HP-UX Reference (11i v2 03/08) - 5 Miscellaneous Topics, 7 Device (Special) Files, 9 General Information, Index (vol 9)

d
dbc_max_pct(5) dbc_max_pct(5)
(Tunable Kernel Parameters)
NAME
dbc_max_pct - maximum percentage of memory to be used for caching file I/O data and metadata
VALUES
Failsafe
5
Default
50
Allowed values
The minimum value allowed is 1 (1% of physical memory). The maximum value allowed is 90 (90% of
physical memory). The value is further constrained in that it must be equal to or greater than
dbc_min_pct.
Specify a positive integer value.
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_max_pct specifies the maximum percentage of physical memory that may be con-
sumed by the dynamic buffer cache. The minimum amount of memory represented by the related tunable
parameter dbc_min_pct is permanently consumed by the buffer cache. As needed, the dynamic buffer
cache will consume memory to grow up to the maximum limit represented by dbc_max_pct if the physi-
cal memory is available, depending on competing requests for physical memory.
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 maximum cache size dictated by
dbc_max_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.
Restrictions on Changing
This tunable is dynamic (tuning will take effect immediately on the running system).
When Should the Value of This Tunable Be Raised?
The user may want to increase the value on systems running a large number of processes actively using
file data.
What are the Side Effects of Raising the Value?
There is no immediate side effect of raising this value. However, as the system requests more physical
pages to grow the buffer cache more memory will be consumed.
When Should the Value of This Tunable 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?
If the specified new maximum limit is lower than the memory currently being consumed by the buffer
cache, the buffer cache will immediately release the additional physical pages to the system to make them
available for other purposes.
On systems with a large number of processes using file data, if
dbc_max_pct is set to too low a value,
very high demand on the buffer cache can cause system performance degradation. This is also true when
using a fixed size buffer cache (when dbc_min_pct is equal to dbc_max_pct).
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.
HP-UX 11i Version 2: August 2003 − 1 − Hewlett-Packard Company Section 5−−67