Large pages

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The main purpose for large page usage is to improve system performance for high performance computing (HPC) applications or any memory-access-intensive application that uses large amounts of virtual memory. The improvement in system performance stems from the reduction of translation lookaside buffer (TLB) misses due to the ability of the TLB to map to a larger virtual memory range.

Large pages also improve memory prefetching by eliminating the need to restart prefetch operations on 4 KB boundaries. AIX® supports large page usage by both 32-bit and 64-bit applications.

The POWER4 large page architecture requires all the virtual pages in a 256 MB segment to be the same size. AIX supports this architecture by using a mixed mode process model such that some segments in a process are backed with 4 KB pages, while other segments are backed with 16 MB pages. Applications can request that their heap segments or memory segments be backed with large pages. For detailed information, refer to Application configuration for large pages.

AIX maintains separate 4 KB and 16 MB physical memory pools. You can specify the amount of physical memory in the 16 MB memory pool using the vmo command. The large page pool is dynamic, so the amount of physical memory that you specify takes effect immediately and does not require a system reboot. The remaining physical memory backs the 4 KB virtual pages.

AIX treats large pages as pinned memory. AIX does not provide paging support for large pages. The data of an application that is backed by large pages remains in physical memory until the application completes. A security access control mechanism prevents unauthorized applications from using large pages or large page physical memory. The security access control mechanism also prevents unauthorized users from using large pages for their applications. For non-root user ids, you must enable the CAP_BYPASS_RAC_VMM capability with the chuser command in order to use large pages. The following example demonstrates how to grant the CAP_BYPASS_RAC_VMM capability as the superuser:

# chuser capabilities=CAP_BYPASS_RAC_VMM,CAP_PROPAGATE <user id>