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CLOCK-SYNCHRONIZATION.md

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Clock Synchronization

The Operating System might use different sources and devices to measure time. For instance, on a bare metal scenario, the OS can program a hardware timer to generate interrupts on a precise frequency, so it's easy to know how much time have passed between interrupts. Factors like temperature and voltage fluctuations might cause some interference leading to a clock drift, but usually it can take months (or even years) to observe seconds of drift on systems without any external clock synchronization. On virtualized systems, however, hardware timers are virtualized by the hypervisor, which cannot ensure the precision guaranteed by physical hardware devices.

The common and best practice to keep clocks synchronized, for both virtualized and non-virtualized systems, is to use the NTP protocol, so clocks are precisely synchronized with external servers equipped with high precision devices, like GPS or Atomic clocks.

EVE always tries to keep its clock synchronized by embedding a NTP Client that synchronizes the system's clock since the boot time. It's recommended to all Edge Applications (containers or regular VMs) to also run a NTP client in order to keep their clocks synchronized.

Clock sources

The Linux kernel can use different sources to measure time: hardware timers, counters or even paravirtualized drivers. On x86 platforms, most common clock sources available are:

  • tsc: Time Stamp Counter, a counter that basically is incremented on each CPU clock cycle. The TSC can vary between platforms and implementations. Some can change frequency while the processor is in power saving state or is completely turned off. Some are stable and never stop to count, even while the processor is sleeping. Even though this counter is stable, its exact frequency is many times not specified, so it requires the kernel to execute a calibration process during initialization. Other available timers can be used in the calibration process. Some processors already specify the TSC frequency through the CPUID instruction, which will not require calibration.

  • acpi_pm: Corresponds to a reliable hardware timer counter defined by the ACPI standard. This counter has a frequency fixed clock at 3.58MHz.

  • HPET: It's a High Precision Event Timer, very precise, still slower to read compared to TSC.

  • kvm-clock: The kvm-clock is a paravirtual driver for KVM (so it's available on guest virtual machines), the host shares information about its clock with guests.

Virtual PTP 1588 clock (PTP_KVM) support

PTP stands for Precision Time Protocol, an IEE standard (IEEE 1588-2019) that defines a protocol for high precision clock synchronization throughout a computer network. PTP hardware devices are usually present in network cards, so they can be used to generate timestamps for many network protocols. EVE supports a virtual PTP clock (for x86_64) that can be exposed to guests as a hardware PTP device able to synchronize Guest's clock with host's (EVE) clock. A client program, such as the chrony daemon, is required in order to synchronize the system's clock with the virtual PTP device.

Recommendations when NTP is not available on the Guests

For some use cases, like when devices are running on very remote locations, network connectivity might not be available all the time, so NTP synchronization can be compromised, or some times, completely unavailable. In order to keep Guest's clock synchronized with Host's clock and avoid clock drifts when NTP is not available, one of the following options are recommended:

  • Use the virtual PTP kvm clock. EVE enables the chrony daemon automatically for containers (it can be disabled defining the environment variable EVE_ENABLE_CHRONY=0, this variable can be setup for each Edge Application through the remote controller). For regulars VMs, the chrony daemon must be installed and configured to sync the guest's clock with the device /dev/ptp0 or /dev/ptp_kvm. The kernel must also support the virtual PTP device driver.

  • Use acpi_pm as a reliable clock source. The parameter "clocksource=acpi_pm" must be added to the bootloader options from the VM.

  • Some hardware platforms have known issues related to the TSC counter, such as report wrong frequencies, present some instability or get the counter reset by BIOS while in some power save mode, for example. Many of these known issues are fixed by Linux kernel or by an updated processor's microcode. It's important to check if the current hardware platform has any known issue regarding TSC. In such a case, another timer source (such as HPET or acpi_pm) can be used as a clocksource. The corresponding kernel command line on GRUB's configuration can be set to change the default clocksource. It's also recommended to check possible fixes and updates in the BIOS provided by the vendor.

References