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Low Memory

Because of specifics of architecture of Intel's processors, the RAM of the server 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.

In Virtuozzo 2.5.1 release, the size of the "low" memory area was limited by 832MB. In Virtuozzo 2.5.1 Service Pack 1 and later versions including Virtuozzo 4.0, the "low" memory area size is limited by 832MB in UP (uniprocessor) version of the kernel and by 3.6GB in SMP and Enterprise versions of the kernel. If the total size of server RAM is less than the limit (832MB or 3.6GB, respectively), then the actual size of the "low" memory area is equal to the total memory size. This actual size is shown in the LowTotal line in /proc/meminfo.

Utilization. The lower bound estimation of "low" memory utilization is

where

Utilization of "low" memory below 1 is normal. Utilization above 1 is not safe, and utilization above 2 is dangerous and is 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

Commitment levels below 1 are normal. Levels between 1 and 1.2 are usually acceptable for systems with about 100 Containers. Systems with more Containers may have the commitment level increased. to about 1.5-2 for 400 environments.

If the Containers actually claim the "low" memory allowed for them and the utilization closes to 1, the performance and stability of the whole system will dramatically drop. So, commitment levels exceeding the suggested ones should be considered unsafe and are not recommended.

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