The macro block-promotion executes body and prevents promotion of objects into generation 2 during this execution. After body is executed, generations 0 and 1 are collected.
This is useful when a significant number of transient objects actually survive all the garbage collections on generation 1. These would normally then be promoted and, by default, never get collected. In such a situation, (mark-and-sweep 2) will free a large amount of space in generation 2. block-promotion can be thought of as doing set-promotion-count on generation 1 with an infinite count , for the duration of body.
block-promotion is suitable only for use in particular operations that are known to create such relatively long-lived, but transient, objects. In typical uses these are objects that live for a few seconds to several hours. An example usage is LispWorks compile-file , to ensure the transient compile-time data gets collected.
block-promotion has global scope and hence may not be useful in an application such as a multi-threaded server. During the execution of body , generation 1 grows to accomodate all the allocated data, which may have some negative effects on the behaviour of the system, in particular on its interactive response.
mark-and-sweep set-promotion-count