04 Lug 11 How to use Huge Pages with Java and Linux

Hi.
Today we get how to use Huge Pages with Java from a Linux powered system.
While a Linux system generally splits memory segments into pages of 4 kb, Huge Pages are memory pages large 2Mb or more.
This is proved to increment speed when the application make use of large quantity of ram, like Java with a large heap (2 GB or more).
It’s correct to say that this is not always the correct configuration choice because the memory setted to be dedicated to Huge Pages cannot be accessed by the kernel (buffer cache) or by the applications, so that memory is subtracted from the virtual memory pool of the system. Since it’s very fast to make it a try and decide if use it or not, let me play with it.

Here in this example we will set 2,5 GB of RAM to be used as Huge Pages. Your mileage may vary.
HPM (huge page memory) is expressed in GB.

First: set the quantity of memory (bytes) to be defined as a shared memory segment
This is quickly found calculating this simple formula: ((HPM * 1024 * 1024 * 1024) – 1).
In our example: ((2,5 * 1024 * 1024 *1024) – 1) = 2684354559
Set it up online with this command:
echo 2684354559 > /proc/sys/kernel/shmmax
If you want to set it permanent at the next system reboot, append those two lines to your /etc/sysctl.conf file:
# Shared memory – max segment size: 2,5 Gb (-1 b)
kernel.shmmax = 2684354559

Second: set the number of reserved large memory pages
This is the number of reserved pages. Each page is large 2 Mb, so finding the number of pages to reserve is simple:
((HPM * 1024) / 2). In our example: ((2,5 * 1024) / 2) = 1280
Set it up online with this command:
echo 1280 > /proc/sys/vm/nr_hugepages
If you want to set it permanent at the next system reboot, append those two lines to your /etc/sysctl.conf file:
# Enable kernel to reserve 2,5GB / 2Mb large pages
vm.nr_hugepages = 1280

Third: set the system group id enabled to use huge pages
Java programs usually should not be fired by the root user. In my case, the group id of my program is “1001”.
Set it up online with this command:
echo 1001 > /proc/sys/vm/hugetlb_shm_group
If you want to set it permanent at the next system reboot, append those two lines to your /etc/sysctl.conf file:
# System group id that can use huge pages (hugepages gid: 1001)
vm.hugetlb_shm_group = 1001

Fourth: run the java program with the Huge Page support
In our example we are using the JVM distributed by Oracle. Other Java vendors may use different parameters to enable Huge Pages. They can even call Huge Pages differently.
The program can now be fired with “-XX:+UseLargePages -XX:LargePageSizeInBytes=2m”
My complete java parameters for my java program are:
java -d64 -server -Xms1900m -Xmx1900m -Xss192k -XX:+UseLargePages -XX:LargePageSizeInBytes=2m -XX:+UseParNewGC

Ciao ciao.
Dino Ciuffetti.