fcache_seqlimit_system.5 (2010 09)

f
fcache_seqlimit_system(5) fcache_seqlimit_system(5)
(Tunable Kernel Parameters)
NAME
fcache_seqlimit_system - percentage of file cache that can be consumed by sequential accesses, per
system-wide limit
VALUES
Failsafe
100
Default
100
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 wip-
ing -- 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 steal-
ing 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.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1