10.1. Configuring Automatic Memory Management Policies¶

10.1.1. Optimizing Virtual Machine Memory with Kernel Same-Page Merging¶

To optimize memory usage by virtual machines, Virtuozzo uses a feature of Linux called Kernel Same-Page Merging (KSM). The KSM daemon ksmd periodically scans memory for pages with identical content and merges those into a single page. Said page is marked as copy-on-write (COW), so when its contents are changed by a virtual machine, the kernel creates a new copy for that virtual machine.

KSM enables the host to:

• avoid swapping due to merging of identical pages;
• run more virtual machines;
• overcommit virtual machine memory;
• speed up RAM and hence certain applications and guest operating systems.

In Virtuozzo 7, KSM is managed by vcmmd and work with both performance and density policies. However, with performance policy enabled, KSM merges identical pages for each NUMA node independently which slows its operation down by as many times as the number of NUMA nodes on the host (e.g., by four times if there are four NUMA nodes).

10.1.2. Managing Host Services with VCMMD¶

In Virtuozzo 7, you can set memory limits and guarantees managed by vcmmd to host services. The daemon will take into account the service memory while allocating resources to virtual environments.

For example, to have a service named service managed by vcmmd, do as follows:

1. Place the service processes in /sys/fs/cgroup/memory/service.slice directory.

2. Register service.slice in vcmmd and set the desired memory parameters. For example:

   # vcmmdctl register SRVC service.slice --guarantee 100000000 --limit 100000000 --swap 100000
   

3. Activate the service:

   # vcmmdctl activate service.slice
   

To check if the service is managed with vcmmd, run

# vcmmdctl list
name                                     type  active  guarantee     limit    swap
<...>
service.slice                            SRVC     yes      97656     97656      97

You can change the service memory parameters at runtime using the vcmmdctl update command.

For example, to change the service.slice parameters, run:

# vcmmdctl update service.slice --guarantee 200000000 --limit 200000000

To check if the parameters have been correctly applied, run

# vcmmdctl list
name                                     type  active  guarantee     limit    swap
<...>
service.slice                            SRVC     yes     195312    195312     max

You can unregister the service from vcmmd at runtime with the vcmmdctl unregister command.

10.1.3. Managing Virtuozzo Storage Services with VCMMD¶

For Virtuozzo Storage hardware nodes, vstorage.slice/vstorage-services.slice is automatically created in the memory cgroup on hardware node start.

To check if vstorage is managed with vcmmd, run

# vcmmdctl list
name                                     type  active  guarantee     limit    swap
<...>
vstorage.slice/vstorage-services.slice   SRVC     yes    4194304 103854973       0

You can temporarily change memory limits and guarantees for vstorage services at runtime using the vcmmdctl update command. For example:

# vcmmdctl update vstorage.slice/vstorage-services.slice --guarantee 100000000 --limit 100000000

The parameters will be reset to the default values on the next vcmmd restart or node reboot.

To configure limits and guarantees for vstorage services permanently, do as follows:

1. Edit the memory parameters in the /etc/vz/vstorage-limits.conf file. (Set -1 for unlimited memory.)

2. Restart vcmmd to apply changes:

   # systemctl restart vcmmd