IBM User Guide for Java V7 on Windows > Run Java applications

Configuring large page memory allocation

You can enable large page support, on systems that support it, by starting Java™ with the -Xlp option.

Large page usage is primarily intended to provide performance improvements to applications that allocate a great deal of memory and frequently access that memory. The large page performance improvements are a result of the reduced number of misses in the Translation Lookaside Buffer (TLB). The TLB maps a larger virtual storage area range and thus causes this improvement.

For the JVM to use large pages, your system must have an adequate number of contiguous large pages available. If large pages cannot be allocated, even when enough pages are available, possibly the large pages are not contiguous.

To use large pages, the user that runs Java must have the authority to "lock pages in memory". To enable this authority, as Administrator go to Control Panel > Administrative Tools > Local Security Policy and then find Local Policies > User Rights Assignment > Lock pages in memory. Alternatively, run secpol.msc. Add the user who runs the Java process, and reboot your machine. For more information, see these websites:

• http://technet2.microsoft.com/WindowsServer/en/library/e72dcdf6-fe17-49dd-a382-02baad31a1371033.mspx
• http://msdn.microsoft.com/en-us/library/aa366720(VS.85).aspx
• http://msdn.microsoft.com/en-us/library/aa366568(VS.85).aspx

Large page allocations only succeed if the local administrative policy for the JVM user has the Lock pages in memory setting enabled.

On Microsoft Windows Vista and later, and Windows 2008 and later, use of large pages is affected by the User Account Control (UAC) feature. When UAC is enabled, a regular user (a member of the Users group) can use the -Xlp option as normal. However, an administrative user (a member of the Administrators group) must run the application as an administrator to gain the privileges required to lock pages in memory. To run as administrator, right-click the application and click Run as administrator. If the user does not have the necessary privileges, the following error message is produced: System configuration does not support option '-Xlp'.

For more information about the -Xlp options, see JVM command-line options.

To obtain the large page sizes available and the current setting, use the -verbose:sizes option. Note the current settings are the requested sizes and not the sizes obtained. For object heap size information, check the -verbose:gc output.

When specifying -Xmx or -Xms, the physical storage allocated is based on the page size. For example, if using 2G large pages with Java options -Xmx1024M and -Xms 512K, the Java heap is allocated on a 2G large page. The real memory for the 2G large page is allocated immediately. Even though the Java heap is consuming a 2G large page, in this example, the maximum Java heap is 1024M with an initial Java heap of 512K as specified. If the 2G pagesize is not pageable, the 2G large page is never paged out as long as the JVM is running. For more information about the -Xmx option, see Garbage Collector command-line options.

Parent: Run Java applications

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