reduce-memory has the same effect as clean-down, except that clean-down may temporarily increase the size of the image in order to be able to promote from lower generations.
reduce-memory never increases the image size, which means that it may fail to promote. This will cause future garbage collections to be slower, until the promotion actually occurs.
reduce-memory is intended to be used when the operating system signals that the memory is low. Using clean-down in this situation may cause a temporary increase in size, which may cause the system to run out of memory, or maybe just kill the Lisp process. In other circumstances clean-down should do a better job (and you might also consider try-move-in-generation).
nil, which is different from clean-down where it defaults to
reduce-memorywith no argument or
(clean-down nil)by trying to reduce the memory.
(clean-down nil)frees and promotes, but does not try to reduce the size (and may actually increase it).
reduce-memoryis implemented only in 32-bit LispWorks.
LispWorks User Guide and Reference Manual - 13 Feb 2015