CPU Power Saving Mode

Onload User Guide (UG1586)

Document ID
Release Date
1.2 English

Modern processors have design features that enable a CPU core to drop into lowering power states when instructed by the operating system that the CPU core is idle. When the OS schedules work on the idle CPU core (or when other CPU cores or devices need to access data currently in the idle CPU core’s data cache) the CPU core is signaled to return to the fully-on power state. These changes in CPU core power states create additional network latency and jitter.

It is therefore recommended that customers wishing to achieve the lowest latency and lowest jitter disable the “C1E power state” or “CPU power saving mode” within the machine's BIOS.

Disabling the CPU power saving modes is required if the application is to realize low latency with low jitter.

Note: To ensure C states are not enabled, overriding the BIOS settings, it is recommended to put the line ‘intel_idle.max_cstate=0 idle=poll’ (for Intel) or ‘processor.max_cstate=0 idle=poll’ (for AMD) into the kernel command line /boot/grub/grub.conf. The settings will produce consistent results and are particularly useful when benchmarking. Allowing some cores to enable Turbo modes while others are idle can produce better latency in some servers. For this, use idle=mwait and enable C-states in the BIOS. Alternatively, on later Linux versions, the tuned service can be enabled and used with the network-latency profile. Users should refer to vendor documentation and experiment with C states for different applications.

Consult your system vendor and documentation for details concerning the disabling of C1E, C states or CPU power saving states.