HP-UX Reference (11i v2 04/09) - 5 Miscellaneous Topics (vol 9)

dbc_min_pct(5) dbc_min_pct(5)

(Tunable Kernel Parameters)

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 ini-

tialization 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 com-

peting 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 parame-

ter 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.