Setting memory limits
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
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
Changes made with the vlimit command are effective only until the vserver is stopped. To make permanent changes, write the value to this file:
To set a soft limit, use the same file name with the
.soft extension. The
rlimits directory is not created by default, so you may need to create it yourself.
If you omit the suffix after the
/etc/vservers/<name>/rlimits/rss file, the value will be set for both, the hard and soft limit.
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.
Displaying current memory limits
To display the currently active RSS limits for a vserver execute the following command:
vlimit -c <xid> -a -d | grep RSS
The above command will display a similar result, whereas the third value (5000) is the soft limit and the last reflects the current hard limit (10000).
RSS N/A 5000 10000
Display memory limits within a vserver
free command will display the RAM and Swap usage of the host while invoked within a vserver. To change this behavior, add the
VIRT_MEM context flag to your vserver configuration:
echo "VIRT_MEM" >> /etc/vservers/<name>/flags
After a successful restart of the related vserver, the total available RAM will equal to the value of
rss.soft while the difference of
rss.soft will be displayed as swap space.
As an example, if you set the
rss.hard limit to 10'000 pages and the
rss.soft limit to 7'000 pages:
vlimit -c <xid> --rss 10000 vlimit -c <xid> -S -rss 7000 vlimit -c basesystem-vserver -a -d | grep RSS
RSS N/A 7000 10000
free will report 28'000 KB of total memory and 12'000 KB of Swap space (assuming that one page is 4 KB):
total used free shared buffers cached Mem: 28000 4396 23604 0 0 0 -/+ buffers/cache: 4396 23604 Swap: 12000 0 12000
Note: According to Herbert, the kernel won't use any real swap as soon as the
rss.soft limit has been reached. Swapping will be done on the host level, not per vserver (see "free" command inside vserver).