Tunable Base Page Size

Note 1

The default block size for the HFS filesystem is 8kB. HFS filesystems

created with a filesystem block size at least equal to the base page size

function correctly. HFS filesystems configured with a 64k block size (and

an 8k fragment size) will work with any supported base page size.

Please refer to the mkfs_hfs(1M) manpage for complete details.

Performance considerations

The kernel operations that manage virtual memory on behalf of applications are more efficient when

the base page size is larger. Starting processes with large code segments and managing the virtual

memory when large data objects are created can be significantly faster with large base pages.

Since physical memory is managed in units of base pages, some memory allocations must be

rounded up to a page boundary. This can cause inefficiencies if the object size is significantly smaller

than a base page. For large objects, those a megabyte in size or larger, the rounding effect is of no

consequence, even for the largest base page size. For objects a few hundred bytes in size, the

rounding impact can be substantial at the larger base page sizes.

Filesystem reads and writes through the Unified File Cache are done at page granularity. If even one

byte is written, the entire base page must be transferred. Depending on the application data

reference pattern, the larger base page sizes can cause more disk traffic than the default size. This

does not hold true if the application is using Direct I/O. Direct I/O bypasses the File Cache and goes

directly to the disk.

Some aspects of large base pages are an unqualified performance benefit. Other aspects depend on

the behavior of applications. On balance, HP has found that the performance "sweet spot" is a 16

kB base page size. Many application workloads will show a slight performance improvement with

16 kB base pages as compared to the default. Some workloads, particularly those that manipulate a

large number of small objects, are better off with 8 kB or 4 kB base pages. It would be unusual for

an application workload to benefit from base pages as large as 64 kB, and therefore we do not

recommend that configuration.

Finally, as customer applications grow to use larger and larger amounts of memory, they would

benefit from an increase in the system base page size.

Summary

The Tunable Base Page Size enhancement to HP-UX 11i v3 Update 3 changes the base page size

managed by the HP-UX kernel on Integrity platforms to be a tunable parameter. This allows customers

to select the base page size that is best suited to their application workload.