The original mark-and-sweep collector of XLISP has been replaced by an in-place two-generation generational collector. This means that there are now two types of garbage collections, minor and major ones. Minor collections occur relatively frequently but are very fast. Major ones occur rather rarely in typical usage, and are only marginally slower than mark-and-sweep collections. The net effect should be to reduce garbage collection pauses significantly.
In addition, the system is more aggressive about allocating more space when a substantial fraction of the allocated space is in use. This should also reduce the number of collections, but may lead to greater process memory growth. The impact on limited memory systems such as a Macintosh with a small partition is not clear at this point.
The new garbage collector requires more storage space per Lisp node. The overall requirements are increased by around 30%-50%. The default partition on the Macintosh will therefore be increased to 3M.