07.01.08 @ 20:58 Dag-Erling Smørgrav wrote:
"Vadim Goncharov" <[EMAIL PROTECTED]> writes:
There were case in our town when on heavy loaded web-server apache
processes were dying on memory pressure - aforementioned man said that
was due to overcommit and OOM killer working.
Well, technically, it was because the server didn't have enough RAM for
the workload it was given. Turning off memory overcommit wouldn't fix
that, it would just change the symptoms.
Yes, but when they have a multi-gigabyte-RAM server, and told that Linux
will be better - no matter they are technically so competent or not, we
can loose users...
I don't know of a single server OS that doesn't overcommit memory. The
only difference between them is how they behave once the shit hits the
fan.
I've heard about disabling it for selected processes or things like memory
reservation backed by temporary files done by OS (afair, it was HP-UX). Or
Linux overcommit switch, for which this ordinary people are happy enough
to not blame (here are defaults):
master:~# cat /proc/sys/vm/overcommit_ratio
50
master:~# cat /proc/sys/vm/overcommit_memory
0
master:~# cat
/usr/src/linux-2.6.16.21-0.8/Documentation/vm/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 slighly 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)
master:~#
Anyway, as somebody else mentioned, the details are in the archives - if
you don't know enough English to find them there, I don't see how having
them summarized in English will help. If the language barrier really is
a problem, ask someone who speaks your language to help you.
1) It is simpler to understand a relatively short summary or ask help from
language-knowing man for this not so big text, than try to ask such man
for many pages
2) Such an article would be good in terms of advocacy, to explain why this
is not a bug, we are not-worse-in-this-place than their-cool-OS, and what
programmer should do. Of course, in case this is really so - or maybe we
want to commit a patch (Kostik Belousov's one may be good start point)
with tunable allowing to disable overcommit?..
--
WBR, Vadim Goncharov
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