NAME
fcache_seqlimit_system — percentage of file cache that can be consumed by sequential accesses, per system-wide limit
VALUES
Allowed values
The minimum value allowed is
0.
The maximum value allowed is
100.
Specify a positive integer value.
DESCRIPTION
This parameter places a system-wide limit on how much memory can be
consumed in the file cache by sequential accesses.
It is important to
note the relationship between this tunable and the per-file limit
tunable,
fcache_seqlimit_file.
These two tunables, in conjunction, control
how much memory in the file cache can be consumed by sequential accesses.
When sequential accesses to a file has exceeded the per-file limit AND
the system-wide limit, the excess file cache pages
are paged out.
Note that both limits have to be reached in order for page
stealing to occur.
For example, a file can exceed its
fcache_seqlimit_file
limit if the
fcache_seqlimit_system
limit has not been reached.
Similarly,
no page stealing occurs if none of the files on the system has exceeded
the per-file limit, even if the system-wide limit has been hit.
The page
stealing feature is specific to improving sequential I/O performance for
large files.
Limiting file cache memory consumption helps alleviate memory
pressure in the file cache.
It also prevents cache wiping -- a single
thread sequentially accessing a large file can wipe out the existing
contents of the file cache.
Who is Expected to Change This Tunable?
System administrators that run applications which do large sequential file
I/O's.
Typically, these files are larger than the size of physical memory on
the system.
Restrictions on Changing
Changes to this tunable take effect immediately.
When Should the Value of This Tunable Be Raised?
When you wish to allow greater file cache consumption due to sequential
accesses.
Setting
fcache_seqlimit_system
and
fcache_seqlimit_file
to 100
effectively disables the sequential access page stealing feature.
What Are the Side Effects of Raising the Value?
Setting the tunable too high may yield poor performance for large sequential
I/O's for large files (larger than physical memory size).
fcache_seqlimit_system
and
fcache_seqlimit_file
should be tuned appropriately for the anticipated workload.
When Should the Value of This Tunable Be Lowered?
When you wish to limit file cache consumption due to sequential accesses.
Setting
fcache_seqlimit_system
and
fcache_seqlimit_file
to 0 forces sequential access page stealing to always occur.
What Are the Side Effects of Lowering the Value?
Could cause more pageouts to occur in an attempt to limit the file cache
consumption.
What Other Tunable Values Should Be Changed at the Same Time?
fcache_seqlimit_system
and
fcache_seqlimit_file
should be changed at the same time.
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.
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
fcache_seqlimit_system
was developed by HP.
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