Out of memory errors with free memory and no limits set

Symptoms

On servers with free memory and no limits in place, errors similar to the following can be generated:

[crit] Memory allocation failed, aborting process. 
(12)Cannot allocate memory: AH00159: fork: Unable to fork new process 

Description 

On servers with the sysctl variable vm.overcommit_memory set to 2, overcommiting of RAM is disallowed. This sets the server to be very conservative with memory-allocation. The setting can be checked with the following command:

# sysctl -a | grep overcommit
vm.overcommit_kbytes = 0
vm.overcommit_memory = 2
vm.overcommit_ratio = 50

More information on these settings can be found in the Linux Kernel documentation.

Workaround

To correct this, change the value of vm.overcommit_memoryto "0" (default) in /etc/sysctl.conf. Run the following command to reload the values from the file:

sysctl -p