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Tunable Kernel Parameters
HP-UX 11i Version 3: February 2007

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fcache_seqlimit_system — percentage of file cache that can be consumed by sequential accesses, per system-wide limit






Allowed values

The minimum value allowed is 0. The maximum value allowed is 100.

Specify a positive integer value.


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.


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.


fcache_seqlimit_system was developed by HP.

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