CONFIG_NO_HZ_FULL_SYSIDLE_SMALL: Number of CPUs above which large-system approach is used

General informations

The Linux kernel configuration item CONFIG_NO_HZ_FULL_SYSIDLE_SMALL:

• prompt: Number of CPUs above which large-system approach is used
• type: int
• depends on: CONFIG_NO_HZ_FULL_SYSIDLE
• defined in kernel/time/Kconfig
• found in Linux kernels: 3.12–3.19, 4.0–4.12

Help text

The full-system idle detection mechanism takes a lazy approach on large systems, as is required to attain decent scalability. However, on smaller systems, scalability is not anywhere near as large a concern as is energy efficiency. The sysidle subsystem therefore uses a fast but non-scalable algorithm for small systems and a lazier but scalable algorithm for large systems. This Kconfig parameter defines the number of CPUs in the largest system that will be considered to be "small".

The default value will be fine in most cases. Battery-powered systems that (1) enable NO_HZ_FULL_SYSIDLE, (2) have larger numbers of CPUs, and (3) are suffering from battery-lifetime problems due to long sysidle latencies might wish to experiment with larger values for this Kconfig parameter. On the other hand, they might be even better served by disabling NO_HZ_FULL entirely, given that NO_HZ_FULL is intended for HPC and real-time workloads that at present do not tend to be run on battery-powered systems.

Take the default if you are unsure.

Hardware

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This page is automaticly generated with free (libre, open) software lkddb(see lkddb-sources).

The data is retrived from:

• Linux kernel
• Linux Kernel Driver DataBase (LKDDb)

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