects buffer, but from the big-chunk area (if it is allocated in generation 0 in the normal way).
Controls the minimum enlargement of generation gen-num , for gen-num > 0. Value 0 means the generation is not expanded. Otherwise, new-generation-size must be a fixnum in the exclusive range (10000, 100000000) and the minimum expansion is then new-generation-size * gen-num words.
Controls the frequency of promotions. Setting minimum-for-promote to a high value causes the system to promote less frequently. This may improve performance for programs that allocate a lot of data for a short term and then delete it.
Controls when a mark and sweep takes place. Setting minimum-for-sweep to a high value causes the system to mark and sweep less often, which means it has to grow. The CPU time spent in garbage collection is mostly smaller, but the process is bigger and may cause more disk access.
LispWorks User Guide and Reference Manual - 21 Dec 2011