Tunable Kernel Parameters
NAME [Toc] [Back]
dbc_max_pct - maximum percentage of memory to be used for caching file
I/O data and metadata
VALUES [Toc] [Back]
Default [Toc] [Back]
Allowed values [Toc] [Back]
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 [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_max_pct specifies the maximum percentage of physical
memory that may be consumed 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 physical 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 [Toc] [Back]
This tunable is dynamic (tuning will take effect immediately on the
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.
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Tunable Kernel Parameters
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
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
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
AUTHOR [Toc] [Back]
dbc_max_pct was developed by HP.
SEE ALSO [Toc] [Back]
kctune(1M), sam(1M), gettune(2), settune(2), dbc_min_pct(5).
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