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diff --git a/Documentation/sysctl/vm.txt b/Documentation/sysctl/vm.txt deleted file mode 100644 index 749322060f10..000000000000 --- a/Documentation/sysctl/vm.txt +++ /dev/null @@ -1,946 +0,0 @@ -Documentation for /proc/sys/vm/* kernel version 2.6.29 - (c) 1998, 1999, Rik van Riel <riel@nl.linux.org> - (c) 2008 Peter W. Morreale <pmorreale@novell.com> - -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 2.6.29. - -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. - -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: - -- admin_reserve_kbytes -- block_dump -- compact_memory -- compact_unevictable_allowed -- dirty_background_bytes -- dirty_background_ratio -- dirty_bytes -- dirty_expire_centisecs -- dirty_ratio -- dirtytime_expire_seconds -- dirty_writeback_centisecs -- drop_caches -- extfrag_threshold -- hugetlb_shm_group -- laptop_mode -- legacy_va_layout -- lowmem_reserve_ratio -- max_map_count -- 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 -- 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 -- panic_on_oom -- percpu_pagelist_fraction -- stat_interval -- stat_refresh -- numa_stat -- swappiness -- unprivileged_userfaultfd -- user_reserve_kbytes -- vfs_cache_pressure -- 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. - -============================================================== - -block_dump - -block_dump enables block I/O debugging when set to a nonzero value. More -information on block I/O debugging is in Documentation/laptops/laptop-mode.txt. - -============================================================== - -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. - -============================================================== - -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. - -============================================================== - -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. - -============================================================== - -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. - -============================================================== - -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. - -============================================================== - -laptop_mode - -laptop_mode is a knob that controls "laptop mode". All the things that are -controlled by this knob are discussed in Documentation/laptops/laptop-mode.txt. - -============================================================== - -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 followings. (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 65536. - -============================================================= - -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 uptodate 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 - -============================================================== - -nr_hugepages - -Change the minimum size of the hugepage pool. - -See Documentation/admin-guide/mm/hugetlbpage.rst - -============================================================== - -nr_hugepages_mempolicy - -Change the size of the hugepage pool at run-time on a specific -set of NUMA nodes. - -See Documentation/admin-guide/mm/hugetlbpage.rst - -============================================================== - -nr_overcommit_hugepages - -Change the maximum size of the hugepage pool. The maximum is -nr_hugepages + nr_overcommit_hugepages. - -See Documentation/admin-guide/mm/hugetlbpage.rst - -============================================================== - -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 Documentation/nommu-mmap.txt 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 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. -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 Documentation/vm/overcommit-accounting.rst 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. - -============================================================= - -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. - -============================================================= - -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. If the user writes '0' to this -sysctl, it will revert to this default behavior. - -============================================================== - -stat_interval - -The time interval between which vm statistics are updated. The default -is 1 second. - -============================================================== - -stat_refresh - -Any read or write (by root only) flushes all the per-cpu vm statistics -into their global totals, for more accurate reports when testing -e.g. cat /proc/sys/vm/stat_refresh /proc/meminfo - -As a side-effect, it also checks for negative totals (elsewhere reported -as 0) and "fails" with EINVAL if any are found, with a warning in dmesg. -(At time of writing, a few stats are known sometimes to be found negative, -with no ill effects: errors and warnings on these stats are suppressed.) - -============================================================== - -numa_stat - -This interface allows runtime configuration of numa statistics. - -When page allocation performance becomes a bottleneck and you can tolerate -some possible tool breakage and decreased numa counter precision, you can -do: - echo 0 > /proc/sys/vm/numa_stat - -When page allocation performance is not a bottleneck and you want all -tooling to work, you can do: - echo 1 > /proc/sys/vm/numa_stat - -============================================================== - -swappiness - -This control is used to define how aggressive the kernel will swap -memory pages. Higher values will increase aggressiveness, lower values -decrease the amount of swap. A value of 0 instructs the kernel not to -initiate swap until the amount of free and file-backed pages is less -than the high water mark in a zone. - -The default value is 60. - -============================================================== - -unprivileged_userfaultfd - -This flag controls whether unprivileged users can use the userfaultfd -system calls. Set this to 1 to allow unprivileged users to use the -userfaultfd system calls, or set this to 0 to restrict userfaultfd to only -privileged users (with SYS_CAP_PTRACE capability). - -The default value is 1. - -============================================================== - -- user_reserve_kbytes - -When overcommit_memory is set to 2, "never overcommit" mode, reserve -min(3% of current process size, user_reserve_kbytes) of free memory. -This is intended to prevent a user from starting a single memory hogging -process, such that they cannot recover (kill the hog). - -user_reserve_kbytes defaults to min(3% of the current process size, 128MB). - -If this is reduced to zero, then the user will be allowed to allocate -all free memory with a single process, minus admin_reserve_kbytes. -Any subsequent attempts to execute a command will result in -"fork: Cannot allocate memory". - -Changing this takes effect whenever an application requests memory. - -============================================================== - -vfs_cache_pressure ------------------- - -This percentage value controls the tendency of the kernel to reclaim -the memory which is used for caching of directory and inode objects. - -At the default value of vfs_cache_pressure=100 the kernel will attempt to -reclaim dentries and inodes at a "fair" rate with respect to pagecache and -swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer -to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will -never reclaim dentries and inodes due to memory pressure and this can easily -lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond 100 -causes the kernel to prefer to reclaim dentries and inodes. - -Increasing vfs_cache_pressure significantly beyond 100 may have negative -performance impact. Reclaim code needs to take various locks to find freeable -directory and inode objects. With vfs_cache_pressure=1000, it will look for -ten times more freeable objects than there are. - -============================================================= - -watermark_boost_factor: - -This factor controls the level of reclaim when memory is being fragmented. -It defines the percentage of the high watermark of a zone that will be -reclaimed if pages of different mobility are being mixed within pageblocks. -The intent is that compaction has less work to do in the future and to -increase the success rate of future high-order allocations such as SLUB -allocations, THP and hugetlbfs pages. - -To make it sensible with respect to the watermark_scale_factor -parameter, the unit is in fractions of 10,000. The default value of -15,000 on !DISCONTIGMEM configurations means that up to 150% of the high -watermark will be reclaimed in the event of a pageblock being mixed due -to fragmentation. The level of reclaim is determined by the number of -fragmentation events that occurred in the recent past. If this value is -smaller than a pageblock then a pageblocks worth of pages will be reclaimed -(e.g. 2MB on 64-bit x86). A boost factor of 0 will disable the feature. - -============================================================= - -watermark_scale_factor: - -This factor controls the aggressiveness of kswapd. It defines the -amount of memory left in a node/system before kswapd is woken up and -how much memory needs to be free before kswapd goes back to sleep. - -The unit is in fractions of 10,000. The default value of 10 means the -distances between watermarks are 0.1% of the available memory in the -node/system. The maximum value is 1000, or 10% of memory. - -A high rate of threads entering direct reclaim (allocstall) or kswapd -going to sleep prematurely (kswapd_low_wmark_hit_quickly) can indicate -that the number of free pages kswapd maintains for latency reasons is -too small for the allocation bursts occurring in the system. This knob -can then be used to tune kswapd aggressiveness accordingly. - -============================================================== - -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 disabled by default. For file servers or workloads -that benefit from having their data cached, zone_reclaim_mode should be -left disabled as the caching effect is likely to be more important than -data locality. - -zone_reclaim may be enabled if it's known that the workload is partitioned -such that each partition fits within a NUMA node and that accessing remote -memory would 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. - -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. - -============ End of Document ================================= |