6.6.4 Changing page sizes in WAS

Enabling an installation to utilize large pages is a multistep process.

The following steps will guide you through enabling large page sizes on a WAS installation:

1. Determine how much large page space is required.

2. Enable large page use in AIX.

3. Enable large page use in WAS.

4. Verify actual large page usage.

In the following sections, we explain each step in more detail.


Step 1 - Determine how much large page space is required on the server

If 16 MB page sizes are used, AIX must be configured to utilize this feature. This will require an estimate of the amount of space that the application will need. Make the size larger than the maximum heap size, while accounting for future application memory needs.


Step 2 - Enable large page sizes in AIX

Refer to Table 6-6 to determine the page sizes supported on your particular server. Alternatively, you can issue the pagesize -af command.

The following example shows how to allocate 4 GB of RAM for use for 16 MB large pages. All commands must be run as root user.

To enable the use of large pages in AIX, follow these steps:

  • Use the Virtual Memory Manager command vmo in conjunction with the -o and -r flags to set the new value permanently. The -o option sets the new size and the -r flag tells the VMM to set this value during boot. You must set both lgpg_regions and lgpg_size. Issue the command:

    vmo -r -o lgpg_regions=256 -o lgpg_size=16777216

    This command sets 256 regions of 16 MB pages, which will result in 4 GB of RAM to be allocated, and only applications that are enabled to utilize 16 MB pages will be able to use this section of memory.
  • These settings do not require a reboot, so they must be added to the boot image in order to take effect on every boot.

    To do this, issue the command:

    bosboot -ad /dev/ipldevice

  • Reboot the server.
  • The user that the WebSphere process executes under will need the capability to utilize the 16 MB large pages. This is accomplished through the command:

    chuser capabilities=CAP_BYPASS_RAC_VMM,CAP_PROPAGATE $USER

    Where $USER is the user ID that WebSphere runs under.

    AIX is now ready for WebSphere to utilize 16 MB large pages.