74 lines
2.2 KiB
Plaintext
74 lines
2.2 KiB
Plaintext
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.
|
|
|
|
2 - Don't overcommit. The total address space commit
|
|
for the system is not permitted to exceed swap + 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 accessing
|
|
pages but will receive errors on memory allocation as
|
|
appropriate.
|
|
|
|
The overcommit policy is set via the sysctl `vm.overcommit_memory'.
|
|
|
|
The overcommit percentage is set via `vm.overcommit_ratio'.
|
|
|
|
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
|
|
------
|
|
|
|
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
|
|
|
|
To Do
|
|
-----
|
|
o Account ptrace pages (this is hard)
|