Compare timestamps for your Linux instances - Amazon Elastic Compute Cloud
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Compare timestamps for your Linux instances

If you're using the Amazon Time Sync Service, you can compare the timestamps on your Amazon EC2 Linux instances with ClockBound to determine the true time of an event. ClockBound measures the clock accuracy of your EC2 instance, and allows you to check if a given timestamp is in the past or future with respect to your instance's current clock. This information is valuable for determining the order and consistency of events and transactions across EC2 instances, independent of each instance's geographic location.

ClockBound is an open source daemon and library. To learn more about ClockBound, including installation instructions, see ClockBound on GitHub.

ClockBound is only supported for Linux instances.

If you're using the direct PTP connection to the PTP hardware clock, your time daemon, such as chrony, will underestimate the clock error bound. This is because a PTP hardware clock does not pass the correct error bound information to chrony, the way that NTP does. As a result, your clock synchronization daemon assumes the clock is accurate to UTC and thus has an error bound of 0. To measure the full error bound, the Nitro System calculates the error bound of the PTP hardware clock, and makes it available to your EC2 instance over the ENA driver sysfs filesystem. You can read this directly as a value, in nanoseconds.

To retrieve the PTP hardware clock error bound

  1. First get the correct location of the PTP hardware clock device by using one of the following commands. The path in the command is different depending on the AMI used to launch the instance.

    • For Amazon Linux 2:

      cat /sys/class/net/eth0/device/uevent | grep PCI_SLOT_NAME

    • For Amazon Linux 2023:

      cat /sys/class/net/ens5/device/uevent | grep PCI_SLOT_NAME

    The output is the PCI slot name, which is the location of the PTP hardware clock device. In this example, the location is 0000:00:03.0.

    PCI_SLOT_NAME=0000:00:03.0

  2. To retrieve the PTP hardware clock error bound, run the following command. Include the PCI slot name from the previous step.

    cat /sys/bus/pci/devices/0000:00:03.0/phc_error_bound

    The output is the clock error bound of the PTP hardware clock, in nanoseconds.

To calculate the correct clock error bound at a specific point in time when using the direct PTP connection to the PTP hardware clock, you must add the clock error bound from chrony or ClockBound at the time that chrony polls the PTP hardware clock. For more information about measuring and monitoring clock accuracy, see Manage Amazon EC2 instance clock accuracy using Amazon Time Sync Service and Amazon CloudWatch – Part 1.