Skip to main content

Inside the Sun's JVM memory

Yesterday, my manager asked me about a problem concerning the eden space memory. So, this is a summary of JVM's memory management.

Heap Size
The allocation of memory for the JVM is specified using -X options when starting ColdFusion

JVM option Meaning
-Xms initial java heap size
-Xmx maximum java heap size
-Xmn the size of the heap for the young generation

(extended options)
-XX:MaxPermSize maximum permanent generation size (for creating object instances)

For efficient garbage collection, the -Xmn value should be lower than the -Xmx value.

Heap size does not determine the amount of memory your process uses
If you monitor your java process with an OS tool like top or taskmanager, you may see the amount of memory you use exceed the amount you have specified for -Xmx. -Xmx limits the java heap size, java will allocate memory for other things, including a stack for each thread. It is not unusual for the total memory consumption of the VM to exceed the value of -Xmx.

Garbage collection
There are essentially two GC threads running. One is a very lightweight thread which does "little" collections primarily on the Eden (a.k.a. Young) generation of the heap. The other is the Full GC thread which traverses the entire heap when there is not enough memory left to allocate space for objects which get promoted from the Eden to the older generation(s).
If there is a memory leak or inadequate heap allocated, eventually the older generation will start to run out of room causing the Full GC thread to run (nearly) continuously. Since this process "stops the world", ColdFusion won't be able to respond to requests and they'll start to back up.
The amount allocated for the Eden generation is the value specified with -Xmn. The amount allocated for the older generation is the value of -Xmx minus the -Xmn. Generally, you don't want the Eden to be too big or it will take too long for the GC to look through it for space that can be reclaimed.

Stack size
Each thread in the VM get's a stack. The stack size will limit the number of threads that you can have, too big of a stack size and you will run out of memory as each thread is allocated more memory than it needs.
JVM option Meaning
-Xss the stack size for each thread
-Xss determines the size of the stack: -Xss1024k. If the stack space is too small, eventually you will see an exception class java.lang.StackOverflowError .

Comments

Shikhar Mishra said…
Thanks, this was very informative.
Rahul said…
Thanks for the great information in your blog Selenium Training in Chennai

Popular posts from this blog

DCcduino usb drivers (CH340 / CH341 chipset)

I've just received my first arduino platform. It's a DCcduino board (a clone of Arduino Uno). As I had some difficulties to have it recognised by my MacBook, I decided to share its drivers. This card has a CH340 USB-to-serial chip. You can find drivers for this chip on the web site of the chinese manufacturer, here :

http://www.wch.cn/downloads.php?name=pro&proid=5

Or download it directly from my Google Drive. The archive contains drivers for Mac, Linux and Windows platforms.

https://drive.google.com/file/d/0B5okZr5AW4gaX2pZaWt6dVNaSFU/edit?usp=sharing

I hope this will help somebody.

UPDATE for Mac users with Yosemite :

Please, follow this extra instructions :
Install the CH340 driverRun the command in Terminal: sudo nvram boot-args="kext-dev-mode=1"Reboot

The great alternative to JRebel

I'm an old user and addict of JRebel. I started to use it on open source projects and in professional contexts. From the beginning, I've been convinced that the licensing mode was wrong because of its lifetime. Asking for license renewal each year is boring. So, I decided to look for FREE alternative solutions and finally I recently found one.

This solution is efficient for maven projects developed with Eclipse. It is base on :

Hotswap Agent project : http://hotswapagent.org/Dynamic Source Lookup plugin for Eclipse : https://github.com/ifedorenko/com.ifedorenko.m2e.sourcelookup
I tested it with Java 7 & Java 8. I work on web applications that run on Tomcat. I developed wih Spring (IoC), sometimes Hibernate and Vaadin



Hotswap Agent installation consists on the deployment of a patch for your JVM. You just have to download the corresponding patch here : https://github.com/dcevm/dcevm/releases Then, download the hotswap-agent.jar from here : https://github.com/HotswapProjects…

How to secure REST services exposed with Jersey (JAX-RS) using Spring Security

This is the challenge I had to perform. I have to open services to business partners and I want to secure them. I use a Java based application with Jersey API (JAX-RS).

The first thing to consider is : what kind of solution offers the best compromise between security and the effort I'll have to provide to maintain this solution?
After some discussions with developers and experts, the conclusion is : expose your services over https and use Basic authentication (Digest authentication and certificate based authentication are too complex for partners)

So, how to implement that? I delegate "https" to my Apache http servers. But I still need to handle authentication (and authorization of course). After long hours on Google, I understood that it is possible to manage security with Jersey by many many ways. Here is a short list :

Delegate simple authentication and authorization to your container (Tomcat) or to your frontend (Apache)Delegate authentication to your container or fro…