This function checks to see if the amount of allocation from the previous call is more than
system:*allocation-interval*, and if it is, performs a mark and sweep and promotion on generation 0. It also tries to reduce the big-chunk area. This is a fairly brief operation, and can be used whenever some operation is finished and may have left some garbage. The system itself uses it after compiling and loading files, when waiting for input, etc.
LispWorks User Guide and Reference Manual - 20 Sep 2017