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OCP-TAP Servo Workstream

Meeting

June 6, 2022

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Agenda

  • Proposed scope and actions
  • Review linuxptp servo-related elements

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Requirements Gathering

  • Problem Formulation
    • Alon to draft initial text
  • Understanding the network & system/hardware
  • Data Center PTP profile is one source of requirements
  • Requirements from other workstreams
    • Reach out to other workstream
  • Application-level (customer) requirements
    • i.e. can we have an algorithm/system that is more adaptive
  • Theory of operations?
  • Create “wishlist” brainstorm on the Wiki
  • Invite experts to present experience / problems to the workstream
    • Example: open source solutions such as linuxptp
  • Define where in the system the Servo can go

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Problem Formulation - 1

  • A time servo attempts to maintain frequency and phase alignment between a time reference and a servo-controlled time source using a closed-loop negative feedback control loop
  • In addition, a time servo may need to support
    • Fast start-up algorithm for correction of large errors
    • Monitoring of the synchronization quality including feedback to users
    • Holdover (open-loop) when reference time is not available or is not usable
    • Switching to a different time reference without any phase jumps or large changes in frequency
    • Potentially support having the algorithm selection and parameterization automatically and dynamically driven by the system’s performance parameters (listed above)
    • Potentially providing feedback to the system such as the required SYNC message rate to achieve the desired performance
  • A time servo’s performance is limited by
    • Stability of the local oscillator
    • Rate and accuracy of the reference inputs
    • Range and resolution of control outputs
    • Delays from sensing to the loop and from loop to effect
    • Algorithm and parameters

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Problem Formulation - 2

  • In a datacenter architecture there are many time servos. Examples include:
    • Within the miniature atomic clock (typically implemented by vendor)
    • Within GNSS acquisition system (typically implemented by vendor)
    • Synchronize Time Appliance PHC (i.e. in NIC) with the time reference (i.e. GNSS) input
    • Synchronize BC to BC’s follower PTP incoming clock
    • Synchronize OC’s system clock to the PTP incoming clock
  • Work with the Servo Workstream includes
    • Understanding the data center servo requirements and performance of components used in said datacenter applications
      • These are defined by the profile, hardware used, application needs, and other inputs
    • Cooperation / coordination with other workflows to understand requirements and dependencies
    • Select, define, implement, and/or test servo algorithms and solutions that meets the data center requirements for the different types/locations of servos in the system

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Proposed initial scope and actions

  • Start by focusing on the Servo synchronizing the Time Appliance PHC (i.e. in NIC) with the time reference (i.e. GNSS) input
    • This is better understood / constrained both in the Data Center Profile as well as the analysis by the oscillators workstream
    • Experiments/changes limited to ptp4u & ts2phc
  • Document requirements
  • Documents algorithm & behavior of existing implementation
  • Research, discuss, compare, and simulate potential algorithms & parameters.
    • This includes algorithms both for the core servo as well as for characterizing the synchronization accuracy, stability, and performance

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Ptp4u analysis

  • This is purely a protocol implementation
  • No actual servo in ptp4u
    • GNSS / PPS input to frequency & timestamp conversion done elsewhere
      • In the OCP timingcard, this is done in the FPGA in the AdjustableClock IP which includes a PI loop
      • In other implementations this can be done in HW, FPGA, ASIC, or SW
    • Sync between time card and NIC PHC done elsewhere
      • phc2sys/ts2phc (or equivalent) to sync time card and NIC
  • No PTM support

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ts2phc

  • Servo is used to synchronize PHC “sinks” to the timestamp source data
  • Uses default PI servo in linuxptp
    • Linuxptp shares the servo code (between ptp4l, phc2sys and ts2phc)
      • Algorithms are PI, Adaptive Linear Regression, NTP SHM, and Null frequency adjust algorithms
        • No option for externally defined servo today
      • Parameters are configurable at startup
    • Ts2phc does not expose any servo configuration
      • Default values are always used

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phc2sys

  • Servo is used to synchronize clock “sink” to the source clock
  • Exposes servo configuration allowing configuration of servo type and parameters
  • Parameters are configured at startup only

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Next Steps

  • Discussion
    • Focusing on ptp4u does not work as there is no servo
    • For ts2phc, we can investigate exposing servo settings if that is useful

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