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.

This tunable sets the upper bound for virtual page size requested by a user application, as set with the chatr command on the binary.

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect at the next reboot.

When Should the Value of This Tunable Be Raised?

This tunable should be raised when a user application 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 of the chatr'ed application, and if many users on the system chatr their applications for no good reason. In the first case, mistakenly chatr'ing an application to use a large page size (512 MB or more) when the application uses 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 64KB of memory total, or a scientific spare 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 will result in several frames of physical memory being wasted if any of a virtual large page is in core memory then all of it must be there. This is true because a virtual large page must be constructed of contiguous physical frames, which may not always be available in the quantity desired, leading to unneeded allocation delays.

In the second case, where several users chatr their application for no good reason, keeping this tunable low minimizes the performance hit on the rest of 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 when using chatr with their applications.

What Are the Side Effects of Lowering the Value?

The only side effect is that applications must run with smaller page sizes.

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.