Wednesday, October 17, 2007

GC-algorithm keywords

http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html is a good intro to generational GC

* Within the Eden (youngest generation area), you can use schedule your garbage collector visits most frequently.
* Within the old generation area, you can schedule your GC visits with least frequency.
* Now, the GC algorithm can be ref count in one area, and a tracer in another.

The *heap* can be divided into old generation area and young (Eden + survivors).

http://articles.techrepublic.com.com/5100-10878_11-6108296.html focuses on java 1.5 GC

--keywords, based on an intro to GC algorithm : http://www.javaworld.com/javaworld/jw-12-2001/jw-1207-java101.html

* copiers -- advantage: no fragmentation

* generational copiers -- a generational algorithm is-a copier(?). Objects are copied from young to old generation subheap.

* subheap gc -- each subheap can have a different gc daemon thread

* concurrent subheap gc -- each subheap's gc daemon thread can run simultaneously

* ref-counters ^ tracers -- basically the 2 main types of algorithms. I think most modern algorithms are tracers, including generational

* mark-and-sweep ---- a tracer feature. Unmarked => reclaimed

* path-from-root ---- a tracer feature.

* overhead -- some algorithms adds dereference-overhead (handle table) to every object access [1], while other algorithms adds extra cpu cycles only when GC thread runs -- daemon-overhead. The first type of overhead is found in ref-counters and handle tables.

[1] read/write

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