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realtime:documentation:howto:tools:cpu-partitioning:start

CPU Partitioning

CPUs can be partitioned to separate the resources of tasks and interrupts with different focus. In a real time system, CPU partitioning can be used to separate CPUs dedicated to real time tasks and their corresponding interrupts.

The base technology for CPU partitioning is CPU affinity. On top of this mechanism further Linux kernel facilities for CPU partitioning are implemented. User space tooling is available as well.

This article gives an short overview about the facilities and tools. Follow the links for detailed information.

Affinity

The processing of tasks or interrupts can be restricted to a specified set of CPUs by setting the affinity. The task CPU affinity affects the scheduler and makes sure that the specific task is executed only on the CPUs which are in the tasks affinity set. The IRQ affinity specifies to which CPU an interrupt is allowed to be routed.

CPU affinity of tasks

In a SMP system the property that binds processes or tasks to one or more processors by the OS scheduler is known as CPU affinity, the capability to override how the processes or tasks are assigned to a particular set of processors by the scheduler is a feature available in several OSes. The idea is to say “always run this process/task on processor one” or “run these processes/tasks on all processors but processor zero”. The scheduler places the processes/tasks on the CPUs which are contained in the affinity set.

Task affinity management can be utilized via the following mechanisms:

  • Kernel command line parameter: isolcpus

IRQ affinity

Hardware interrupts can interrupt kernel and user space computations at any given time, except when the kernel disables interrupt processing to protect resources. When a hardware interrupt is handled the CPU switches into a separate context and executes the handler code and switches back to the interrupted context and resumes the execution.

Depending on the interrupt hardware, interrupts can be routed to any CPU or delivery can be rotated between CPUs. Most interrupt controllers allow to restrict the set of CPUs to which a particular interrupt can be delivered by setting the IRQ affinity.

When the CPU receives an interrupt, a context switch to interrupt context is executed and the current task has to wait until the IRQ is handled. The possibility to allow only a set of CPUs to handle dedicated IRQ is called IRQ affinity. Thereby the hardware routing of the interrupt to the CPUs is affected.

IRQ affinity management can be utilized via the following mechanisms:

Realtime application example

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realtime/documentation/howto/tools/cpu-partitioning/start.txt · Last modified: 2017/07/17 16:03 by anna-maria