13900K PCH latency

[hoeralu]

Hello!

My system consists of a 13900K CPU on an industrial mainboard with the R680E chipset running a proprietary real-time OS.

The application needs to send ethernet frames at a certain rate, preferrably with very low timing jitter.

When the network card is in the PCIe slot that connects directly to the CPU it works perfectly fine, however when it's in a PCIe slot connected to the PCH there is relatively big spikes in latency (~7µs) when there was no communication to the PCH in the previous 10µs.

Of course going through the PCH there will be additional latency and that is fine, it's just the spike when the PCH has to wake up that is the problem.

When I read an unrelated memory mapped register 10µs before I need to send the frame something wakes up and the latency disappears. (in this case I read offset 0xD4 of the SPI controller at Bus 0, Device 31, Function 5)

It seems that something in the PCH or the DMI interface goes to sleep, but from the CPU and Chipset datasheets I could not find any settings that influence this behavior. I checked the following:

• C-States disabled in BIOS
• ASPM disabled, both on CPU side and chipset side of the DMI
• Hyperthreading is disabled
• E-cores disabled 
• CPU Frequency is locked
• uncore ratio is locked

Is there any energy saving settings I missed that could influence the DMI or PCH access times?

Thank you

[BrenzL_Intel]

Hello hoeralu,

Thank you for reaching out to the Intel Community Forum

I will do further research on this matter and post the response on this thread once it is available.

If you have questions, please let us know. Thank you.

Best regards,

Brenz L.

Intel Customer Support Technician

[BrenzL_Intel]

Hello hoeralu,

Thank you for your continued cooperation with Intel. To better understand your system configuration, please provide the following information:

• Have you already applied the Intel default settings in your BIOS?
• Is the processor currently overclocked?
• Is your BIOS updated to the latest version?

Once I have this information, we can determine the next steps and identify what recommendations or actions would be most appropriate for this concern.

Best regards,

Brenz L.

Intel Customer Support Technician

[hoeralu]

Hello Brenz,

the BIOS is the latest version the board manufacturer provides (with build date 07/05/2024), resetting to default settings made no improvement.

The Processor is usually locked to 5.2GHz on all cores with temperatures between 40 and 60C depending on load. Setting all cores to the base frequency of 3GHz made no difference, except for the expected overall performance loss.

Best regards,

Lu

[BrenzL_Intel]

Hello hoeralu,

Thank you for your continued cooperation.

Regarding energy‑saving settings that may affect DMI or PCH access times, please note that the Platform Controller Hub (PCH) is part of the motherboard’s chipset. Because of this, any power‑saving or latency‑related settings that influence PCH behavior—such as DMI ASPM, chipset power gating, or PCH power states—are controlled by the motherboard manufacturer, not by the processor.

For this reason, we recommend contacting your motherboard manufacturer’s support team for guidance on the specific BIOS settings available for your board that may impact DMI/PCH performance.

Since the concern falls under motherboard‑specific configuration, we will proceed to close this thread. Please keep in mind that this thread will no longer be monitored by Intel. Thank you for your understanding.

If you need further assistance in the future, please feel free to open a new thread.

Thank you and have a great day.

Best regards,

Brenz L.

Intel Customer Support Technician