filecache_max(5)filecache_max(5)NAME
filecache_max, filecache_min - maximum or minimum amount of physical
memory used for caching file I/O data
VALUES
Failsafe
Default
The defaults are computed and adjusted automatically, based on the
amount physical memory on the system.
The current default value, when displayed by will be expressed as the
current value used internally by the system (representing bytes). The
displayed value will vary as the internal value is automatically
adjusted.
Allowed values
not less than the equivalent of 1 megabyte
not less than the equivalent of 1 megabyte
not greater than the equivalent of 70% of physical memory
not greater than the equivalent of 90% of physical memory
See below.
Values can be specified as:
1) Default
2) A percentage of total physical memory:
A positive whole number followed by a percent symbol (for
example, 70%).
3) A constant value:
A positive whole number that represents number of bytes
of physical memory, optionally followed by a multiplier
suffix, where and
Recommended values
It is recommended that these tunables are left in the automatic
(default) state, to allow the system to better balance the memory usage
among filesystem I/O-intensive processes and other types of processes.
DESCRIPTION
These tunables control the amount of physical memory that can be used
for caching file data during file system I/O operations.
The amount of physical memory that is specified by the tunable is
reserved and guaranteed to be available for file caching.
The amount of physical memory used for file caching can grow beyond up
to depending on I/O load and competing requests for physical memory.
When these tunables are set to default, or set to a percent value, they
automatically adjust with Online Addition or Deletion (OL*) of physical
memory, as appropriate.
Who Is Expected to Change These Tunables?
The automatic (default) state should be appropriate for most environ‐
ments.
You must set these tunables to a constant value (not default or per‐
cent) if you want to specify file cache limits with finer granularity
than percent of physical memory (for example, a minimum size or fixed
size of <1% of physical memory).
You must set these tunables to a constant value (not default or per‐
cent) if you do not want the limits of the file cache to adjust with
OL* of physical memory.
To discriminate in favor of deterministic I/O on systems with large
file I/O activity, or on the contrary, to discriminate in favor of bet‐
ter performance of non-I/O-intensive processes, you can consider chang‐
ing the values of these tunables, keeping in mind the side effects as
described below.
To determine a reasonable value for the cache size you should consider
the file I/O-intensive applications on your system, and the size of
their working set. Depending on the type of applications, the working
set size can be based on the size of a transaction, or data size in
given unit of time. For example, for a conservative value of in
megabytes, you can use the following formula:
number-of-records-in-working-set
Only those processes that actively use disk I/O for 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, 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, login shells, system daemons, or connec‐
tions, and so on. These processes use very little, if any, disk
I/O for file data.
Restrictions on Changing
These tunables are dynamic and automatic.
The system rounds the specified tunable value(s) down to the closest
physical page boundary.
The amount of physical memory represented by must be equal to or less
than the memory represented by tunable
Setting these tunables to a constant value will de-couple them from OL*
events.
Tuning up of the parameter may fail if there is not enough free physi‐
cal memory to satisfy the request.
These tunables, and must both be set to a relative state (either
default of percent state), or must both be set to a constant value.
For example, the following are acceptable settings:
The following will result in an error:
If is currently set to default, the following is acceptable:
But the following will result in an error:
See other restrictions in the section above.
When Should the Value of These Tunables Be Raised?
Low system performance at initialization time and/or on a system with
filesystem I/O-intensive processes may be an indication that the values
of these tunables are too low. If there is a large number of processes
actively and constantly using file data I/O, you should raise the value
of for more deterministic I/O. In most cases, especially when the file
data I/O is expected to peak only occasionally, it is recommended that
the value of the maximum limit, is raised instead.
What Are the Side Effects of Raising the Values?
The amount of memory reserved for the minimum file cache size, dictated
by cannot be used on the system for other purposes. Be careful not to
raise this value so high that it can eventually cause memory pressure
and overall system performance degradation.
When Should the Values of These Tunables Be Lowered?
The value of the minimum limit, can be lowered to allow a larger per‐
centage of memory to be used for purposes other than filesystem I/O
caching, depending on competing requests. By lowering the value of a
larger amount of memory is available for other purposes, without com‐
peting with file I/O requests.
What Are the Side Effects of Lowering the Values?
If there are many competing requests for physical memory, and the file
cache tunables are set to too low a value, very high demand on file I/O
operations can eventually cause filesystem I/O performance degradation.
EXAMPLES
Set the file cache minimum to 10% of physical memory:
Set a fixed size file cache of 1 gigabyte:
Set the file cache minimum to 15% of physical memory, and the maximum
to 65% of the physical memory:
WARNINGS
All HP-UX kernel tunable parameters are release specific. These param‐
eters may be removed or have their meaning changed in future releases
of HP-UX.
Other tunable parameters related to sizing the buffer cache that
existed in previous HP-UX releases are now obsolete. The tunables and
should be used to set limits to the file cache. Note that, on any
given system, the optimum values of these two new tunables are not nec‐
essarily equivalent to the optimum values of the obsolete tunable val‐
ues in the older systems. You should first determine if the new
default values yield acceptable performance on your system, before
attempting to change the values of the new file cache tunables.
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 val‐
ues. For information about the effects of installation on tunable val‐
ues, consult the documentation for the kernel software being installed.
For information about optional kernel software that was factory
installed on your system, see at
AUTHOR
and were developed by HP.
SEE ALSOkctune(1M), sam(1M), gettune(2), settune(2).
Tunable Kernel Parameters filecache_max(5)