dbc_min_pct(5)

NAME

dbc_min_pct — minimum percentage of memory used for caching file I/O data and metadata

VALUES

Failsafe

5

Default

5

Allowed values

The minimum value allowed is 1 (1% of physical memory). The maximum value allowed is 70 (70% of physical memory). The value is further constrained in that it must be equal to or less than dbc_max_pct.

Specify a positive integer value.

Recommended values

The minimum cache size dictated by dbc_min_pct rarely needs to be modified from its default value. However, users may need to change this tunable on systems with exceptionally large or small amount of physical memory, or on systems with an exceptionally large or small number of system processes that actively use disk I/O for file data.

To determine a reasonable (and conservative) value for the minimum cache size in megabytes use the following formula:

(number-of-system-processes)*(largest-file-system-block-size)/1024

To determine the value for dbc_min_pct, divide the result by the number of megabytes of physical memory installed in the computer and multiply that value by 100 to obtain the correct value in percent.

Only those processes that actively use file data should be included in the calculation. All others can be excluded. Here are some examples of what processes should be included in or excluded from the calculation:

Include:: NFS daemons, text formatters such as nroff, database management applications, text editors, compilers, and so on, that access or use source and/or output files stored in one or more file systems mounted on the system.
Exclude:: X-display applications, hpterm, rlogin, login shells, system daemons, telnet or uucp connections, and so on. These processes use very little, if any, disk I/O for file data.

DESCRIPTION

During file-system I/O operations, data is stored in a buffer cache, the size of which can be fixed or dynamically allocated. When the parameters dbc_min_pct and dbc_max_pct are different, the size of the buffer cache grows or shrinks dynamically depending on competing requests for system memory.

The value of dbc_min_pct specifies the minimum percentage of physical memory that is reserved for use by the dynamic buffer cache.

By setting both dbc_min_pct and dbc_max_pct to the same value the kernel creates a fixed size buffer cache that uses exactly that percentage of physical memory and does not change in size.

Who Is Expected to Change This Tunable?

The minimum cache size dictated by dbc_min_pct rarely needs to be modified from its default value. However, users may need to change this value in systems with exceptionally large or small amount of physical memory, or on systems with exceptionally large or small number of system processes that actively use disk I/O for file data.

Restrictions on Changing

This tunable is dynamic (tuning will take effect immediately on the running system). Tuning of this parameter will fail if there is not enough free physical memory to grow the file system buffer cache to the requested new minimum value.

When Should the Value of This Tunable Be Raised?

The value of this minimum limit rarely needs to be increased. Low system performance (especially at initialization time) on a system with file system I/O-intensive processes may be an indication that the value of dbc_min_pct is too low. However, in most cases it is recommended that the value of the maximum limit, dbc_max_pct, is raised instead.

What are the Side Effects of Raising the Value?

The amount of memory reserved for the static (minimum) portion of the dynamic buffer cache, dictated by dbc_min_pct, can not be used on the system for other purposes. The specified percentage of memory is consumed immediately when this tunable value is raised. The user must be careful not to raise this value so high that it can eventually cause memory pressure.

When Should the Value of This Tunable Be Lowered?

The default value of this minimum limit rarely needs to be lowered. The value can be lowered to allow a larger percentage of memory to be used for purposes other than file system I/O caching, depending on competing requests.

What are the Side Effects of Lowering the Value?

There is no immediate side effect when dynamically lowering this tunable value. If there are many competing requests for physical memory, and dbc_min_pct is set to too low a value, very high demand on the buffer cache can eventually cause system performance degradation.

What Other Tunable Values Should Be Changed at the Same Time?

The value of dbc_min_pct must be equal to or less than the value of dbc_max_pct. Kernel checks during tunable setting ensure this.

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.

Other tunable parameters related to the static buffer cache that existed in previous HP-UX releases are now considered obsolete. The recommended way to obtain a fixed size buffer cache is to set dbc_min_pct equal to dbc_max_pct.

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

dbc_min_pct was developed by HP.