Server Gurus Discussions

[Centos7]

I have several rack mounted servers running AMD Epyc processors (most of them are 7313). These are powering a real-time app which pins critical threads to isolated cores. I also have to move interrupts to other cores to avoid jitter. Unfortunately, when I do so the processor is stuck at its base speed and will no longer boost. Is there a way to get boosted speeds on isolated cores?

I have narrowed the problem down to nohz_full setting. When set as follows, jitter goes away but I am stuck at base speed. This is my GRUB_CMDLINE:

GRUB_CMDLINE_LINUX="rhgb quiet rd.auto=1 crashkernel=1G-64G:448M,64G-:512M isolcpus=4-15 nohz_full=4-15 rcu-nocbs=4-15 mce=ignore_ce nosoftlockup audit=0 mitigations=off iommu=off nmi_watchdog=0 enforcemodulesig=0 transparent_hugepage=never"

[mentos22]

when isolating cores and moving interrupts to other cores on amd epyc proccesore .  it's possible that the boosted speeds are not achieved due to the configuration settings. The "nohz_full" setting you mentioned can potentially impact the processor's ability to boost its clock speed on isolated cores.

The "nohz_full" setting is used to reduce timer interrupts on selected isolated cores to minimize latency and improve real-time performance. However, this setting may also prevent the processor from entering higher clock speed states and staying at the base speed.

To address this issue and potentially enable boosted speeds on isolated cores, you can try the following steps:

1. Experiment with different configurations: Adjust the "nohz_full" setting to exclude some of the isolated cores and see if it allows the processor to reach boosted speeds. For example, you can try setting "nohz_full=4-11" instead of "nohz_full=4-15" to exclude a few cores from the "nohz_full" configuration.

2. Review BIOS settings: Check if your server's BIOS has any specific settings related to core isolation and clock speed boosting. Some BIOS versions may have options to enable or adjust the behavior of boosted speeds on isolated cores.

3. Update BIOS and firmware: Ensure that you have the latest BIOS version and firmware updates installed for your AMD Epyc processors. Sometimes, updates can address performance-related issues and improve compatibility with specific configurations.

4. Consult AMD documentation and support: Refer to the documentation and support resources provided by AMD for guidance on optimizing performance in real-time applications on Epyc processors. They may offer specific recommendations or settings to achieve boosted speeds on isolated cores.

It's important to note that making changes to BIOS settings and processor configurations can have implications for stability and compatibility. It's recommended to perform thorough testing and ensure that the changes align with your specific requirements and the recommendations provided by AMD or your hardware manufacturer.

Additionally, you may consider consulting with a specialist in real-time computing or reaching out to AMD's technical support for further assistance tailored to your specific use case and configuration.

i hope this information is very helpful for you.

[Centos7]

> Experiment with different configurations: Adjust the "nohz_full" setting to exclude some of the isolated cores and see if it allows the processor to reach boosted speeds. For example, you can try setting "nohz_full=4-11" instead of "nohz_full=4-15" to exclude a few cores from the "nohz_full" configuration.

Yes, that's exactly what I did as reported in the original post. Cores which are not specified by nohz_full will boost to higher speeds.

We are looking to get boosted speeds without interrupts. I am not sure where to look in the BIOS settings for anything that might affect this behavior and the manufacturer (SuperMicro) has not been able to offer any suggestions through their support channels.