VSwap

New RHEL6-based OpenVZ kernel has a new memory management model, which supersedes User beancounters. It is called VSwap.

Now you can set two primary parameters: physpages and swappages, while all the other beancounters become secondary and optional.

• physpages

This parameter limits the physical memory (RAM) available to processes inside a container.

The barrier is ignored and should be set to 0, and the limit sets the limit.

Currently (as of >= 042stab042) the user memory, the kernel memory and the page cache are accounted into physpages.

• swappages

This parameter limits the amount of swap space which can be used for processes inside a container.

The barrier is ignored and should be set to 0, and the limit sets the limit.

The sum of physpages.limit and swappages.limit limits the maximum amount of allocated memory which can be used by a container. When physpages limit is reached, memory pages belonging to the container are pushed out to so called virtual swap (vswap). The difference between normal swap and vswap is that with vswap no actual disk I/O usually occurs. Instead, a container is artificially slowed down, to emulate the effect of the real swapping. Actual swap out occurs only if there is a global memory shortage on the system.

Setting

Note: for VSwap, you need vswap-enabled kernel, ie RHEL6-based OpenVZ kernel.

Since vzctl 3.0.30, you can use --ram and --swap parameters, like this:

vzctl set 777 --ram 512M --swap 1G --save

If you are converting an existing CT from old (non-vswap) configuration, you can either

• leave all the other UBC parameters as-is,
• or set those to 'unlimited',
• or remove those from configuration file.

Convert non-vswap CT to Vswap

If you have an existing container with usual UBC parameters set, and you want to convert this one into VSwap enabled config, here's what you need to do.

1. Decide on how much RAM and swap you want this CT to have. Generally, sum of your new RAM+swap should be more or less equal to sum of old PRIVVMPAGES and KMEMSIZE.
2. Manually remove all UBC parameters from config. This is optional, you can still have UBC limits applied if you want.
3. Add PHYSPAGES and SWAPPAGES parameters to config. Easiest way is to use vzctl set $CTID --ram N --swap M --save

Now your config is vswap enabled, and when you (re)start it, vswap mechanism will be used by the kernel for this CT.

How to distinguish between vswap and non-vswap configs?

Both vzctl and the kernel treats a configuration file as vswap one if PHYSPAGES limit is not set to unlimited (a.k.a. LONG_MAX).

In addition, vzctl checks if kernel support vswap, and refuses to start a vswap-enabled container on a non vswap capable kernel. The check is presence of /proc/vz/vswap file.

See also

• Recent improvements in VSwap