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2.9.2. Process Behavior in the Root Control Group

Certain blkio and cpu configuration options affect processes (tasks) running in the root cgroup in a different way than those in a subgroup. Consider the following example:
  1. Create two subgroups under one root group: /rootgroup/red/ and /rootgroup/blue/
  2. In each subgroup and in the root group, define the cpu.shares configuration option and set it to 1.
In the scenario configured above, one process placed in each group (that is, one task in /rootgroup/tasks, /rootgroup/red/tasks and /rootgroup/blue/tasks) consumes 33.33% of the CPU:
/rootgroup/ process:      33.33%
/rootgroup/blue/ process: 33.33%
/rootgroup/red/ process:  33.33%
Any other processes placed in subgroups blue and red result in the 33.33% percent of the CPU assigned to that specific subgroup to be split among the multiple processes in that subgroup.
However, multiple processes placed in the root group cause the CPU resource to be split per process, rather than per group. For example, if /rootgroup/ contains three processes, /rootgroup/red/ contains one process and /rootgroup/blue/ contains one process, and the cpu.shares option is set to 1 in all groups, the CPU resource is divided as follows:
/rootgroup/ processes:    20% + 20% + 20%
/rootgroup/blue/ process: 20%
/rootgroup/red/ process:  20%
Therefore, it is recommended to move all processes from the root group to a specific subgroup when using the blkio and cpu configuration options which divide an available resource based on a weight or a share (for example, cpu.shares or blkio.weight). To move all tasks from the root group into a specific subgroup, you can use the following commands:
rootgroup]# cat tasks >> red/tasks
rootgroup]# echo > tasks