Commit 8d83d826 authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet

docs/vm: overcommit-accounting: convert to ReST format

Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent cb5e4376
.. _overcommit_accounting:
=====================
Overcommit Accounting
=====================
The Linux kernel supports the following overcommit handling modes
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.
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. Classic example is code using sparse arrays
and just relying on the virtual memory consisting almost
entirely of zero pages.
1
Always overcommit. Appropriate for some scientific
applications. Classic example is code using sparse arrays and
just relying on the virtual memory consisting almost entirely
of zero pages.
2 - Don't overcommit. The total address space commit
for the system is not permitted to exceed swap + a
configurable amount (default is 50%) of physical RAM.
Depending on the amount you use, in most situations
this means a process will not be killed while accessing
pages but will receive errors on memory allocation as
appropriate.
2
Don't overcommit. The total address space commit for the
system is not permitted to exceed swap + a configurable amount
(default is 50%) of physical RAM. Depending on the amount you
use, in most situations this means a process will not be
killed while accessing pages but will receive errors on memory
allocation as appropriate.
Useful for applications that want to guarantee their
memory allocations will be available in the future
without having to initialize every page.
Useful for applications that want to guarantee their memory
allocations will be available in the future without having to
initialize every page.
The overcommit policy is set via the sysctl `vm.overcommit_memory'.
The overcommit policy is set via the sysctl ``vm.overcommit_memory``.
The overcommit amount can be set via `vm.overcommit_ratio' (percentage)
or `vm.overcommit_kbytes' (absolute value).
The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage)
or ``vm.overcommit_kbytes`` (absolute value).
The current overcommit limit and amount committed are viewable in
/proc/meminfo as CommitLimit and Committed_AS respectively.
``/proc/meminfo`` as CommitLimit and Committed_AS respectively.
Gotchas
-------
=======
The C language stack growth does an implicit mremap. If you want absolute
guarantees and run close to the edge you MUST mmap your stack for the
guarantees and run close to the edge you MUST mmap your stack for the
largest size you think you will need. For typical stack usage this does
not matter much but it's a corner case if you really really care
In mode 2 the MAP_NORESERVE flag is ignored.
In mode 2 the MAP_NORESERVE flag is ignored.
How It Works
------------
============
The overcommit is based on the following rules
For a file backed map
SHARED or READ-only - 0 cost (the file is the map not swap)
PRIVATE WRITABLE - size of mapping per instance
| SHARED or READ-only - 0 cost (the file is the map not swap)
| PRIVATE WRITABLE - size of mapping per instance
For an anonymous or /dev/zero map
SHARED - size of mapping
PRIVATE READ-only - 0 cost (but of little use)
PRIVATE WRITABLE - size of mapping per instance
For an anonymous or ``/dev/zero`` map
| SHARED - size of mapping
| PRIVATE READ-only - 0 cost (but of little use)
| PRIVATE WRITABLE - size of mapping per instance
Additional accounting
Pages made writable copies by mmap
shmfs memory drawn from the same pool
| Pages made writable copies by mmap
| shmfs memory drawn from the same pool
Status
------
o We account mmap memory mappings
o We account mprotect changes in commit
o We account mremap changes in size
o We account brk
o We account munmap
o We report the commit status in /proc
o Account and check on fork
o Review stack handling/building on exec
o SHMfs accounting
o Implement actual limit enforcement
======
* We account mmap memory mappings
* We account mprotect changes in commit
* We account mremap changes in size
* We account brk
* We account munmap
* We report the commit status in /proc
* Account and check on fork
* Review stack handling/building on exec
* SHMfs accounting
* Implement actual limit enforcement
To Do
-----
o Account ptrace pages (this is hard)
=====
* Account ptrace pages (this is hard)
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