3.2.2. RT Tunable Parameters
The RT scheduler works in a similar way to the ceiling enforcement control of the CFS (for more information, refer to Section 3.2.1, “CFS Tunable Parameters”) but limits CPU access to real-time tasks only. The amount of time for which real-time task can access the CPU is decided by allocating a run time and a period for each cgroup. All tasks in a cgroup are then allowed to access the CPU for the defined period of time for one run time (for example, tasks in a cgroup can be allowed to run 0.1 seconds in every 1 second). Please note that the following parameters take into account all available logical CPUs, therefore the access times specified with these parameters are in fact multiplied by the number of CPUs.
- cpu.rt_period_us
- applicable to real-time scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "
us") for how regularly a cgroup's access to CPU resources is reallocated. - cpu.rt_runtime_us
- applicable to real-time scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "
us") for the longest continuous period in which the tasks in a cgroup have access to CPU resources. Establishing this limit prevents tasks in one cgroup from monopolizing CPU time. As mentioned above, the access times are multiplied by the number of logical CPUs. For example, settingcpu.rt_runtime_usto200000andcpu.rt_period_usto1000000translates to the task being able to access a single CPU for 0.4 seconds out of every 1 second on systems with two CPUs (0.2 x 2), or 0.8 seconds on systems with four CPUs (0.2 x 4).
