Documentation for /proc/sys/vm/¶

kernel version 2.6.29

For general info and legal blurb, please look in Documentation for /proc/sys.

This file contains the documentation for the sysctl files in /proc/sys/vm and is valid for Linux kernel version 2.6.29.

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.

Currently, these files are in /proc/sys/vm:

admin_reserve_kbytes
compact_memory
compaction_proactiveness
compact_unevictable_allowed
defrag_mode
dirty_background_bytes
dirty_background_ratio
dirty_bytes
dirty_expire_centisecs
dirty_ratio
dirtytime_expire_seconds
dirty_writeback_centisecs
drop_caches
enable_soft_offline
extfrag_threshold
highmem_is_dirtyable
hugetlb_shm_group
legacy_va_layout
lowmem_reserve_ratio
max_map_count
mem_profiling (only if CONFIG_MEM_ALLOC_PROFILING=y)
memory_failure_early_kill
memory_failure_recovery
min_free_kbytes
min_slab_ratio
min_unmapped_ratio
mmap_min_addr
mmap_rnd_bits
mmap_rnd_compat_bits
movable_gigantic_pages
nr_hugepages
nr_hugepages_mempolicy
nr_overcommit_hugepages
nr_trim_pages (only if CONFIG_MMU=n)
numa_zonelist_order
oom_dump_tasks
oom_kill_allocating_task
overcommit_kbytes
overcommit_memory
overcommit_ratio
page-cluster
page_lock_unfairness
panic_on_oom
panic_on_unrecoverable_memory_failure
percpu_pagelist_high_fraction
stat_interval
stat_refresh
numa_stat
swappiness
unprivileged_userfaultfd
user_reserve_kbytes
vfs_cache_pressure
vfs_cache_pressure_denom
watermark_boost_factor
watermark_scale_factor
zone_reclaim_mode

admin_reserve_kbytes¶

The amount of free memory in the system that should be reserved for users with the capability cap_sys_admin.

admin_reserve_kbytes defaults to min(3% of free pages, 8MB)

That should provide enough for the admin to log in and kill a process, if necessary, under the default overcommit ‘guess’ mode.

Systems running under overcommit ‘never’ should increase this to account for the full Virtual Memory Size of programs used to recover. Otherwise, root may not be able to log in to recover the system.

How do you calculate a minimum useful reserve?

sshd or login + bash (or some other shell) + top (or ps, kill, etc.)

For overcommit ‘guess’, we can sum resident set sizes (RSS). On x86_64 this is about 8MB.

For overcommit ‘never’, we can take the max of their virtual sizes (VSZ) and add the sum of their RSS. On x86_64 this is about 128MB.

Changing this takes effect whenever an application requests memory.

compact_memory¶

Available only when CONFIG_COMPACTION is set. When 1 is written to the file, all zones are compacted such that free memory is available in contiguous blocks where possible. This can be important for example in the allocation of huge pages although processes will also directly compact memory as required.

compaction_proactiveness¶

This tunable takes a value in the range [0, 100] with a default value of 20. This tunable determines how aggressively compaction is done in the background. Write of a non zero value to this tunable will immediately trigger the proactive compaction. Setting it to 0 disables proactive compaction.

Note that compaction has a non-trivial system-wide impact as pages belonging to different processes are moved around, which could also lead to latency spikes in unsuspecting applications. The kernel employs various heuristics to avoid wasting CPU cycles if it detects that proactive compaction is not being effective.

Setting the value above 80 will, in addition to lowering the acceptable level of fragmentation, make the compaction code more sensitive to increases in fragmentation, i.e. compaction will trigger more often, but reduce fragmentation by a smaller amount. This makes the fragmentation level more stable over time.

Be careful when setting it to extreme values like 100, as that may cause excessive background compaction activity.

compact_unevictable_allowed¶

Available only when CONFIG_COMPACTION is set. When set to 1, compaction is allowed to examine the unevictable lru (mlocked pages) for pages to compact. This should be used on systems where stalls for minor page faults are an acceptable trade for large contiguous free memory. Set to 0 to prevent compaction from moving pages that are unevictable. Default value is 1. On CONFIG_PREEMPT_RT the default value is 0 in order to avoid a page fault, due to compaction, which would block the task from becoming active until the fault is resolved.

defrag_mode¶

When set to 1, the page allocator tries harder to avoid fragmentation and maintain the ability to produce huge pages / higher-order pages.

It is recommended to enable this right after boot, as fragmentation, once it occurred, can be long-lasting or even permanent.

dirty_background_bytes¶

Contains the amount of dirty memory at which the background kernel flusher threads will start writeback.

Note:: dirty_background_bytes is the counterpart of dirty_background_ratio. Only one of them may be specified at a time. When one sysctl is written it is immediately taken into account to evaluate the dirty memory limits and the other appears as 0 when read.

dirty_background_ratio¶

Contains, as a percentage of total available memory that contains free pages and reclaimable pages, the number of pages at which the background kernel flusher threads will start writing out dirty data.

The total available memory is not equal to total system memory.

dirty_bytes¶

Contains the amount of dirty memory at which a process generating disk writes will itself start writeback.

Note: dirty_bytes is the counterpart of dirty_ratio. Only one of them may be specified at a time. When one sysctl is written it is immediately taken into account to evaluate the dirty memory limits and the other appears as 0 when read.

Note: the minimum value allowed for dirty_bytes is two pages (in bytes); any value lower than this limit will be ignored and the old configuration will be retained.

dirty_expire_centisecs¶

This tunable is used to define when dirty data is old enough to be eligible for writeout by the kernel flusher threads. It is expressed in 100’ths of a second. Data which has been dirty in-memory for longer than this interval will be written out next time a flusher thread wakes up.

dirty_ratio¶

Contains, as a percentage of total available memory that contains free pages and reclaimable pages, the number of pages at which a process which is generating disk writes will itself start writing out dirty data.

The total available memory is not equal to total system memory.

dirtytime_expire_seconds¶

When a lazytime inode is constantly having its pages dirtied, the inode with an updated timestamp will never get chance to be written out. And, if the only thing that has happened on the file system is a dirtytime inode caused by an atime update, a worker will be scheduled to make sure that inode eventually gets pushed out to disk. This tunable is used to define when dirty inode is old enough to be eligible for writeback by the kernel flusher threads. And, it is also used as the interval to wakeup dirtytime_writeback thread.

Setting this to zero disables periodic dirtytime writeback.

dirty_writeback_centisecs¶

The kernel flusher threads will periodically wake up and write old data out to disk. This tunable expresses the interval between those wakeups, in 100’ths of a second.

Setting this to zero disables periodic writeback altogether.

drop_caches¶

Writing to this will cause the kernel to drop clean caches, as well as reclaimable slab objects like dentries and inodes. Once dropped, their memory becomes free.

To free pagecache:

echo 1 > /proc/sys/vm/drop_caches

To free reclaimable slab objects (includes dentries and inodes):

echo 2 > /proc/sys/vm/drop_caches

To free slab objects and pagecache:

echo 3 > /proc/sys/vm/drop_caches

This is a non-destructive operation and will not free any dirty objects. To increase the number of objects freed by this operation, the user may run sync prior to writing to /proc/sys/vm/drop_caches. This will minimize the number of dirty objects on the system and create more candidates to be dropped.

This file is not a means to control the growth of the various kernel caches (inodes, dentries, pagecache, etc...) These objects are automatically reclaimed by the kernel when memory is needed elsewhere on the system.

Use of this file can cause performance problems. Since it discards cached objects, it may cost a significant amount of I/O and CPU to recreate the dropped objects, especially if they were under heavy use. Because of this, use outside of a testing or debugging environment is not recommended.

You may see informational messages in your kernel log when this file is used:

cat (1234): drop_caches: 3

These are informational only. They do not mean that anything is wrong with your system. To disable them, echo 4 (bit 2) into drop_caches.

enable_soft_offline¶

Correctable memory errors are very common on servers. Soft-offline is kernel’s solution for memory pages having (excessive) corrected memory errors.

For different types of page, soft-offline has different behaviors / costs.

For a raw error page, soft-offline migrates the in-use page’s content to a new raw page.
For a page that is part of a transparent hugepage, soft-offline splits the transparent hugepage into raw pages, then migrates only the raw error page. As a result, user is transparently backed by 1 less hugepage, impacting memory access performance.
For a page that is part of a HugeTLB hugepage, soft-offline first migrates the entire HugeTLB hugepage, during which a free hugepage will be consumed as migration target. Then the original hugepage is dissolved into raw pages without compensation, reducing the capacity of the HugeTLB pool by 1.

It is user’s call to choose between reliability (staying away from fragile physical memory) vs performance / capacity implications in transparent and HugeTLB cases.

For all architectures, enable_soft_offline controls whether to soft offline memory pages. When set to 1, kernel attempts to soft offline the pages whenever it thinks needed. When set to 0, kernel returns EOPNOTSUPP to the request to soft offline the pages. Its default value is 1.

It is worth mentioning that after setting enable_soft_offline to 0, the following requests to soft offline pages will not be performed:

Request to soft offline pages from RAS Correctable Errors Collector.
On ARM, the request to soft offline pages from GHES driver.
On PARISC, the request to soft offline pages from Page Deallocation Table.

extfrag_threshold¶

This parameter affects whether the kernel will compact memory or direct reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in debugfs shows what the fragmentation index for each order is in each zone in the system. Values tending towards 0 imply allocations would fail due to lack of memory, values towards 1000 imply failures are due to fragmentation and -1 implies that the allocation will succeed as long as watermarks are met.

The kernel will not compact memory in a zone if the fragmentation index is <= extfrag_threshold. The default value is 500.

highmem_is_dirtyable¶

Available only for systems with CONFIG_HIGHMEM enabled (32b systems).

This parameter controls whether the high memory is considered for dirty writers throttling. This is not the case by default which means that only the amount of memory directly visible/usable by the kernel can be dirtied. As a result, on systems with a large amount of memory and lowmem basically depleted writers might be throttled too early and streaming writes can get very slow.

Changing the value to non zero would allow more memory to be dirtied and thus allow writers to write more data which can be flushed to the storage more effectively. Note this also comes with a risk of pre-mature OOM killer because some writers (e.g. direct block device writes) can only use the low memory and they can fill it up with dirty data without any throttling.

hugetlb_shm_group¶

hugetlb_shm_group contains group id that is allowed to create SysV shared memory segment using hugetlb page.

legacy_va_layout¶

If non-zero, this sysctl disables the new 32-bit mmap layout - the kernel will use the legacy (2.4) layout for all processes.

lowmem_reserve_ratio¶

For some specialised workloads on highmem machines it is dangerous for the kernel to allow process memory to be allocated from the “lowmem” zone. This is because that memory could then be pinned via the mlock() system call, or by unavailability of swapspace.

And on large highmem machines this lack of reclaimable lowmem memory can be fatal.

So the Linux page allocator has a mechanism which prevents allocations which could use highmem from using too much lowmem. This means that a certain amount of lowmem is defended from the possibility of being captured into pinned user memory.

(The same argument applies to the old 16 megabyte ISA DMA region. This mechanism will also defend that region from allocations which could use highmem or lowmem).

The lowmem_reserve_ratio tunable determines how aggressive the kernel is in defending these lower zones.

If you have a machine which uses highmem or ISA DMA and your applications are using mlock(), or if you are running with no swap then you probably should change the lowmem_reserve_ratio setting.

The lowmem_reserve_ratio is an array. You can see them by reading this file:

% cat /proc/sys/vm/lowmem_reserve_ratio
256     256     32

But, these values are not used directly. The kernel calculates # of protection pages for each zones from them. These are shown as array of protection pages in /proc/zoneinfo like the following. (This is an example of x86-64 box). Each zone has an array of protection pages like this:

Node 0, zone      DMA
  pages free     1355
        min      3
        low      3
        high     4
      :
      :
    numa_other   0
        protection: (0, 2004, 2004, 2004)
      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  pagesets
    cpu: 0 pcp: 0
        :

These protections are added to score to judge whether this zone should be used for page allocation or should be reclaimed.

In this example, if normal pages (index=2) are required to this DMA zone and watermark[WMARK_HIGH] is used for watermark, the kernel judges this zone should not be used because pages_free(1355) is smaller than watermark + protection[2] (4 + 2004 = 2008). If this protection value is 0, this zone would be used for normal page requirement. If requirement is DMA zone(index=0), protection[0] (=0) is used.

zone[i]’s protection[j] is calculated by following expression:

(i < j):
  zone[i]->protection[j]
  = (total sums of managed_pages from zone[i+1] to zone[j] on the node)
    / lowmem_reserve_ratio[i];
(i = j):
   (should not be protected. = 0;
(i > j):
   (not necessary, but looks 0)

The default values of lowmem_reserve_ratio[i] are

256

(if zone[i] means DMA or DMA32 zone)

32

(others)

As above expression, they are reciprocal number of ratio. 256 means 1/256. # of protection pages becomes about “0.39%” of total managed pages of higher zones on the node.

If you would like to protect more pages, smaller values are effective. The minimum value is 1 (1/1 -> 100%). The value less than 1 completely disables protection of the pages.

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, mprotect, and madvise, 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 65530.

mem_profiling¶

Enable memory profiling (when CONFIG_MEM_ALLOC_PROFILING=y)

1: Enable memory profiling.

0: Disable memory profiling.

Enabling memory profiling introduces a small performance overhead for all memory allocations.

The default value depends on CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT.

When CONFIG_MEM_ALLOC_PROFILING_DEBUG=y, this control is read-only to avoid warnings produced by allocations made while profiling is disabled and freed when it’s enabled.

memory_failure_early_kill¶

Control how to kill processes when uncorrected memory error (typically a 2bit error in a memory module) is detected in the background by hardware that cannot be handled by the kernel. In some cases (like the page still having a valid copy on disk) the kernel will handle the failure transparently without affecting any applications. But if there is no other up-to-date copy of the data it will kill to prevent any data corruptions from propagating.

1: Kill all processes that have the corrupted and not reloadable page mapped as soon as the corruption is detected. Note this is not supported for a few types of pages, like kernel internally allocated data or the swap cache, but works for the majority of user pages.

0: Only unmap the corrupted page from all processes and only kill a process who tries to access it.

The kill is done using a catchable SIGBUS with BUS_MCEERR_AO, so processes can handle this if they want to.

This is only active on architectures/platforms with advanced machine check handling and depends on the hardware capabilities.

Applications can override this setting individually with the PR_MCE_KILL prctl

memory_failure_recovery¶

Enable memory failure recovery (when supported by the platform)

1: Attempt recovery.

0: Always panic on a memory failure.

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 watermark[WMARK_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 amount 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.

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.

min_unmapped_ratio¶

This is available only on NUMA kernels.

This is a percentage of the total pages in each zone. Zone reclaim will only occur if more than this percentage of pages are in a state that zone_reclaim_mode allows to be reclaimed.

If zone_reclaim_mode has the value 4 OR’d, then the percentage is compared against all file-backed unmapped pages including swapcache pages and tmpfs files. Otherwise, only unmapped pages backed by normal files but not tmpfs files and similar are considered.

The default is 1 percent.

mmap_min_addr¶

This file indicates the amount of address space which a user process will be restricted from mmapping. 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.

mmap_rnd_bits¶

This value can be used to select the number of bits to use to determine the random offset to the base address of vma regions resulting from mmap allocations on architectures which support tuning address space randomization. This value will be bounded by the architecture’s minimum and maximum supported values.

This value can be changed after boot using the /proc/sys/vm/mmap_rnd_bits tunable

mmap_rnd_compat_bits¶

This value can be used to select the number of bits to use to determine the random offset to the base address of vma regions resulting from mmap allocations for applications run in compatibility mode on architectures which support tuning address space randomization. This value will be bounded by the architecture’s minimum and maximum supported values.

This value can be changed after boot using the /proc/sys/vm/mmap_rnd_compat_bits tunable

movable_gigantic_pages¶

This parameter controls whether gigantic pages may be allocated from ZONE_MOVABLE. If set to non-zero, gigantic pages can be allocated from ZONE_MOVABLE. ZONE_MOVABLE memory may be created via the kernel boot parameter kernelcore or via memory hotplug as discussed in Memory Hot(Un)Plug.

Support may depend on specific architecture.

Note that using ZONE_MOVABLE gigantic pages make memory hotremove unreliable.

Memory hot-remove operations will block indefinitely until the admin reserves sufficient gigantic pages to service migration requests associated with the memory offlining process. As HugeTLB gigantic page reservation is a manual process (via nodeN/hugepages/.../nr_hugepages interfaces) this may not be obvious when just attempting to offline a block of memory.

Additionally, as multiple gigantic pages may be reserved on a single block, it may appear that gigantic pages are available for migration when in reality they are in the process of being removed. For example if memoryN contains two gigantic pages, one reserved and one allocated, and an admin attempts to offline that block, this operations may hang indefinitely unless another reserved gigantic page is available on another block memoryM.

nr_hugepages¶

Change the minimum size of the hugepage pool.

See HugeTLB Pages

hugetlb_optimize_vmemmap¶

This knob is not available when the size of ‘struct page’ (a structure defined in include/linux/mm_types.h) is not power of two (an unusual system config could result in this).

Enable (set to 1) or disable (set to 0) HugeTLB Vmemmap Optimization (HVO).

Once enabled, the vmemmap pages of subsequent allocation of HugeTLB pages from buddy allocator will be optimized (7 pages per 2MB HugeTLB page and 4095 pages per 1GB HugeTLB page), whereas already allocated HugeTLB pages will not be optimized. When those optimized HugeTLB pages are freed from the HugeTLB pool to the buddy allocator, the vmemmap pages representing that range needs to be remapped again and the vmemmap pages discarded earlier need to be rellocated again. If your use case is that HugeTLB pages are allocated ‘on the fly’ (e.g. never explicitly allocating HugeTLB pages with ‘nr_hugepages’ but only set ‘nr_overcommit_hugepages’, those overcommitted HugeTLB pages are allocated ‘on the fly’) instead of being pulled from the HugeTLB pool, you should weigh the benefits of memory savings against the more overhead (~2x slower than before) of allocation or freeing HugeTLB pages between the HugeTLB pool and the buddy allocator. Another behavior to note is that if the system is under heavy memory pressure, it could prevent the user from freeing HugeTLB pages from the HugeTLB pool to the buddy allocator since the allocation of vmemmap pages could be failed, you have to retry later if your system encounter this situation.

Once disabled, the vmemmap pages of subsequent allocation of HugeTLB pages from buddy allocator will not be optimized meaning the extra overhead at allocation time from buddy allocator disappears, whereas already optimized HugeTLB pages will not be affected. If you want to make sure there are no optimized HugeTLB pages, you can set “nr_hugepages” to 0 first and then disable this. Note that writing 0 to nr_hugepages will make any “in use” HugeTLB pages become surplus pages. So, those surplus pages are still optimized until they are no longer in use. You would need to wait for those surplus pages to be released before there are no optimized pages in the system.

nr_hugepages_mempolicy¶

Change the size of the hugepage pool at run-time on a specific set of NUMA nodes.

See HugeTLB Pages

nr_overcommit_hugepages¶

Change the maximum size of the hugepage pool. The maximum is nr_hugepages + nr_overcommit_hugepages.

See HugeTLB Pages

nr_trim_pages¶

This is available only on NOMMU kernels.

This value adjusts the excess page trimming behaviour of power-of-2 aligned NOMMU mmap allocations.

A value of 0 disables trimming of allocations entirely, while a value of 1 trims excess pages aggressively. Any value >= 1 acts as the watermark where trimming of allocations is initiated.

The default value is 1.

See No-MMU memory mapping support for more information.

numa_zonelist_order¶

This sysctl is only for NUMA and it is deprecated. Anything but Node order will fail!

‘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 node order

“Zone Order” orders the zonelists by zone type, then by node within each zone. Specify “[Zz]one” for zone order.

Specify “[Dd]efault” to request automatic configuration.

On 32-bit, the Normal zone needs to be preserved for allocations accessible by the kernel, so “zone” order will be selected.

On 64-bit, devices that require DMA32/DMA are relatively rare, so “node” order will be selected.

Default order is recommended unless this is causing problems for your system/application.

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, pgtables_bytes, swapents, oom_score_adj score, and name. This is helpful to determine why the OOM killer was invoked, to identify the rogue task that caused it, and to determine why the OOM killer chose the task it did to kill.

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 1 (enabled).

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.

overcommit_kbytes¶

When overcommit_memory is set to 2, the committed address space is not permitted to exceed swap plus this amount of physical RAM. See below.

Note: overcommit_kbytes is the counterpart of overcommit_ratio. Only one of them may be specified at a time. Setting one disables the other (which then appears as 0 when read).

overcommit_memory¶

This value contains a flag that enables memory overcommitment.

When this flag is 0, the kernel compares the userspace memory request size against total memory plus swap and rejects obvious overcommits.

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. Note that user_reserve_kbytes affects this policy.

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 Overcommit Accounting and mm/util.c::__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¶

page-cluster controls the number of pages up to which consecutive pages are read in from swap in a single attempt. This is the swap counterpart to page cache readahead. The mentioned consecutivity is not in terms of virtual/physical addresses, but consecutive on swap space - that means they were swapped out together.

It is a logarithmic value - setting it to zero means “1 page”, setting it to 1 means “2 pages”, setting it to 2 means “4 pages”, etc. Zero disables swap readahead completely.

The default value is three (eight pages at a time). There may be some small benefits in tuning this to a different value if your workload is swap-intensive.

Lower values mean lower latencies for initial faults, but at the same time extra faults and I/O delays for following faults if they would have been part of that consecutive pages readahead would have brought in.

page_lock_unfairness¶

This value determines the number of times that the page lock can be stolen from under a waiter. After the lock is stolen the number of times specified in this file (default is 5), the “fair lock handoff” semantics will apply, and the waiter will only be awakened if the lock can be taken.

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. Even oom happens under memory cgroup, the whole system panics.

The default value is 0.

1 and 2 are for failover of clustering. Please select either according to your policy of failover.

panic_on_oom=2+kdump gives you very strong tool to investigate why oom happens. You can get snapshot.

panic_on_unrecoverable_memory_failure¶

When a hardware memory error (e.g. multi-bit ECC) hits a kernel page that cannot be recovered by the memory failure handler, the default behaviour is to ignore the error and continue operation. This is dangerous because the corrupted data remains accessible to the kernel, risking silent data corruption or a delayed crash when the poisoned memory is next accessed.