GARBAGE COLLECTION IN JAVA - PERFORMANCE CONSIDERATION

There are two primary measures of garbage collection performance. Throughput is the percentage of total time not spent in garbage collection, considered over long periods of time. Throughput includes time spent in allocation (but tuning for speed of allocation is generally not needed.) Pauses are the times when an application appears unresponsive because garbage collection is occurring.

Users have different requirements of garbage collection. For example, some consider the right metric for a web server to be throughput, since pauses during garbage collection may be tolerable. However, in an interactive graphics program even short pauses may negatively affect the user experience.

Some users are sensitive to other considerations. Footprint is the working set of a process, measured in pages and cache lines. On systems with limited physical memory or many processes, footprint may dictate scalability. Promptness is the time between when an object becomes dead and when the memory becomes available, an important consideration for distributed systems, including remote method invocation (RMI).

In general, a particular generation sizing chooses a trade-off between these considerations. For example, a very large young generation may maximize throughput, but does so at the expense of footprint, promptness, and pause times. young generation pauses can be minimized by using a small young generation at the expense of throughput. To a first approximation, the sizing of one generation does not affect the collection frequency and pause times for another generation.

There is no one right way to size generations. The best choice is determined by the way the application uses memory as well as user requirements. For this reason the virtual machine's choice of a garbage collector are not always optimal, and may be overridden by the user in the form of command line options.

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GARBAGE COLLECTION IN JAVA - GENERATIONS

At initialization, a maximum address space is virtually reserved but not allocated to physical memory unless it is needed. The complete address space reserved for object memory can be divided into the young and tenured generations.

The young generation consists of eden plus two survivor spaces. Objects are initially allocated in eden. One survivor space is empty at any time, and serves as a destination of the next, copying collection of any live objects in eden and the other survivor space. Objects are copied between survivor spaces in this way until they are old enough to be tenured, or copied to the tenured generation.

Other virtual machines, including the production virtual machine for the J2SE Platform version 1.2 for the Solaris Operating System, used two equally sized spaces for copying rather than one large eden plus two small spaces. This means the options for sizing the young generation are not directly comparable.

A third generation closely related to the tenured generation is the permanent generation. The permanent generation is special because it holds data needed by the virtual machine to describe objects that do not have an equivalence at the Java language level. For example objects describing classes and methods are stored in the permanent generation.

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JSESSIONID & APACHE STICKY SESSION

Recently I and my team implemented the sticky session feature from apache to our application. After that we started getting frequent session time out issue from testing team. After a long analysis we found, the browser is not flushing the JSESSIONID each time the new session get created from the server. So if the cached session is of node 1 and new session is get created from node 2 the user used to get the session time out problem as apache used to redirect the request to node 1, where there is no session available.We found out this after analyzing the request header and server logs.

So we came up with a solution to flush the JSESSIONID each time the user comes to login page. Bellow is the java script code for the same.


<script>
setCookiesecure("JSESSIONID", getCookie("JSESSIONID"), -1, "/", null, true);
</script>


function setCookiesecure (name,value,expires,path,domain,secure) {

document.cookie = name + "=" + value + ((expires) ? "; expires=" + expires : "") + ((path) ? ";path=" +      path : "") + ((domain) ? "; domain=" +domain :"") + ((secure) ? "; secure" : "");
}


function getCookie( name )
{
        var start = document.cookie.indexOf( name + "=" );
        var len = start + name.length + 1;
        if ( ( !start ) && ( name != document.cookie.substring(0, name.length ) ) ) { 
        return null; 
        }

        if ( start == -1 ) return null;

        var end = document.cookie.indexOf( ";", len ); 
        if ( end == -1 ) end = document.cookie.length;
        return unescape( document.cookie.substring( len, end ) );
}

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RESOURCE CACHING FROM APACHE

1. Uncomment the following lines from the "$apache_home/conf/httpd.conf".

LoadModule expires_module modules/mod_expires.so

LoadModule headers_module modules/mod_headers.so

LoadModule deflate_module modules/mod_deflate.so

2. Add the following lines to "$apache_home/conf/mod-jk.conf" at the last.

#cache settings

ExpiresActive On

    Options FollowSymLinks MultiViews

    AllowOverride All

    Order allow,deny

    Allow from all

    ExpiresDefault A25920003

Now you can check the request header, the resources can be in browser's cache for 30 days.

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STATIC RESOURCE APACHE PROXY

1. Dump your application static resources i.e images, css, html, javascript to "$apache_home\htdocs\".

2. Add the following lines to the "$apache_home/conf/mod-jk.conf" bellow "# Let Apache serve the images"

JkUnMount /__application__/css/* node1
JkUnMount /__application__/images/* node1
JkUnMount /__application__/xmlhttp/* node1
JkUnMount /__application__/js/* node1

Note: In case of multiple nodes, the load balancer name in place of node1.

3. Add the following lines to the "$apache_home/conf/uriworkermap.properties"  at the last.

!/*/css/*=*
!/*/images/*=*
!/*/xmlhttp/*=*
!/*/js/*=*

4. Now remove the static resources from your war/ ear.

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APACHE SETUP FOR JBOSS

1. Download Apache HTTP 2.2 from here(http://olex.openlogic.com/package_versions/download/9478?package_version_id=5577&path=openlogic%2Fapache%2F2.2.17%2Fopenlogic-apache-2.2.17-windows-ins-no-ssl-1.zip)

2. Unzip the file and double click on the installation file. Next follow the screen instructions to install.

3. After installation click the test configuration to test if Apache is working or not.

4. Now start the apache by clicking start from Apache sub menu.

5. Request for http://localhost/ to check the status.

6. Now time to confirure the proxy set up for Jboss. To do so we are going to use the the mod jk module in apache http.

7. Download themod_jk-1.2.31-httpd-2.2.3.s from here(http://www.apache.org/dist/tomcat/tomcat-connectors/jk/binaries/win32/jk-1.2.31/mod_jk-1.2.31-httpd-2.2.3.so).

8. Rename the file to mod_jk.so and put to the "$apache_home/modules/" folder.

9. Now open the "$apache_home/conf/httpd.conf" and add the following lines at the end.

<IfModule !mod_jk.c>

  LoadModule jk_module modules/mod_jk.so

</IfModule>

#Include mod_jk configuration file

Include "C:/Program Files/Apache Software Foundation/Apache2.2/conf/mod-jk.conf"

Note: change the path acording to your environment.

10. Create a file "$apache_home/conf/mod-jk.conf" and add the following content to that.

# Load mod_jk module

# Specify the filename of the mod_jk lib

#LoadModule jk_module modules/mod_jk.so

# Where to find workers.properties

#JkWorkersFile conf/workers.properties

JkWorkersFile conf/workers.properties

# Where to put jk logs

#JkLogFile logs/mod_jk.log

JkLogFile logs/mod_jk.log

# Set the jk log level [debug/error/info]

JkLogLevel info

# Select the log format

JkLogStampFormat "[%a %b %d %H:%M:%S %Y]"

# JkOptions indicates to send SSK KEY SIZE

# Notes:

# 1) Changed from +ForwardURICompat.

# 2) For mod_rewrite compatibility, use +ForwardURIProxy (default since 1.2.24)

# See http://tomcat.apache.org/security-jk.html

JkOptions +ForwardKeySize +ForwardURICompatUnparsed -ForwardDirectories

# JkRequestLogFormat

JkRequestLogFormat "%w %V %T"

# Mount your applications

JkMount /__application__/* node1

# Let Apache serve the images

#JkUnMount /__application__/images/* node1

# You can use external file for mount points.

# It will be checked for updates each 60 seconds.

# The format of the file is: /url=worker

# /examples/*=ajp13

JkMountFile  conf/uriworkermap.properties

# Add shared memory.

# This directive is present with 1.2.10 and

# later versions of mod_jk, and is needed for

# for load balancing to work properly

# Note: Replaced JkShmFile logs/jk.shm due to SELinux issues. Refer to

# https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=225452

JkShmFile run/jk.shm

# Add jkstatus for managing runtime data

<Location /jkstatus>

JkMount status

Order deny,allow

Deny from all

Allow from 127.0.0.1

</Location>

11. Now create "$apache_home/conf/workers.property" and add the following lines to that.

# Define list of workers that will be used

# For mapping requests

worker.list=node1,status

# Define Node1

worker.node1.port=8009

worker.node1.host=10.5.1.26

worker.node1.type=ajp13

#ping_mode as of mod_jk

worker.node1.ping_mode=A

#worker.node1.socket_timeout=10

        

# Status worker for managing load balancer

worker.status.type=status

#worker.localhost.host=10.5.1.26

Note: Change the host IP according to your IP. This configuration has been done for only one node. So there is no load balancer. You can add more than two nodes and a load balancer to manage. You can also enable sticky session if Jboss clustering and session replication is not enabled. 

12. Now create "$apache_home/conf/uriworkermap.properties" and add the following lines to that.

# Mount the Servlet context to the ajp13 worker

/jmx-console=node1

/jmx-console/*=node1

/web-console=node1

/web-console/*=node1

/test=node1

/test/*=node1

13. Now Jboss has to be configured to work with Apache.

A) Open "$jboss_Home/server//deploy/jboss.web-deployer/server.xml"

search for the following line and specify jvmRoot attribute to node1 as bellow.

Engine name = ”jboss.web” default host = “hostname” jvmRoute = “node1”

B)  Open "$jboss_Home/server/<instance_name>/deploy/jboss.web-deployer/meta-INF/jboss-service.xml"

search for "useJK" and make the value to "true" as bellow.

<attributename = “UseJK” > true</attribute>

14. Now restart the both web and app server. To test access test from web server DNS ie http://localhost/test.

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