VSwap

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

Primary parameters

With VSwap, there are two required parameters: ram and swap (a.k.a. physpages and swappages). All the other beancounters become optional.

• physpages

This parameter sets the amount of fast physical memory (RAM) available to processes inside a container, in memory pages. Currently (as of 042stab042 kernel) the user memory, the kernel memory and the page cache are accounted into physpages.

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

• ram

is an easy shortcut for physpages.limit, and is measured in bytes

• swappages

This parameter sets the amount of "slower memory" (vswap) available to processes inside a container, in memory pages.

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

• swap

is an easy shortcut for swappages.limit, and is measured in bytes

The sum of physpages.limit and swappages.limit limits the maximum amount of 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.

Note: swap used by a container can exceed swappages.limit, but is always within sum of physpages.limit and swappages.limit.

Implicit UBC parameters

Since vzctl 4.6, if some optional beancounters are not set, vzctl sets them implicitly, based on ram and swap settings.

The following formulae are used:

$lockedpages_{bar}=oomguarpages_{bar}=ram$

$lockedpages_{lim}=oomguarpages_{lim}=\infty$

$vmguarpages_{bar}=vmguarpages_{lim}=ram+swap$

VM overcommit and privvmpages

vzctl 4.6 adds a new parameter, --vm_overcommit. Its only purpose is to be used in privvmpages calculation, in case VSwap is used and there is no explicit setting for privvmpages.

If $vm\_overcommit$ is set:

$privvmpages_{bar}=privvmpages_{lim}=(ram+swap)\times vm\_overcommit$

If it is not set:

$privvmpages_{bar}=privvmpages_{lim}=\infty$

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

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 (or use --reset_ub), vswap mechanism will be used by the kernel for this CT.

Here is an example of the above steps:

CTID=123
RAM=1G
SWAP=2G
CFG=/etc/vz/conf/${CTID}.conf
cp $CFG $CFG.pre-vswap
grep -Ev '^(KMEMSIZE|LOCKEDPAGES|PRIVVMPAGES|SHMPAGES|NUMPROC|PHYSPAGES|VMGUARPAGES|OOMGUARPAGES|NUMTCPSOCK|NUMFLOCK|NUMPTY|NUMSIGINFO|TCPSNDBUF|TCPRCVBUF|OTHERSOCKBUF|DGRAMRCVBUF|NUMOTHERSOCK|DCACHESIZE|NUMFILE|AVNUMPROC|NUMIPTENT|ORIGIN_SAMPLE|SWAPPAGES)=' > $CFG <  $CFG.pre-vswap
vzctl set $CTID --ram $RAM --swap $SWAP --save
vzctl set $CTID --reset_ub

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). You can also use the following command:

# vzlist -o vswap $CTID

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.

Details about vSwap implementation in Virtuozzo 7

The Container swapping process is similar to that on a standalone computer.
This means in particular that some pages may get into the swap even if there is some free memory reported in the Container.
This may validly happen in case kernel memory management system detects some anonymous memory which is not touched for a long time by Container processes and decides that it's more efficient to put these anonymous pages into the swap and use more caches in a Container instead.

The Container swap space resides in physical node swap file.
When the swap-out for a Container starts, appropriate number of pages are allocated in physical swap on the host. Next

1. if there is no free memory on the host, the real swap-out of Container's memory to physical swap happens
2. if there is free memory on the host, the Container's memory is saved in a special swap cache in host's RAM and no real write to host's physical swap occurs

Note: The physical swap space is allocated in both cases anyway, this guarantees all amount of host's swap cache for Containers memory can be written into physical swap on host in case the host gets short of RAM.

• Consequence 1: without any configured node swap file the Container's `SWAPPAGES` parameter will be ignored.

• Consequence 2: if node's swap size is less than sum of all Containers' swap sizes on the node, Containers won't be able to use 100% of their swap simultaneously - similar to RAM settings.

See also

• On vSwap and 042stab04x kernel improvements
• Recent improvements in vzctl