Editing Tracing garbage collection (section)

=== Stop-the-world vs. incremental vs. concurrent ===

Simple ''stop-the-world'' garbage collectors completely halt execution of the program to run a collection cycle, thus guaranteeing that new objects are not allocated and objects do not suddenly become unreachable while the collector is running.

This has the disadvantage that the program can perform no useful work while a collection cycle is running (sometimes called the "embarrassing pause"<ref>{{cite journal |last1=Steele |first1=Guy L. |author-link=Guy L. Steele Jr. |title=Multiprocessing Compactifying Garbage Collection |journal=Communications of the ACM |date=September 1975 |volume=18 |issue=9 |pages=495–508 |doi=10.1145/361002.361005 |doi-access=free |s2cid=29412244}}</ref>). Stop-the-world garbage collection is therefore mainly suitable for non-interactive programs. Its advantage is that it is both simpler to implement and faster than incremental garbage collection.

''Incremental'' and ''concurrent'' garbage collectors are designed to reduce this disruption by interleaving their work with activity from the main program. ''Incremental'' garbage collectors perform the garbage collection cycle in discrete phases, with program execution permitted between each phase (and sometimes during some phases). ''Concurrent'' garbage collectors do not stop program execution at all, except perhaps briefly when the program's execution stack is scanned. However, the sum of the incremental phases takes longer to complete than one batch garbage collection pass, so these garbage collectors may yield lower total throughput.

Careful design is necessary with these techniques to ensure that the main program does not interfere with the garbage collector and vice versa; for example, when the program needs to allocate a new object, the runtime system may either need to suspend it until the collection cycle is complete, or somehow notify the garbage collector that there exists a new, reachable object.