The PostgreSQL 9.0 Reference Manual - Volume 3 - Server Administration Guide by The PostgreSQL Global Development Group Paperback (6"x9"), 274 pages ISBN 9781906966072 RRP £9.95 ($14.95) Sales of this book support the PostgreSQL project! Get a printed copy>>>

PostgreSQL 9.0 Reference Manual Volume 3 Buy the book here! >>> support free documentation

3.4.3 Linux Memory Overcommit

In Linux 2.4 and later, the default virtual memory behavior is not optimal for PostgreSQL. Because of the way that the kernel implements memory overcommit, the kernel might terminate the PostgreSQL server (the master server process) if the memory demands of another process cause the system to run out of virtual memory.

If this happens, you will see a kernel message that looks like this (consult your system documentation and configuration on where to look for such a message):

Out of Memory: Killed process 12345 (postgres).

This indicates that the ‘postgres’ process has been terminated due to memory pressure. Although existing database connections will continue to function normally, no new connections will be accepted. To recover, PostgreSQL will need to be restarted.

One way to avoid this problem is to run PostgreSQL on a machine where you can be sure that other processes will not run the machine out of memory. If memory is tight, increasing the swap space of the operating system can help avoid the problem, because the out-of-memory (OOM) killer is invoked only when physical memory and swap space are exhausted.

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

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_adj value for the postmaster process to -17, thereby guaranteeing it will not be targeted by the OOM killer. The simplest way to do this is to execute

echo -17 > /proc/self/oom_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_ADJ=0 added to CFLAGS. That will cause postmaster child processes to run with the normal oom_adj value of zero, so that the OOM killer can still target them at need.

Note: Some vendors' Linux 2.4 kernels are reported to have early versions of the 2.6 overcommit sysctl parameter. However, setting vm.overcommit_memory to 2 on a 2.4 kernel that does not have the relevant code will make things worse, not better. It is recommended that you inspect the actual kernel source code (see the function vm_enough_memory in the file ‘mm/mmap.c’) to verify what is supported in your kernel before you try this in a 2.4 installation. The presence of the ‘overcommit-accounting’ documentation file should not be taken as evidence that the feature is there. If in any doubt, consult a kernel expert or your kernel vendor.