Symptoms
After switching VPS memory mode from SLM to 'Combined SLM + UBC mode' and setting swappages parameter, container misbehaves and shows memory-related errors, e.g.:
[root@vz ~]# vzctl enter 111
Unable to fork: Cannot allocate memory
Switching back to SLM mode makes the container stable again.
Cause
'Combined SLM + UBC mode' utilizes the whole set of UBC parameters, so it is needed to configure each parameter appropriately.
In case vSwap mode is used on the Virtuozzo server (Virtuozzo containers 4.7 and Virtuozzo 6.0), it is required to set physpages (RAM in 4KB pages), swappages (SWAP in 4KB pages) and privvmpages (memory overcommit in 4KB pages) parameters.
SLM mode, in turn, uses a single parameter SLMMEMORYLIMIT, which eases the memory management of such containers. With vSwap mode, the limit is converted automatically to RAM (physpages) and memory overcommit parameter is set to 1.5 * RAM (privvmpages) by default, unless other limits are defined for the VPS in its configuration file.
Resolution
When switching to 'Combined SLM + UBC mode', make sure the following parameters are configured properly:
- physpages - should be set in accordance with memory limit in 4 KB pages, e.g. if RAM limit is 1 GB, physpages should be set to 262144
- swappages - can be set according to the demand, in 4 KB pages as well
- privvmpages - to emulate the default Linux behavior, it should be set to 1.5 * (RAM + SWAP)
All other UBC parameters can be left without changes, OA applies automatically calculated values on them.
Additional information
Check the following articles for additional information on vSwap:
Is it possible to manage 'vswap' parameter for VPS in POA?
How vSwap memory management scheme configures UBC parameters in POA?
Memory limits in Virtuozzo Containers for Linux