/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.

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:

  /usr/share/doc/kernel-doc-<version>/Documentation/filesystems/proc.txt

• max_map_count - Configures the maximum number of memory map areas a process may have. In most cases, the default value of 65536 is appropriate.
• min_free_kbytes - Forces the Linux VM (virtual memory manager) to keep a minimum number of kilobytes free. The VM uses this number to compute a pages_min value for each lowmem zone in the system. The default value is in respect to the total memory on the machine.
• nr_hugepages - Indicates the current number of configured hugetlb pages in the kernel.

  For more information, refer to the following installed documentation:

  /usr/share/doc/kernel-doc-<version>/Documentation/vm/hugetlbpage.txt

• nr_pdflush_threads - Indicates the number of pdflush daemons that are currently running. This file is read-only, and should not be changed by the user. Under heavy I/O loads, the default value of two is increased by the kernel.
• overcommit_memory - Configures the conditions under which a large memory request is accepted or denied. The following three modes are available:
  • 0 - The kernel performs heuristic memory over commit handling by estimating the amount of memory available and failing requests that are blatantly invalid. Unfortunately, since memory is allocated using a heuristic rather than a precise algorithm, this setting can sometimes allow available memory on the system to be overloaded. This is the default setting.
• 1 - The kernel performs no memory over commit handling. Under this setting, the potential for memory overload is increased, but so is performance for memory intensive tasks (such as those executed by some scientific software).
• 2 - The kernel fails requests for memory that add up to all of swap plus the percent of physical RAM specified in /proc/sys/vm/overcommit_ratio. This setting is best for those who desire less risk of memory overcommitment.

  This setting is only recommended for systems with swap areas larger than physical memory.

• overcommit_ratio - Specifies the percentage of physical RAM considered when /proc/sys/vm/overcommit_memory is set to 2. The default value is 50.
• page-cluster - Sets the number of pages read in a single attempt. The default value of 3, which actually relates to 16 pages, is appropriate for most systems.
• swappiness - Determines how much a machine should swap. The higher the value, the more swapping occurs. The default value, as a percentage, is set to 60.

All kernel-based documentation can be found in the following locally installed location:

/usr/share/doc/kernel-doc-<version>/Documentation/, which contains additional information.