Introduction
Putting a JVM in a read only ramdisk file system improve the overall performance of Java on Ubuntu Linux.
It doesn't replace a SSD but rooming 200M of RAM to get some substantial performance is always welcome.
However there are some limitations with this technic:
- tmpfs is swappable. Under heavy loaded machine our ramdisk could be transferred in a mechanical hard drive.
- SquashFS is building a highly compressed file, reducing the IO and the size of the ramdisk. Not sure if an unsquashed JVM image would outperform a squashed image.
SquashFS
In order to put a JVM in a RAM disk we are going to use the tool SquashFS.
Use the following command to install the Squash FS tools.
sudo apt-get update sudo apt-get install squashfs-tools
Make a squashfs file out of your desired JVM, use LZMA2 compression rather than gzip.
cd /usr/lib/jvm sudo mkdir ramdisk mksquashfs /usr/lib/jvm/java-8-oracle /usr/lib/jvm/ramdisk/java-1.8.0.91-oracle.sqsh -comp xz
At this point you have an existing JVM squashed in .sqsh file with a roughly size of 160M:
user@machine:/media$ ls --block-size=M -al /usr/lib/jvm/ramdisk/java-1.8.0.91-oracle.sqsh -rw-r--r-- 1 user users 160M May 12 15:53 /usr/lib/jvm/ramdisk/java-1.8.0.91-oracle.sqsh
Mount Point
Create a mountpoint for the ramdisk:
sudo mkdir /media/ramdisk-java-8-oracle
Edit your /etc/fstab, define your mount point as a tmpfs file system and run your squashed JVM against this mount point by adding the following declarations:
# define a tmps mount point for Java ramdisk tmpfs /media/ramdisk-java-8-oracle tmpfs defaults,size=170M,mode=1777 0 0 # run a squashed JVM against our tmps mount point /usr/lib/jvm/ramdisk/java-1.8.0.91-oracle.sqsh /media/ramdisk-java-8-oracle squashfs ro,defaults,loop 0 0
Installation
At this point you need to reboot to initialize the ramdisk with your Squashed JVM. Now you have to proceed with a standard JVM installation.
- Check your Java environment
- Define a jinfo file
- Prepare a set of shell scripts to install and uninstall your new JVM.
in the /usr/lib/jvm directory we define the default-java to point to a local symlink who will point to our ramdisk:
user@machine:/usr/lib/jvm$ ls -al total 92 drwxr-xr-x 7 root root 4096 Jul 13 13:40 . drwxr-xr-x 205 root root 53248 Jul 13 09:22 .. lrwxrwxrwx 1 root root 34 Jul 7 15:33 default-java -> /usr/lib/jvm/ramdisk-java-8-oracle lrwxrwxrwx 1 root root 28 Jul 13 12:15 .default-java.jinfo -> .ramdisk-java-8-oracle.jinfo lrwxrwxrwx 1 root root 20 Jul 7 11:46 ramdisk-java-8-oracle -> /media/ramdisk-java-8-oracle -rw-r--r-- 1 root root 3569 Jul 13 10:24 .ramdisk-java-8-oracle.jinfo
We create a .ramdisk-java-oracle.jinfo with a unique priority id:
name=ramdisk-java-8-oracle priority=1073 section=main jre ControlPanel /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/ControlPanel jre java /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/java jre javaws /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/javaws jre jcontrol /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/jcontrol jre jjs /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/jjs jre keytool /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/keytool jre orbd /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/orbd jre pack200 /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/pack200 jre policytool /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/policytool jre rmid /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/rmid jre rmiregistry /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/rmiregistry jre servertool /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/servertool jre tnameserv /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/tnameserv jre unpack200 /usr/lib/jvm/ramdisk-java-8-oracle/jre/bin/unpack200 jre jexec /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/jexec jdk appletviewer /usr/lib/jvm/ramdisk-java-8-oracle/bin/appletviewer jdk extcheck /usr/lib/jvm/ramdisk-java-8-oracle/bin/extcheck jdk idlj /usr/lib/jvm/ramdisk-java-8-oracle/bin/idlj jdk jar /usr/lib/jvm/ramdisk-java-8-oracle/bin/jar jdk jarsigner /usr/lib/jvm/ramdisk-java-8-oracle/bin/jarsigner jdk javac /usr/lib/jvm/ramdisk-java-8-oracle/bin/javac jdk javadoc /usr/lib/jvm/ramdisk-java-8-oracle/bin/javadoc jdk javafxpackager /usr/lib/jvm/ramdisk-java-8-oracle/bin/javafxpackager jdk javah /usr/lib/jvm/ramdisk-java-8-oracle/bin/javah jdk javap /usr/lib/jvm/ramdisk-java-8-oracle/bin/javap jdk javapackager /usr/lib/jvm/ramdisk-java-8-oracle/bin/javapackager jdk jcmd /usr/lib/jvm/ramdisk-java-8-oracle/bin/jcmd jdk jconsole /usr/lib/jvm/ramdisk-java-8-oracle/bin/jconsole jdk jdb /usr/lib/jvm/ramdisk-java-8-oracle/bin/jdb jdk jdeps /usr/lib/jvm/ramdisk-java-8-oracle/bin/jdeps jdk jhat /usr/lib/jvm/ramdisk-java-8-oracle/bin/jhat jdk jinfo /usr/lib/jvm/ramdisk-java-8-oracle/bin/jinfo jdk jmap /usr/lib/jvm/ramdisk-java-8-oracle/bin/jmap jdk jmc /usr/lib/jvm/ramdisk-java-8-oracle/bin/jmc jdk jps /usr/lib/jvm/ramdisk-java-8-oracle/bin/jps jdk jrunscript /usr/lib/jvm/ramdisk-java-8-oracle/bin/jrunscript jdk jsadebugd /usr/lib/jvm/ramdisk-java-8-oracle/bin/jsadebugd jdk jstack /usr/lib/jvm/ramdisk-java-8-oracle/bin/jstack jdk jstat /usr/lib/jvm/ramdisk-java-8-oracle/bin/jstat jdk jstatd /usr/lib/jvm/ramdisk-java-8-oracle/bin/jstatd jdk jvisualvm /usr/lib/jvm/ramdisk-java-8-oracle/bin/jvisualvm jdk native2ascii /usr/lib/jvm/ramdisk-java-8-oracle/bin/native2ascii jdk rmic /usr/lib/jvm/ramdisk-java-8-oracle/bin/rmic jdk schemagen /usr/lib/jvm/ramdisk-java-8-oracle/bin/schemagen jdk serialver /usr/lib/jvm/ramdisk-java-8-oracle/bin/serialver jdk wsgen /usr/lib/jvm/ramdisk-java-8-oracle/bin/wsgen jdk wsimport /usr/lib/jvm/ramdisk-java-8-oracle/bin/wsimport jdk xjc /usr/lib/jvm/ramdisk-java-8-oracle/bin/xjc #plugin xulrunner-1.9-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so plugin firefox-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so #plugin iceape-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so #plugin iceweasel-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so #plugin mozilla-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so #plugin midbrowser-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so #plugin xulrunner-javaplugin.so /usr/lib/jvm/ramdisk-java-8-oracle/jre/lib/i386/libnpjp2.so
Link the .default-java.jinfo with .ramdisk-java-oracle.jinfo.
Then we create a bunch of install and remove shell scripts to update-alternatives either the default-java and the ramdisk-java-8-oracle.
First we update-alternatives ramdisk-java-8-oracle then default-java and finally update-java-alternatives our default-java.