When an application starts in a container, it allocates a certain amount of memory for its needs. Usually, the allocated memory is much more than the application actually requires for its execution. This may lead to a situation when you cannot run an application in the container even if it has enough free memory. To deal with such situations, the VSwap memory management scheme introduces the new vm_overcommit option. Using it, you can configure the amount of memory applications in a container may allocate, irrespective of the amount of RAM and swap space assigned to the container.
The amount of memory that can be allocated by applications of a container is the sum of RAM and swap space set for this container multiplied by a memory overcommit factor. In the default (basic) container configuration file, this factor is set to 1.5. For example, if a container is based on the default configuration file and assigned 1 GB of RAM and 512 MB of swap, the memory allocation limit for the container will be 2304 MB. You can configure this limit and set it, for example, to 3 GB by running this command:
# vzctl set MyCT --vm_overcommit 2 --save
This command uses the factor of 2 to increase the memory allocation limit to 3 GB: (1 GB of RAM + 512 MB of swap) * 2 = 3 GB
Now applications in the container MyCT can allocate up to 3 GB of memory, if necessary.
