vps_chatr_ceiling(5)

NAME

vps_chatr_ceiling — maximum (in kilobytes) of user selectable page size

VALUES

Default

1048576 (KB)

Allowed values

Minimum: 4 (KB)

Maximum: 4194304 (KB)

DESCRIPTION

The Translation Look-aside Buffer (TLB) is a microprocessor feature for virtual memory, where the most recent physical to virtual address translations are cached, in the expectation that these translations are likely to be needed again soon. This is based on the principles of spatial and temporal locality of address references in programs. Historically, the TLB was entirely managed within hardware to achieve speed optimizations while sacrificing the flexibility of software implementations, for example, easily changed algorithms or table implementations.

In recent years, the flexibility of a software implementation of the TLB has regained importance over pure hardware speed. Specifically, the idea of logical grouping of physical frames (whose size is fixed in hardware) into "superpages" or "large pages", which can be represented in software TLB algorithms using a single base address translation for many physical frames, significantly reduces the lost cycles due to page faults assuming reasonable spatial and temporal locality. For example, consider a scientific application working on an array where each element requires 1K of memory. Using the usual 4K physical frame size, and referencing the array sequentially causes a page fault that requires the page be read into memory from disk or swap, and loads the TLB with the frame base address translation every fifth element.

The vps_chatr_ceiling tunable parameter sets the upper bound for virtual page size on files that a user specifies, through chatr, or any programmatic interfaces or other mechanisms. (Programmatic interface refers to an interface that can be called from a program.)

Originally, this tunable was created to set an upper bound for virtual page size set by the user through the chatr command on the binary. Hence the parameter name vps_chatr_ceiling (where "vps" refers to variable page size).

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect for any subsequent physical memory allocations. It does not affect any physical memory that has already been allocated.

When Should the Value of This Tunable Be Raised?

This tunable should be raised when a user application, or in general, the system with known large memory set usage (such as a database) is expected to need larger pages than the current value allows.

What Are the Side Effects of Raising the Value?

The side effects depend on the actual memory usage in the system and whether large page sizes are used indiscriminately, that is, with no performance justification. In the first case, mistakenly indicating that an application use a large page size (512 MB or more, for example) when the application accesses memory in a sparse pattern, or has a much smaller working set, in general. For example, an application uses a shell script which only needs 64 KB of memory total, or a scientific sparse array analysis program that works on large data sets, but only on very small portions of the data, where the rest can be swapped out or not even allocated. Setting this value for the application results in several frames of physical memory being wasted, because if any of a virtual large page is in core memory, then all of it must be loaded. This could potentially lead to unnecessary pageouts and pageins, that is, excessive disk activity, leading to performance degradation.

In the second case, where several users request large page sizes for their application for no good reason, keeping this tunable low minimizes the performance hit on the rest of the system.

Modern architectures support very large page sizes (up to 4 GB for IA and up to 1 GB for PA-RISC). Setting vps_chatr_ceiling to very high values (anything above 64 KB) should be done with extreme caution. It can lead to excessive memory consumption and quickly deplete the amount of free memory available on the system.

When Should the Value of This Tunable Be Lowered?

This tunable should be lowered if no user application actually needs large pages sized at the current tunable value, to minimize the chance of a mistaken or malicious user causing wasted physical frames by requesting unnecessarily large page sizes.

What Are the Side Effects of Lowering the Value?

One side effect is that applications run with smaller page sizes. Also, the system uses smaller page sizes for certain system pages.

What Other Tunables Should Be Changed at the Same Time?

None.

WARNINGS

All HP-UX kernel tunable parameters are release specific. This parameter may be removed or have its meaning changed in future releases of HP-UX.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter values. After installation, some tunable parameters may no longer be at the default or recommended values. For information about the effects of installation on tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

vps_chatr_ceiling was developed by HP.