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Update of Documentation: vm.txt and proc.txt
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Update Documentation/sysctl/vm.txt and Documentation/filesystems/proc.txt.
 More specifically, the section on /proc/sys/vm in
Documentation/filesystems/proc.txt was removed and a link to
Documentation/sysctl/vm.txt added.

Most of the verbiage from proc.txt was simply moved in vm.txt, with new
addtional text for "swappiness" and "stat_interval".

Signed-off-by: Peter W Morreale <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
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Peter W Morreale authored and torvalds committed Jan 16, 2009
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288 changes: 2 additions & 286 deletions Documentation/filesystems/proc.txt
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Expand Up @@ -1371,292 +1371,8 @@ auto_msgmni default value is 1.
2.4 /proc/sys/vm - The virtual memory subsystem
-----------------------------------------------

The files in this directory can be used to tune the operation of the virtual
memory (VM) subsystem of the Linux kernel.

vfs_cache_pressure
------------------

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. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.

dirty_background_bytes
----------------------

Contains the amount of dirty memory at which the pdflush background writeback
daemon will start writeback.

If dirty_background_bytes is written, dirty_background_ratio becomes a function
of its value (dirty_background_bytes / the amount of dirtyable system memory).

dirty_background_ratio
----------------------

Contains, as a percentage of the dirtyable system memory (free pages + mapped
pages + file cache, not including locked pages and HugePages), the number of
pages at which the pdflush background writeback daemon will start writing out
dirty data.

If dirty_background_ratio is written, dirty_background_bytes becomes a function
of its value (dirty_background_ratio * the amount of dirtyable system memory).

dirty_bytes
-----------

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

If dirty_bytes is written, dirty_ratio becomes a function of its value
(dirty_bytes / the amount of dirtyable system memory).

dirty_ratio
-----------

Contains, as a percentage of the dirtyable system memory (free pages + mapped
pages + file cache, not including locked pages and HugePages), the number of
pages at which a process which is generating disk writes will itself start
writing out dirty data.

If dirty_ratio is written, dirty_bytes becomes a function of its value
(dirty_ratio * the amount of dirtyable system memory).

dirty_writeback_centisecs
-------------------------

The pdflush writeback daemons 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.

dirty_expire_centisecs
----------------------

This tunable is used to define when dirty data is old enough to be eligible
for writeout by the pdflush daemons. 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 pdflush daemon wakes up.

highmem_is_dirtyable
--------------------

Only present if CONFIG_HIGHMEM is set.

This defaults to 0 (false), meaning that the ratios set above are calculated
as a percentage of lowmem only. This protects against excessive scanning
in page reclaim, swapping and general VM distress.

Setting this to 1 can be useful on 32 bit machines where you want to make
random changes within an MMAPed file that is larger than your available
lowmem without causing large quantities of random IO. Is is safe if the
behavior of all programs running on the machine is known and memory will
not be otherwise stressed.

legacy_va_layout
----------------

If non-zero, this sysctl disables the new 32-bit mmap 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
-
Note: # of this elements is one fewer than number of zones. Because the highest
zone's value is not necessary for following calculation.

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
pages_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 present_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 present
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%).

page-cluster
------------

page-cluster controls the number of pages which are written to swap in
a single attempt. The swap I/O size.

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.

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.

overcommit_memory
-----------------

Controls overcommit of system memory, possibly allowing processes
to allocate (but not use) more memory than is actually available.


0 - Heuristic overcommit handling. Obvious overcommits of
address space are refused. Used for a typical system. It
ensures a seriously wild allocation fails while allowing
overcommit to reduce swap usage. root is allowed to
allocate slightly more memory in this mode. This is the
default.

1 - Always overcommit. Appropriate for some scientific
applications.

2 - Don't overcommit. The total address space commit
for the system is not permitted to exceed swap plus a
configurable percentage (default is 50) of physical RAM.
Depending on the percentage you use, in most situations
this means a process will not be killed while attempting
to use already-allocated memory but will receive errors
on memory allocation as appropriate.

overcommit_ratio
----------------

Percentage of physical memory size to include in overcommit calculations
(see above.)

Memory allocation limit = swapspace + physmem * (overcommit_ratio / 100)

swapspace = total size of all swap areas
physmem = size of physical memory in system

nr_hugepages and hugetlb_shm_group
----------------------------------

nr_hugepages configures number of hugetlb page reserved for the system.

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

hugepages_treat_as_movable
--------------------------

This parameter is only useful when kernelcore= is specified at boot time to
create ZONE_MOVABLE for pages that may be reclaimed or migrated. Huge pages
are not movable so are not normally allocated from ZONE_MOVABLE. A non-zero
value written to hugepages_treat_as_movable allows huge pages to be allocated
from ZONE_MOVABLE.

Once enabled, the ZONE_MOVABLE is treated as an area of memory the huge
pages pool can easily grow or shrink within. Assuming that applications are
not running that mlock() a lot of memory, it is likely the huge pages pool
can grow to the size of ZONE_MOVABLE by repeatedly entering the desired value
into nr_hugepages and triggering page reclaim.

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.

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.

swap_token_timeout
------------------

This file contains valid hold time of swap out protection token. The Linux
VM has token based thrashing control mechanism and uses the token to prevent
unnecessary page faults in thrashing situation. The unit of the value is
second. The value would be useful to tune thrashing behavior.

drop_caches
-----------

Writing to this will cause the kernel to drop clean caches, dentries and
inodes from memory, causing that memory to become free.

To free pagecache:
echo 1 > /proc/sys/vm/drop_caches
To free dentries and inodes:
echo 2 > /proc/sys/vm/drop_caches
To free pagecache, dentries and inodes:
echo 3 > /proc/sys/vm/drop_caches

As this is a non-destructive operation and dirty objects are not freeable, the
user should run `sync' first.
Please see: Documentation/sysctls/vm.txt for a description of these
entries.


2.5 /proc/sys/dev - Device specific parameters
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