WSL2-Linux-Kernel/Documentation/sysctl/vm.txt

Documentation for /proc/sys/vm/*	kernel version 2.2.10
	(c) 1998, 1999,  Rik van Riel <riel@nl.linux.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
- max_map_count
- min_free_kbytes
- laptop_mode
- block_dump
- drop-caches
- zone_reclaim_mode
- min_unmapped_ratio
- min_slab_ratio
- panic_on_oom
- mmap_min_address
- numa_zonelist_order

==============================================================

dirty_ratio, dirty_background_ratio, dirty_expire_centisecs,
dirty_writeback_centisecs, 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.

==============================================================

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.

==============================================================

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.