Memory Limits

A vserver kernel keeps track many resources used by each guest (context). Some of these relate to memory usage by the guest. You can place limits on these resources to prevent guests from using all the host memory and making the host unusable.

Two resources are particularly important in this regard:

• The Resident Set Size (rss) is the amount of pages currently present in RAM.
• The Address Space (as) is the total amount of memory (pages) mapped in each process in the context.

Both are measured in pages, which are 4 kB each on Intel machines (i386). So a value of 200000 means a limit of 800,000 kB, a little less than 800 MB.

Each resource has a soft and a hard limit.

• If a guest exceeds the rss hard limit, the kernel will invoke the Out-of-Memory (OOM) killer to kill some process in the guest.
• The rss soft limit is shown inside the guest as the maximum available memory. If a guest exceeds the rss soft limit, it will get an extra "bonus" for the OOM killer (proportional to the oversize).
• If a guest exceeds the as hard limit, memory allocation attempts will return an error, but no process is killed.
• The as soft limit is not in utilized until now. In the future it may be used to penalizing guests over that limit or it could be used to force swapping on them and such ...

Bertl explained the difference between rss and as with the following example. If two processes share 100 MB of memory, then only 100 MB worth of virtual memory pages can be used at most, so the RSS use of the guest increases by 100 MB. However, two processes are using it, so the AS use increases by 200 MB.

This makes me think that limiting AS is less useful than limiting RSS, since it doesn't directly reflect real, limited resources (RAM and swap) on the host, that deprive other virtual machines of those resources. Bertl says that AS limits can be used to give guests a "gentle" warning that they are running out of memory, but I don't know how much more gentle it is, or how to set it accurately.

For example, 100 processes each mapping a 100 MB file would consume a total of 10 GB of address space (AS), but no more than 100 MB of resources on the host. But if you set the AS limit to 10 GB, then it will not stop one process from allocating 4 GB of RAM, which could kill the host or result in that process being killed by the OOM killer.

You can set the hard limit on a particular context, effective immediately, with this command:

/usr/sbin/vlimit -c <xid> --<resource> <value>

<xid> is the context ID of the guest, which you can determine with the /usr/sbin/vserver-stat command.

For example, if you want to change the rss hard limit for the vserver with <xid> 49000, and limit it to 10,000 pages (40 MB), you could use this command:

/usr/sbin/vlimit -c 49000 --rss 10000

You can change the soft limit instead by adding the -S parameter.

Changes made with the vlimit command are effective only until the vserver is stopped. To make permanent changes, write the value to this file:

/etc/vservers/<name>/rlimits/<resource>.hard

To set a soft limit, use the same file name without the .hard extension. The rlimits directory is not created by default, so you may need to create it yourself.

Changes to these files take effect only when the vserver is started. To make immediate and permanent changes to a running vserver, you need to run vlimit and update the rlimits file.

The safest setting, to prevent any guest from interfering with any other, is to set the total of all RSS hard limits (across all running guests) to be less than the total virtual memory (RAM and swap) on the host. It should be sufficiently less to leave room for processes running on the host, and some disk cache, perhaps 100 MB.

However, this is very conservative, since it assumes the worst case where all guests are using the maximum amount of memory at one time. In practice, you can usually get away with contended resources, i.e. allowing guests to use more than this value.