Monday, March 9, 2009

what triggers GC -- various comments

Some big trading engine (UBS?) runs just 1 GC cycle each day. I wonder how often GC runs normally.

P155 [[SanFrancisco]] nicely outlines that GC can start
* upon alloc-failure
* when jvm explicitly waits (IO, sleep ...)
* every 10 seconds, but at a low priority

Synchronous GC is predominant, but google "asynchronous garbage collection idle periods of low activity" and you see GC can start when system is idle. Creditex java guys also hinted at that.

In my experiment, I do see GC springs into action when app is paused.

http://java.sun.com/docs/hotspot/gc1.4.2/faq.html says about JDK1.4 --
* In the default garbage collector a generation is collected when it is full (i.e., when no further allocations can be done from that generation).
* The concurrent low pause collector starts a collection when the occupancy of the tenured generation reaches a specified value (by default 68%).
* The incremental low pause collector collects a portion of the tenured generation during each young generation collection.
* A collection can also be started explicitly by the application.

http://www.softwareengineeringsolutions.com/blogs/2010/04/30/garbage-collection-in-java-part-2/ drills into alloc-failure --

The JVM is unable to fulfill the request (alloc-failure), and so it awakens the garbage collector to come to its aid. This results in a Young Generation collection cycle.

a Young Generation collection is called for because there isn't enough space available in Eden to satisfy this allocation request (alloc-failure). However, unlike the earlier scenario, the promotion of the middle-aged object from the active Survivor space to being an elderly object in the old generation cannot proceed because there is no free space available in the Old Generation. In this case, the Young Generation collection is put on hold, while an Old Generation collection cycle is spun up to free up space in the Old Generation.

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