89 строки
2.7 KiB
ReStructuredText
89 строки
2.7 KiB
ReStructuredText
.. _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.
|
|
|
|
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.
|
|
|
|
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 amount can be set via ``vm.overcommit_ratio`` (percentage)
|
|
or ``vm.overcommit_kbytes`` (absolute value). These only have an effect
|
|
when ``vm.overcommit_memory`` is set to 2.
|
|
|
|
The current overcommit limit and amount committed are viewable in
|
|
``/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
|
|
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.
|
|
|
|
|
|
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
|
|
|
|
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
|
|
|
|
Status
|
|
======
|
|
|
|
* 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
|
|
=====
|
|
* Account ptrace pages (this is hard)
|