While logging to a VPS from the host server(main node) , you see the message:

Unable to fork: Cannot allocate memory

This usually occurs when VPS is running out of resources especially RAM. Restarting the vps may be a temporary fix which allows you to enter the server . But it prevents executing some commands throwing the same error message .

You can increase privvmpages and kmemsize values for the VPS in order to solve the issue .From the client node in order to see each resource , run the command

cat /proc/user_beancounters

It has five values associated with it: current usage, maximum usage , barrier, limit, and fail counter. The meaning of barrier and limit is parameter-dependent; in short, those can be thought of as a soft limit and a hard limit. If any resource hits the limit, the fail counter for it is increased. This allows the owner to detect problems by monitoring /proc/user_beancounters in the container.

uid      resource         held        maxheld    barrier          limit failcnt

101:    kmemsize       5741113  8347532   2147483646  2147483646 453225323642

privvmpages    33656     44989       98304          98304 106507

In order to increase the value of the two parameters edit the below file from the main node

/etc/sysconfig/vz-scripts/101.conf ( 101 is my vps node)

You can see the value as below

PRIVVMPAGES=”98304:98304″

Increase the value as you need

PRIVVMPAGES=”100000:100000″

Restart your vps after the changes .

# vzctl restart 101

You can also raise the limit as sown below from the host node .

vzctl set 101 –privvmpages 100000:100000 –save

vzctl restart 101

kmemsize

Size of unswappable memory in bytes, allocated by the operating system kernel.

privvmpages

Memory allocation limit. Privvmpages parameter allows controlling the amount of memory allocated by applications.