dbc_min_pct(5) dbc_min_pct(5)
Tunable Kernel Parameters
NAME [Toc] [Back]
dbc_min_pct - minimum percentage of memory used for caching file I/O
data and metadata
VALUES [Toc] [Back]
Failsafe
5
Default [Toc] [Back]
5
Allowed values [Toc] [Back]
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 [Toc] [Back]
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.
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dbc_min_pct(5) dbc_min_pct(5)
Tunable Kernel Parameters
DESCRIPTION [Toc] [Back]
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 [Toc] [Back]
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
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dbc_min_pct(5) dbc_min_pct(5)
Tunable Kernel Parameters
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 [Toc] [Back]
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 [Toc] [Back]
dbc_min_pct was developed by HP.
SEE ALSO [Toc] [Back]
kctune(1M), sam(1M) hpterm(1), nroff(1), rlogin(1), telnet(1),
uucp(1), gettune(2), settune(2), dbc_max_pct(5).
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