Documentation / sysctl / vm.txt

Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.

	Documentation for /proc/sys/vm/*	kernel version 2.2.10
		(c) 1998, 1999,  Rik van Riel <riel[AT]nl.linux[DOT]org>
	
	For general info and legal blurb, please look in README.
	
	==============================================================
	
	This file contains the documentation for the sysctl files in
	/proc/sys/vm and is valid for Linux kernel version 2.2.
	
	The files in this directory can be used to tune the operation
	of the virtual memory (VM) subsystem of the Linux kernel and
	the writeout of dirty data to disk.
	
	Default values and initialization routines for most of these
	files can be found in mm/swap.c.
	
	Currently, these files are in /proc/sys/vm:
	- overcommit_memory
	- page-cluster
	- dirty_ratio
	- dirty_background_ratio
	- dirty_expire_centisecs
	- dirty_writeback_centisecs
	- highmem_is_dirtyable   (only if CONFIG_HIGHMEM set)
	- max_map_count
	- min_free_kbytes
	- laptop_mode
	- block_dump
	- drop-caches
	- zone_reclaim_mode
	- min_unmapped_ratio
	- min_slab_ratio
	- panic_on_oom
	- oom_dump_tasks
	- oom_kill_allocating_task
	- mmap_min_address
	- numa_zonelist_order
	- nr_hugepages
	- nr_overcommit_hugepages
	
	==============================================================
	
	dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
	dirty_writeback_centisecs, highmem_is_dirtyable,
	vfs_cache_pressure, laptop_mode, block_dump, swap_token_timeout,
	drop-caches, hugepages_treat_as_movable:
	
	See Documentation/filesystems/proc.txt
	
	==============================================================
	
	overcommit_memory:
	
	This value contains a flag that enables memory overcommitment.
	
	When this flag is 0, the kernel attempts to estimate the amount
	of free memory left when userspace requests more memory.
	
	When this flag is 1, the kernel pretends there is always enough
	memory until it actually runs out.
	
	When this flag is 2, the kernel uses a "never overcommit"
	policy that attempts to prevent any overcommit of memory.  
	
	This feature can be very useful because there are a lot of
	programs that malloc() huge amounts of memory "just-in-case"
	and don't use much of it.
	
	The default value is 0.
	
	See Documentation/vm/overcommit-accounting and
	security/commoncap.c::cap_vm_enough_memory() for more information.
	
	==============================================================
	
	overcommit_ratio:
	
	When overcommit_memory is set to 2, the committed address
	space is not permitted to exceed swap plus this percentage
	of physical RAM.  See above.
	
	==============================================================
	
	page-cluster:
	
	The Linux VM subsystem avoids excessive disk seeks by reading
	multiple pages on a page fault. The number of pages it reads
	is dependent on the amount of memory in your machine.
	
	The number of pages the kernel reads in at once is equal to
	2 ^ page-cluster. Values above 2 ^ 5 don't make much sense
	for swap because we only cluster swap data in 32-page groups.
	
	==============================================================
	
	max_map_count:
	
	This file contains the maximum number of memory map areas a process
	may have. Memory map areas are used as a side-effect of calling
	malloc, directly by mmap and mprotect, and also when loading shared
	libraries.
	
	While most applications need less than a thousand maps, certain
	programs, particularly malloc debuggers, may consume lots of them,
	e.g., up to one or two maps per allocation.
	
	The default value is 65536.
	
	==============================================================
	
	min_free_kbytes:
	
	This is used to force the Linux VM 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.  Each lowmem zone gets 
	a number of reserved free pages based proportionally on its size.
	
	Some minimal ammount of memory is needed to satisfy PF_MEMALLOC
	allocations; if you set this to lower than 1024KB, your system will
	become subtly broken, and prone to deadlock under high loads.
	
	Setting this too high will OOM your machine instantly.
	
	==============================================================
	
	percpu_pagelist_fraction
	
	This is the fraction of pages at most (high mark pcp->high) in each zone that
	are allocated for each per cpu page list.  The min value for this is 8.  It
	means that we don't allow more than 1/8th of pages in each zone to be
	allocated in any single per_cpu_pagelist.  This entry only changes the value
	of hot per cpu pagelists.  User can specify a number like 100 to allocate
	1/100th of each zone to each per cpu page list.
	
	The batch value of each per cpu pagelist is also updated as a result.  It is
	set to pcp->high/4.  The upper limit of batch is (PAGE_SHIFT * 8)
	
	The initial value is zero.  Kernel does not use this value at boot time to set
	the high water marks for each per cpu page list.
	
	===============================================================
	
	zone_reclaim_mode:
	
	Zone_reclaim_mode allows someone to set more or less aggressive approaches to
	reclaim memory when a zone runs out of memory. If it is set to zero then no
	zone reclaim occurs. Allocations will be satisfied from other zones / nodes
	in the system.
	
	This is value ORed together of
	
	1	= Zone reclaim on
	2	= Zone reclaim writes dirty pages out
	4	= Zone reclaim swaps pages
	
	zone_reclaim_mode is set during bootup to 1 if it is determined that pages
	from remote zones will cause a measurable performance reduction. The
	page allocator will then reclaim easily reusable pages (those page
	cache pages that are currently not used) before allocating off node pages.
	
	It may be beneficial to switch off zone reclaim if the system is
	used for a file server and all of memory should be used for caching files
	from disk. In that case the caching effect is more important than
	data locality.
	
	Allowing zone reclaim to write out pages stops processes that are
	writing large amounts of data from dirtying pages on other nodes. Zone
	reclaim will write out dirty pages if a zone fills up and so effectively
	throttle the process. This may decrease the performance of a single process
	since it cannot use all of system memory to buffer the outgoing writes
	anymore but it preserve the memory on other nodes so that the performance
	of other processes running on other nodes will not be affected.
	
	Allowing regular swap effectively restricts allocations to the local
	node unless explicitly overridden by memory policies or cpuset
	configurations.
	
	=============================================================
	
	min_unmapped_ratio:
	
	This is available only on NUMA kernels.
	
	A percentage of the total pages in each zone.  Zone reclaim will only
	occur if more than this percentage of pages are file backed and unmapped.
	This is to insure that a minimal amount of local pages is still available for
	file I/O even if the node is overallocated.
	
	The default is 1 percent.
	
	=============================================================
	
	min_slab_ratio:
	
	This is available only on NUMA kernels.
	
	A percentage of the total pages in each zone.  On Zone reclaim
	(fallback from the local zone occurs) slabs will be reclaimed if more
	than this percentage of pages in a zone are reclaimable slab pages.
	This insures that the slab growth stays under control even in NUMA
	systems that rarely perform global reclaim.
	
	The default is 5 percent.
	
	Note that slab reclaim is triggered in a per zone / node fashion.
	The process of reclaiming slab memory is currently not node specific
	and may not be fast.
	
	=============================================================
	
	panic_on_oom
	
	This enables or disables panic on out-of-memory feature.
	
	If this is set to 0, the kernel will kill some rogue process,
	called oom_killer.  Usually, oom_killer can kill rogue processes and
	system will survive.
	
	If this is set to 1, the kernel panics when out-of-memory happens.
	However, if a process limits using nodes by mempolicy/cpusets,
	and those nodes become memory exhaustion status, one process
	may be killed by oom-killer. No panic occurs in this case.
	Because other nodes' memory may be free. This means system total status
	may be not fatal yet.
	
	If this is set to 2, the kernel panics compulsorily even on the
	above-mentioned.
	
	The default value is 0.
	1 and 2 are for failover of clustering. Please select either
	according to your policy of failover.
	
	=============================================================
	
	oom_dump_tasks
	
	Enables a system-wide task dump (excluding kernel threads) to be
	produced when the kernel performs an OOM-killing and includes such
	information as pid, uid, tgid, vm size, rss, cpu, oom_adj score, and
	name.  This is helpful to determine why the OOM killer was invoked
	and to identify the rogue task that caused it.
	
	If this is set to zero, this information is suppressed.  On very
	large systems with thousands of tasks it may not be feasible to dump
	the memory state information for each one.  Such systems should not
	be forced to incur a performance penalty in OOM conditions when the
	information may not be desired.
	
	If this is set to non-zero, this information is shown whenever the
	OOM killer actually kills a memory-hogging task.
	
	The default value is 0.
	
	=============================================================
	
	oom_kill_allocating_task
	
	This enables or disables killing the OOM-triggering task in
	out-of-memory situations.
	
	If this is set to zero, the OOM killer will scan through the entire
	tasklist and select a task based on heuristics to kill.  This normally
	selects a rogue memory-hogging task that frees up a large amount of
	memory when killed.
	
	If this is set to non-zero, the OOM killer simply kills the task that
	triggered the out-of-memory condition.  This avoids the expensive
	tasklist scan.
	
	If panic_on_oom is selected, it takes precedence over whatever value
	is used in oom_kill_allocating_task.
	
	The default value is 0.
	
	==============================================================
	
	mmap_min_addr
	
	This file indicates the amount of address space  which a user process will
	be restricted from mmaping.  Since kernel null dereference bugs could
	accidentally operate based on the information in the first couple of pages
	of memory userspace processes should not be allowed to write to them.  By
	default this value is set to 0 and no protections will be enforced by the
	security module.  Setting this value to something like 64k will allow the
	vast majority of applications to work correctly and provide defense in depth
	against future potential kernel bugs.
	
	==============================================================
	
	numa_zonelist_order
	
	This sysctl is only for NUMA.
	'where the memory is allocated from' is controlled by zonelists.
	(This documentation ignores ZONE_HIGHMEM/ZONE_DMA32 for simple explanation.
	 you may be able to read ZONE_DMA as ZONE_DMA32...)
	
	In non-NUMA case, a zonelist for GFP_KERNEL is ordered as following.
	ZONE_NORMAL -> ZONE_DMA
	This means that a memory allocation request for GFP_KERNEL will
	get memory from ZONE_DMA only when ZONE_NORMAL is not available.
	
	In NUMA case, you can think of following 2 types of order.
	Assume 2 node NUMA and below is zonelist of Node(0)'s GFP_KERNEL
	
	(A) Node(0) ZONE_NORMAL -> Node(0) ZONE_DMA -> Node(1) ZONE_NORMAL
	(B) Node(0) ZONE_NORMAL -> Node(1) ZONE_NORMAL -> Node(0) ZONE_DMA.
	
	Type(A) offers the best locality for processes on Node(0), but ZONE_DMA
	will be used before ZONE_NORMAL exhaustion. This increases possibility of
	out-of-memory(OOM) of ZONE_DMA because ZONE_DMA is tend to be small.
	
	Type(B) cannot offer the best locality but is more robust against OOM of
	the DMA zone.
	
	Type(A) is called as "Node" order. Type (B) is "Zone" order.
	
	"Node order" orders the zonelists by node, then by zone within each node.
	Specify "[Nn]ode" for zone order
	
	"Zone Order" orders the zonelists by zone type, then by node within each
	zone.  Specify "[Zz]one"for zode order.
	
	Specify "[Dd]efault" to request automatic configuration.  Autoconfiguration
	will select "node" order in following case.
	(1) if the DMA zone does not exist or
	(2) if the DMA zone comprises greater than 50% of the available memory or
	(3) if any node's DMA zone comprises greater than 60% of its local memory and
	    the amount of local memory is big enough.
	
	Otherwise, "zone" order will be selected. Default order is recommended unless
	this is causing problems for your system/application.
	
	==============================================================
	
	nr_hugepages
	
	Change the minimum size of the hugepage pool.
	
	See Documentation/vm/hugetlbpage.txt
	
	==============================================================
	
	nr_overcommit_hugepages
	
	Change the maximum size of the hugepage pool. The maximum is
	nr_hugepages + nr_overcommit_hugepages.
	
	See Documentation/vm/hugetlbpage.txt

• [ sysctl ]
• 00-INDEX
• abi.txt
• ctl_unnumbered.txt
• fs.txt
• kernel.txt
• README
• sunrpc.txt
• vm.txt
•