18 Feb
2009
18 Feb
'09
2:13 a.m.
I usually have found that the garbage collector is the hardest-working thing
in a java-web app (unfortunately) so when I noticed this new GC option in *Java
HotSpot 14*, I figured it might help (potentially a lot w/ high-volume
sites, which are the real GC churners).
Has anybody tried the "Garbage-First garbage collector (G1)" with Xwiki?
http://platform.xwiki.org/xwiki/bin/view/AdminGuide/Performances suggests
CATALINA_OPTS="-server -Xms128m -Xmx1024m -XX:MaxPermSize=128m -Dfile.encoding=utf-8
-Djava.awt.headless=true -XX:+UseParallelGC -XX:MaxGCPauseMillis=100"
However these instructions don't work, as-is, since UseParallelGC is the default
nowadays:
https://jdk6.dev.java.net/6uNea.html says:
The parallel collector is still the default GC and is the most efficient GC
for common household usage. G1 is meant to be an
alternative for the
concurrent collector. It is designed to be more predictable and enable fast
allocation with memory regions design.
Having actually worked on a variety of strange garbage collectors (for Lisp)
where all of java's problems had already been solved decades ago ...this
feels like a 1985-1990 flashback. :-)
Here's more info
Java SE 6 Update 14 Milestone Schedule b01 (1st available build) 02/11/09 FCS
Q2, 2009 *Feature List:*
- *Service Tag creation for Windows JRE (CR
6676849<http://bugs.sun.com/view_bug.do?bug_id=6676840>
)*: For more information about Service Tag, check out this technical
article
<http://java.sun.com/developer/technicalArticles/productregistration/>.
- *Java HotSpot 14*: including the new garbage collector, G1, Big
Decimal enhancements, TreeMap and HashMap enhancements, optimized compressed
OOP code generation.
The Garbage-First garbage collector (G1) is currently in beta. It is not
enabled by default. The parallel collector is still the default GC and is
the most efficient GC for common household usage. G1 is meant to be an
alternative for the concurrent collector. It is designed to be more
predictable and enable fast allocation with memory regions design.
To use it: -XX:+UnlockExperimentalVMOptions -XX:+UseG1GC
For more information about GC & G1, please see:
- G1 Technical Session in JavaOne
2008<http://developers.sun.com/learning/javaoneonline/j1sessn.jsp?sessn=…
- Java SE 6 GC
tunning<http://java.sun.com/javase/technologies/hotspot/gc/gc_tuning_6.h…
- Analysis of the Garbage-First Garbage
Collection<http://research.sun.com/jtech/pubs/04-g1-paper-ismm.pdf>
Niels
http://nielsmayer.com
PS: and will a different GC prevent sequences like this from happening?? see
http://nielsmayer.com/happy-happy-joy-joy.txt
PPS: some things that cause big-growth, like importing and exporting, might
not grow as large with a "generation scavenging" style GC as provided by
"Garbage-First Collection." Sometimes, just GCing a large structure being
iterated-over uses a lot more memory than it needs to; because the gc is
letting objects that should get collected early, like the incremental
results of an iteration, "build up" and increase overall memory size while
decreasing locality and cache-hits. This seems to cause a nearly exponential
performance dropoff when very little memory churn might occur if only things
got collected "at the right time."
http://research.sun.com/jtech/pubs/04-g1-paper-ismm.pdf suggests this new GC
will help:
2.4 Generational GarbageFirst
Generational garbage collection [34, 26] has several advantages, which a
collection strategy ignores at its peril. Newly allocated objects are
usually more likely to become garbage than older objects, and newly
allocated objects are also more likely to be the target of pointer
modi cations, if only because of initialization. We can take advantage of
both of these properties in Garbage-First in a flexible way. We can
heuristically designate a region as young when it is chosen as a mutator
allocation region. This commits the region to be a member of the next
collection set. In return for this loss of heuristic flexibility, we gain an
important benefit: remembered set processing is not required to consider
modifications in young regions. Reachable young objects will be scanned
after they are evacuated as a normal part of the next evacuation pause.
Note that a collection set can contain a mix of young and non-young
regions. Other than the special treatment for remembered sets described
above, both kinds of regions are treated uniformly.
...
2.5 Concurrent Marking
Concurrent marking is an important component of the system. It provides
collector completeness without imposing any order on region choice for
collection sets (as, for example, the Train algorithm of Hudson and Moss
[22] does). Further, it provides the live data information that allows
regions to be collected in \garbage-first" order. This section describes our
concurrent marking algorithm.