Why does hardware clock (PHC0) not return UTC

Steven Sommars 0

Running a Linux (Ubuntu 24.02.2 LTS) VM on an Azure hypervisor. The default time synchronization configuration includes chrony 4.5 with one source, a PHC reference clock. This would be great, except that the host clock is in error by 10-30 msec. Dropping PHC and using the four Google anycast NTP servers works much better.

What did I do wrong?

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Steven Sommars 0 Reputation points

2025-03-03T14:18:27.5366667+00:00

This graph shows the long-term difference between the PHC(/dev/ptp0) and NTP-disciplined clocks. There are strong short term variations as well.

If it matters: cat /sys/devices/system/clocksource/clocksource0/current_clocksource
hyperv_clocksource_tsc_page

Either I'm doing something wrong, or PHC is broken.
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Geethasri.V 15 Reputation points Microsoft External Staff

2025-03-04T17:26:31.39+00:00

Hi Steven Sommars,
Thank you for reaching out to us on the Microsoft Q&A forum.

It seems you're facing issues with the Precision Hardware Clock (PHC) not returning UTC on your Ubuntu 24.02.2 LTS VM running on an Azure hypervisor. Here are some insights and possible solutions:

From Your Organization:

COSMIC Linux Troubleshooting: There may be challenges with Linux debuggability tools and their integration with Azure Watson and Azure Profiler for automatic crash reports and profiler traces. These limitations could be complicating the diagnosis of the PHC issue.

Azure Profiler Overview: The Azure Profiler is a useful tool to identify performance bottlenecks, such as high idle time or hotspots in code. It might help you identify any underlying performance issues that are affecting the PHC's accuracy. From the Web:

Ensure UTC Configuration: To ensure that your hardware clock is set to UTC, check and adjust your /etc/default/rcS file. Set UTC=yes to ensure the hardware clock follows UTC. Alternatively, you can use the hwclock tool to adjust the hardware clock to UTC if necessary.

PHC and NTP Synchronization: The discrepancy between the PHC and NTP-disciplined clocks could be due to the host clock's inherent error. In this case, using multiple NTP sources—such as Google's anycast NTP servers—can provide more reliable time synchronization.

Chrony Configuration: It may also be worth reviewing your Chrony configuration to ensure that it is properly referencing the PHC as a source. If the issue persists, consider reaching out to Azure support or your internal team for further assistance. Would you like more specific guidance on any of these steps, or do you need help troubleshooting the configuration further?

Please refer blow Documentation for your refence. https://learn.microsoft.com/en-us/azure/virtual-machines/linux/time-sync
https://learn.microsoft.com/en-us/azure/virtual-machines/windows/time-sync If the information is helpful, please consider by clicking the "Upvote" on the post. If you have any further queries, please let us know in the comment. Thank you.

Steven Sommars 0 Reputation points

2025-03-05T00:48:06.8233333+00:00

Thanks for the suggestions.

This is not a performance, debugging, or crash issue. The concern is PHC (PTP hardware clock) which, by design, should return time with an offset of at most a couple of microseconds compared to UTC.

"The discrepancy between the PHC and NTP-disciplined clocks could be due to the host clock's inherent error."

Are you saying that Microsoft Azure PHC doesn't work? If so, Microsoft documentation should state this, plus the default Linux images should be changed to not use PHC.

I have tried several Chrony configurations:

Synchronized to PHC only. Result: System time is in error when comparing to stratum 1 clocks.

Synchronized to NTP stratum 1 only. (Google stratum 1 servers) Result: The above graph shows the excessive error I observe

Synchronized to NTP stratum 1 and PHC. Result: Chrony synchronizes to NTP stratum 1. PHC is marked as a falseticker.

I suspect that PHC is not working. Can Microsoft confirm or refute this? The offset reached 100 msec today!

Geethasri.V 15 Reputation points Microsoft External Staff

2025-03-05T06:07:59.2766667+00:00

Hi Steven Sommars,

i am looking into the issue i will get back to you soon.

Thank you

Geethasri.V 15 Reputation points Microsoft External Staff

2025-03-05T16:57:49.93+00:00

Hi Steven Sommars,

Microsoft provides PTP (Precision Time Protocol) support in Windows Server 2019 and Windows 10 (version 1809 and later), mainly through the Windows Time Service (W32Time). This service synchronizes the system clock with an external time source, such as a PTP grandmaster clock.

Windows Time Service helps synchronize clocks on devices in a domain, ensuring consistency in time across systems. With PTP support, Windows can synchronize time with high precision, typically within microseconds, especially when using specialized hardware like PTP-capable network interfaces.

To configure PTP on Windows, the W32Time service can be adjusted to use PTP or NTP servers. You can set up a Windows system as a PTP client (slave), allowing it to sync with an external PTP server. Please go through this document for your reference https://learn.microsoft.com/en-us/windows-server/networking/windows-time-service/windows-time-service-tech-ref https://learn.microsoft.com/en-us/windows-server/networking/windows-time-service/windows-time-service-tools-and-settings?tabs=config If the comment was helpful, please don't forget to upvote. Thank you.

Steven Sommars 0 Reputation points

2025-03-05T17:13:42.16+00:00

I'm not running Windows, so this comment does not apply

Geethasri.V 15 Reputation points Microsoft External Staff

2025-03-07T13:25:26.5066667+00:00

Hi Steven Sommars,

Edit the Chrony Configuration:

sudo nano /etc/chrony/chrony.conf

Comment Out the PHC Line: Add '# 'before the PHC reference clock line.

Add Google NTP Servers: Add the following lines:
server time.google.com iburst server time1.google.com iburst server time2.google.com iburst server time3.google.com iburst

Save and Exit (in nano, press CTRL + X, then Y, and Enter).

Restart Chrony:

sudo systemctl restart chronyd

Verify Configuration:

chronyc sources

If the information is helpful, please consider by clicking the "Upvote".

If you have any further queries, please let us know we are glad to help you.

Thank you.

Steven Sommars 0 Reputation points

2025-03-07T14:15:04.3166667+00:00

Already did this.

Please tell development that PHC is broken. Looks like a system bug.
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