Postgresql 启动报错 FATAL: could not map anonymous shared memory
FATAL: could not map anonymous shared memory: Cannot allocate memory
HINT: This error usually means that PostgreSQL’s request for a shared memory segment exceeded available memory, swap space, or huge pages. To reduce the request size (currently 8757182464 bytes), reduce PostgreSQL’s shared memory usage, perhaps by reducing shared_buffers or max_connections.
修改大页后，修改了shared_buffers参数为8G,启动数据库报错
解决方法：
修改内核参数
vm.overcommit_memory=2
或者修改
echo -1000 > /proc/self/oom_score_adj
或者减少shared buffers参数设置
If PostgreSQL itself is the cause of the system running out of memory, you can avoid the problem by changing your configuration. In some cases, it may help to lower memory-related configuration parameters, particularly shared_buffers and work_mem. In other cases, the problem may be caused by allowing too many connections to the database server itself. In many cases, it may be better to reduce max_connections and instead make use of external connection-pooling software.
On Linux 2.6 and later, it is possible to modify the kernel’s behavior so that it will not “overcommit” memory. Although this setting will not prevent the OOM killer from being invoked altogether, it will lower the chances significantly and will therefore lead to more robust system behavior. This is done by selecting strict overcommit mode via sysctl:
sysctl -w vm.overcommit_memory=2 –临时修改，重启无效
vi /etc/sysctl.conf 修改配置文件
加入vm.overcommit_memory=2
sysctl -p 载入配置文件，永久生效
or placing an equivalent entry in /etc/sysctl.conf. You might also wish to modify the related setting vm.overcommit_ratio. For details see the kernel documentation file Documentation/vm/overcommit-accounting.
Another approach, which can be used with or without altering vm.overcommit_memory, is to set the process-specific oom_score_adj value for the postmaster process to -1000, thereby guaranteeing it will not be targeted by the OOM killer. The simplest way to do this is to execute
echo -1000 > /proc/self/oom_score_adj
in the postmaster’s startup script just before invoking the postmaster. Note that this action must be done as root, or it will have no effect; so a root-owned startup script is the easiest place to do it. If you do this, you may also wish to build PostgreSQL with -DLINUX_OOM_SCORE_ADJ=0 added to CPPFLAGS. That will cause postmaster child processes to run with the normal oom_score_adj value of zero, so that the OOM killer can still target them at need.
Older Linux kernels do not offer /proc/self/oom_score_adj, but may have a previous version of the same functionality called /proc/self/oom_adj. This works the same except the disable value is -17 not -1000. The corresponding build flag for PostgreSQL is -DLINUX_OOM_ADJ=0.
https://www.kernel.org/doc/Documentation/vm/overcommit-accounting
vm.overcommit_memory的值为0时，不允许用户overcommit
vm.overcommit_memory的值为1时，总是可以overcommit
vm.overcommit_memory的值为2时，不允许超过swap+物理内存的*50%(overcommit_ratio决定这个百分比)
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) vm.overcommit_kbytes’ (absolute value).
or
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)
参考：
https://www.kernel.org/doc/Documentation/vm/overcommit-accounting
http://www.postgresql.org/docs/9.4/static/kernel-resources.html –17.4.3 linux memory overcommit 章节