WhiteHedge Blog

In earlier part we have seen how memory is allocated to JVM on start up and brief about what GC is, let’s look at what are the parameters are available to tune the Heap size and Garbage collector, to work efficiently.
Normally parameters selected default by JVM helps application to run smooth it is called Ergonomics where it dynamically adjust heap space, but when you require high performance with applications that has high load of concurrent users or large volume of data, The best performance can be achieved in two steps 

1. by tweaking the parameters for the heap size first and then
2. To use appropriate GC collectors for better throughput.

We will see both. There are many parameters available for JVM but I will present only few relevant here. 

To Change Heap Size

If we pass following parameters to JVM at startup see how memory is allocated
export JAVA_OPTS='-server -Xms1175m -Xmx1175m -XX:NewSize=550m -XX:MaxNewSize=550m -XX:PermSize=125m -XX:MaxPermSize=125m'  

note: m stands for size in megabytes  

heap size allocation

Note that if newratio is not mentioned, then Old/tenured size is calculated as (total heap) – (NewSize). Also note that survivorratio is taken as 8 here calculated dynamically  

For sizing Heap try to first find out existing allocation use JMAP/JVISUALVM or other tools for existing sizing. (I have mentioned below note about it) 

 GC Tunning Parameters 

Note + sign invokes multi threaded and – invokes single threaded GC

Points for tuning 

1. Try first Ergonomics — Automatic Selections and Behavior Tuning
2. If you cannot measure the application measure the machine. Means use large size of heap available on the machine that your hardware supports.
3. There is no problem having large heap if you know the GC pauses and are acceptable for application.
4. Swapping memory means starvation, it’s a hell.
5. GC performance is proportion to heap size dedicated to young.
6. Using serial collector (1cpu) make sure you do not allocate more than half of heap size. When using parallel collector it is better to specify desired behavior rather than exact heap size values. Let the collector automatically and dynamically modify the heap sizes in order to achieve that behavior
7. Most cases it is preferred to allocate 40-60 /30-70 ratio to young and old size.
8. Do not allocate more memory to parmgen, its waste… Try to find out how much it uses by accessing application first and then add 20% extra to it. Generally it will be 10-15% for your total heap size. Use jstat or jmap to find usage.

Tools to monitor GC performance and heap sizing 

JSTAT : this tool is a command line tool available with JDK5 it provides the GC statistic, helps to understand GC behavior. It can be invoked as below. 

jstat –gcutil <processeid> <n seconds to repeat> <n no of repeats>
e.g.  root$:> /home/jdk/bin/jstat –gcutil 6584  3s 5

Above command will give output as below after every 3 seconds five times

Following is the column headings 

S0/s1 – survivor space, E – Eden, O- Old, P- Permgen, YGC – Young GC, YGCT – YGC Time, FGC – Full GC, FGCT – FGC Time, GCT – Cumulative GC Time. 

JMAP : this tool is a command line tool available with JDK5 it provides heap information allocated to JVM, it also helps to find live objects in the memory by taking heap dump. Command syntax is as follows

jmap –<heap/histo> <processeid>

e.g. 

root$:> /home/jdk/bin/jmap –heap 6584 
root$:> /home/jdk/bin/jmap –histo 6584 
root$:> /home/jdk/bin/jmap –histo:live 6584 

First example gives summery info of heap size allocation and also used and free space info. 

Second and third example gives dump of objects, its instance count and bytes allocated figures, if use “:live” option gives only live objects.   

JVISUALVM: this tool is a visual representation of GC and heap size. 

Read more articles:
More about GC and how it works in detail
http://java.sun.com/javase/technologies/hotspot/gc/gc_tuning_6.html#par_gc.oom
http://developers.sun.com/mobility/midp/articles/garbage/index.html 

Different tuning ideas and sizing examples mentioned
http://java.sun.com/performance/reference/whitepapers/tuning.html
http://blogs.sun.com/partnertech/entry/a_short_primer_to_java 

all jvm parameter options
http://blogs.sun.com/watt/resource/jvm-options-list.html
http://java.sun.com/javase/technologies/hotspot/vmoptions.jsp#PerformanceTuning 

FAQ about GC
http://java.sun.com/docs/hotspot/gc1.4.2/faq.html 

Mislenious
http://blogs.sun.com/fkieviet/entry/classloader_leaks_the_dreaded_java