Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual page 75

/usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt
3.3.9.5. /proc/sys/vm/
This directory facilitates the configuration of the Linux kernel's virtual memory (VM) subsystem. The
kernel makes extensive and intelligent use of virtual memory, which is commonly referred to as swap
space.
The following files are commonly found in the /proc/sys/vm/ directory:
• block_dump — Configures block I/O debugging when enabled. All read/write and block dirtying
operations done to files are logged accordingly. This can be useful if diagnosing disk spin up
and spin downs for laptop battery conservation. All output when block_dump is enabled can be
retrieved via dmesg. The default value is 0.
Tip
If block_dump is enabled at the same time as kernel debugging, it is prudent to stop
the klogd daemon, as it generates erroneous disk activity caused by block_dump.
• dirty_background_ratio — Starts background writeback of dirty data at this percentage of
total memory, via a pdflush daemon. The default value is 10.
• dirty_expire_centisecs — Defines when dirty in-memory data is old enough to be eligible for
writeout. Data which has been dirty in-memory for longer than this interval is written out next time a
pdflush daemon wakes up. The default value is 3000, expressed in hundredths of a second.
• dirty_ratio — Starts active writeback of dirty data at this percentage of total memory for the
generator of dirty data, via pdflush. The default value is 40.
• dirty_writeback_centisecs — Defines the interval between pdflush daemon wakeups,
which periodically writes dirty in-memory data out to disk. The default value is 500, expressed in
hundredths of a second.
• laptop_mode — Minimizes the number of times that a hard disk needs to spin up by keeping
the disk spun down for as long as possible, therefore conserving battery power on laptops. This
increases efficiency by combining all future I/O processes together, reducing the frequency of spin
ups. The default value is 0, but is automatically enabled in case a battery on a laptop is used.
This value is controlled automatically by the acpid daemon once a user is notified battery power
is enabled. No user modifications or interactions are necessary if the laptop supports the ACPI
(Advanced Configuration and Power Interface) specification.
For more information, refer to the following installed documentation:
/usr/share/doc/kernel-doc-<version>/Documentation/laptop-mode.txt
• lower_zone_protection — Determines how aggressive the kernel is in defending lower
memory allocation zones. This is effective when utilized with machines configured with highmem
memory space enabled. The default value is 0, no protection at all. All other integer values are in
megabytes, and lowmem memory is therefore protected from being allocated by users.
For more information, refer to the following installed documentation:
/proc/sys/
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