You see a message “Unable to fork: Cannot allocate memory” while logging to a VPS from the host server. The reason is the VPS is running out of resources especially RAM. To temporary solve the issue, you may restart the VPS by executing

vzctl restart VEID

OR increase RAM for the VPS by increasing privvmpages and kmemsize for the VPS.

Edit the configuration file of the VPS

vi /etc/sysconfig/vz-scripts/veid.conf

increase the value of the above two parameters and restart the VPS.

This parameter defines how many simultaneous request can be served. Any connection request from browsers that come in after that will be queued.
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The MaxClients directive sets the limit on the number of simultaneous requests that can be supported. No more than this number of child server processes will be created. To configure more than 256 clients, you must edit the HARD_SERVER_LIMIT entry in httpd.h and recompile. In our case we want this variable to be as small as possible, because in this way we can limit the resources used by the server children. Since we can restrict each child’s process size (see Preventing Your Processes from Growing), the calculation of MaxClients is pretty straightforward:

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Q. How do I identify Linux server ECC memory modules from a shell prompt?

A. You need to count the chips on the module. If memory module has 9 chips per sides, it is ECC memory (EDAC protected memory). It may or may not be registered. If the memory module has 8 chips per side, it is not ECC memory modules.

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