UBC systemwide configuration

User Beancounters Definition /proc/user_beancounters /proc/bc/ General information Units of measurement VSwap Parameters description Primary parameters numproc, numtcpsock, numothersock, vmguarpages Secondary parameters kmemsize, tcpsndbuf, tcprcvbuf, othersockbuf, dgramrcvbuf, oomguarpages, privvmpages Auxiliary parameters lockedpages, shmpages, physpages, numfile, numflock, numpty, numsiginfo, dcachesize, numiptent, swappages Internals User pages accounting RSS fractions accounting On-demand accounting UBC consistency Consistency formulae System-wide configuration vzubc(8) Configuration examples Basic Derived Intermediate configurations Tables List of parameters Parameter properties Consistency Config examples

The UBC consistency check article discussed validation of a resource control configuration for a single Virtual Environment. This article discusses checks that the configuration of the whole system is valid.

Configurations where resources allowed for Virtual Environments exceed system capacity are not valid and dangerous from stability point of view. They may result in abnormal termination of the applications, bad responsiveness of the system and, sometimes, system hangs. Whereas the configuration validation discussed in UBC consistency check addressed application functionality, the validation considered in this section is aimed at security and stability of the whole system.

The best way to make sure that the configuration of the whole system is valid is to run periodic automatic checks, based on the formulae described below.

Resource utilization and commitment level

Several resources of the whole system (such as RAM) are discussed below in terms of utilization and commitment level.

Utilization shows the amount of resources consumed by all Virtual Environments at the given time. In general, low utilization values mean that the system is under-utilized. Often, it means that the system is capable of supporting more Virtual Environment if the existing VEs continue to maintain the same load and resource consumption level. High utilization values (in general, more than 1) mean the the system is overloaded and the service level of the Virtual Environments is degraded.

Commitment level shows how much resources are “promised” to the existing Virtual Environments. Low commitment levels mean that the system is capable of supporting more Virtual Environments. Commitment levels more than 1 mean that the Virtual Environments are promised more resources than the system has, and in this case the system is said to be overcommitted. If the system runs a lot of Virtual Environments, it is usually acceptable to have some overcommitment, because it is unlikely that all Virtual Environments will request resources at the same time. However, higher commitment levels (as discussed below for each resource individually) will cause Virtual Environmentss to experience failures to allocate and use the resources promised to them.

“Low memory”

Because of specifics of architecture of Intel's processors, the RAM of the computer can't be used uniformly. The most important memory area is so called “low memory”, a part of memory residing at lower addresses and directly accessible by the kernel. For current Linux kernels, the size of low memory area is 832MB (or, if the computer has less RAM than 832MB, the size of the RAM).

Utilization

The lower bound estimation of low memory utilization is

${\frac {\displaystyle \sum _{all\ VE}(kmemsize_{cur}+allsocketbuf_{cur})}{0.4\cdot \min(RAM\ size,{\rm {832MB}})}}{\rm {,}}$

where

$allsocketbuf=tcprcvbuf+tcpsndbuf+dgramrcvbuf+othersockbuf{\rm {.}}$

Utilization of low memory below $1$ is normal. Utilization above $1$ is not safe, and utilization above $2$ is dangerous is very likely to cause bad system responsiveness, application stalls for seconds or more and termination of some applications.

Commitment level

The commitment level can be computed as

${\frac {\displaystyle \sum _{all\ VE}(kmemsize_{lim}+allsocketbuf_{lim})}{0.4\cdot \min(RAM\ size,{\rm {832MB}})}}{\rm {.}}$

Commitment levels below $1$ are normal. Levels between $1$ and $1.2$ are usually acceptable for systems with about 100 Virtual Environments. Systems with more Virtual Environments may have the commitment level increased, to about $1.5$—$2$ for 400 VEs. Higher commitment levels for this resource are not recommended, because the consequences of exceeding the low memory capacities are severe and affect the whole system and all the Virtual Environments.

Total RAM

This subsection discusses usage of the whole RAM and its utilization. Usage of swap space and the sum of used RAM and swap space are discussed below in subsection Memory and swap space.

Current version of OpenVZ can't guarantee availability of certain amount of memory (in opposite to the sum of memory and swap space), so the commitment level is not applicable to the total RAM. Such guarantees will be implemented in future versions.

Utilization

The amount of RAM consumed by all Virtual Environments can be computed as

$\sum _{all\ VE}(physpages_{cur}\cdot 4096+kmemsize_{cur}+allsocketbuf_{cur}){\rm {.}}$

The difference between memory usage shown by free(1) or /proc/meminfo and the total amount of RAM consumed by Virtual Environments is the memory used by system daemons and different caches.

The memory utilization can be computed as

${\frac {\displaystyle \sum _{all\ VE}(physpages_{cur}\cdot 4096+kmemsize_{cur}+allsocketbuf_{cur})}{RAM\ size}}{\rm {.}}$

Utilization levels from $0.8$ to $1$ are normal. Lower utilization means that the system is under-utilized, and, if other system resources and their commitment levels permit, the system can host more Virtual Environments. By the nature of the accounting of physpages and other parameters, total RAM utilization can't be bigger than $1$.

Memory and swap space

The main resource of the computer determining the amount of memory applications can use is the sum of RAM and swap sizes. If the total size of the used memory exceeds the RAM size, Linux kernel moves some data to swap and and loads it back when the application needs it. More frequently used data tends to stay in RAM, less frequently used data spends more time in swap.

Swap-in and swap-out activity reduces the system performance to some extent. However, if this activity is not excessive, the performance decrease is not very noticeable. On the other hand, the benefits of using swap space are quite big, allowing to increase the number of Virtual Private Servers in the system about 2 times.

Swap space is essential for handling system load bursts. System with enough swap just slows down at high load bursts, whereas the system without swap reacts to high load bursts by refusing memory allocations (causing applications to refuse to accept clients or terminate) and by direct killing of some applications. Additionally, the presence of swap space helps the system to better balance memory and move data between “low memory” and the rest of the RAM.

In all OpenVZ installations it is strongly recommended to have swap of size not less than the RAM size.

Also, it is not recommended to create swap space of the size of more than 4 times RAM size because of performance degradation related to swap-in and swap-out activity. That is, the system should be configured so that $RAM\ size\leq swap\ size\leq 4\cdot RAM\ size{\rm {.}}$ The optimal configuration is when swap size is twice more than the RAM size.

Utilization

${\frac {\displaystyle \sum _{all\ VE}(oomguarpages_{cur}\cdot 4096+kmemsize_{cur}+allsocketbuf_{cur})}{RAM\ size+swap\ size}}{\rm {.}}$

The normal utilization of memory plus swap ranges between ${\frac {RAM\ size}{RAM\ size+swap\ size}}{\rm {\quad and\quad }}{\frac {RAM\ size+{\frac {1}{2}}swap\ size}{RAM\ size+swap\ size}}.$

Lower utilization means that the system memory is under-utilized at the moment of checking the utilization. Higher utilization is likely to cause gradual performance degradation because of swap-in and swap-out activity and is a sign of overloading of the system.

Commitment level

${\frac {\displaystyle \sum _{all\ VE}(oomguarpages_{bar}\cdot 4096+kmemsize_{lim}+allsocketbuf_{lim})}{RAM\ size+swap\ size}}{\rm {.}}$

The normal commitment level is about $0.8$—$1$.

Commitment levels more than $1$ means that the Virtual Environmens are guaranteed more memory than the system has. Such overcommitment is strongly not recommended, because in that case if all the memory is consumed, random applications, including the ones belonging to the host system, may be killed and the system may become inaccessible by ssh(1) and lose other important functionality.

It is better to guarantee Virtual Environments less and have less commitment levels than to accidently overcommit the system by memory plus swap. If the system has spare memory and swap, Virtual Environments will transparently be able to use the memory and swap above their guarantees. Guarantees given to Virtual Environments should not be big, and it is normal if memory and swap usage for some VEs stays above their guarantee. It is also normal to give guarantees only to Virtual Environments with preferred service. But administrators should not guarantee Virtual Environment more

than the system actually has.