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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.
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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.
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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 176M160M:
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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 176M160M May 12 15:53 /usr/lib/jvm/ramdisk/java-1.8.0.91-oracle.sqsh |
Mount Point
Create a mountpoint for the ramdisk:
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sudo mkdir /media/ramdisk-java-8-oracle |
Edit your /etc/fstab and , define your mount point as a tmpfs file system and run your squashed JVM against this mount point by adding the following declarations:
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# changedefine a thetmps defaultmount sizepoint for theJava ramdisk tmpfs /media/ramdisk-java-8-oracle tmpfs defaults,size=350M170M,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 |
If you reboot at this point you will have a ramdisk of 350M defined at /media/ramdisk-java-8-oracle filled with your squashed JVM:
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user@machine:/media$ du -hs ramdisk-java-8-oracle/
349M ramdisk-java-8-oracle/ |
That way we know how correctly size what we need for our ramdisk.
350M for a java-1.8.0.91 from Oracle.
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:
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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 |
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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.