NAME
dbc_min_pct — the minimum percentage of memory to be used by the dynamic buffer cache
VALUES
Default
5
Specify an 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
bufpages
and
nbuf
are both set to their default values of 0,
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.
It is possible to set both
dbc_min_pct
and
dbc_max_pct
to the same value, 12 for example, and create a kernel
that uses exactly that percentage of physical memory for the buffer cache,
regardless of the size of physical memory.
Selecting an Appropriate Value
If
dbc_min_pct
is set to too low a value,
very high demand on the buffer cache can effectively hang the system.
The is also true when using fixed buffer cache.
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 disk I/O
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.
Related Parameters
dbc_min_pct
must be less than or equal to
dbc_max_pct.
To use dynamic buffer caching,
bufpages
and
nbuf
must both be set to zero.
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.
AUTHOR
dbc_min_pct
was developed by HP.
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