Time Synchronization

A collection of notes and links from looking at projects and papers on time synchronization.

NTP

Overview

Usage

The API for fetching NTP’s state is ntp_adjtime(2). Significantly shortened:

SYNOPSIS
       #include <sys/timex.h>

       int ntp_adjtime(struct timex *buf);

DESCRIPTION
           struct timex {
               // ...
               /* Maximum error (microseconds) */
               long maxerror;
               /* Current time (read-only, except for
                  ADJ_SETOFFSET); upon return, time.tv_usec
                  contains nanoseconds, if STA_NANO status
                  flag is set, otherwise microseconds */
               struct timeval time;
               // ...
           };

[Kudu] Wait for NTP synchronization on startup before checking the time

Time system time can experience a large jump (forwards or backwards) when a system synchronizes with NTP for the first time. It is wise to wait for that synchronization to occur first before checking the system time.

Specifically, chrony will never step the system clock after the initial synchronization, so time will always be monotonic thereafter (as long as chrony remains in control of the system clock).

[Kudu] Call sched_yield() before checking and sending the time

Any unpredictable delay between fetching the current time and sending it across the network will increase the error on clock synchronization. One of the potential causes of unpredictable delay is preemption. Invoking sched_yield() before fetching and sending the time will make it highly unlikely that the time quantum is exceeded between checking and sending the time.

[Kudu] Detect cloud type, and configure NTP to cloud’s NTP provider

A large number of users are running on the cloud, and a number of cloud providers offer highly accurate NTP endpoints. Therefore, it’s a nice addition to auto-detect the fact that the program is running on a cloud provider, and automatically configure NTP to point at the special highly-accurate NTP endpoint.

[Kudu] Always re-check STA_NANO, as it can change at runtime.

NTP will either offer your time in milliseconds or nanoseconds. Some NTP implementations will toggle that choice at runtime, so your code must tolerate transitions between the two.

[Kudu] Tolerate transient failures in ntp_adjtime

It’s possible that ntp_adjtime will fail for a brief window as the clock temporarily loses synchronization. During this time, you can continue to advance the clock and maxerror manually, and time out after a short window.

Open Questions

SCM_DROP_IF_LATE

When pulling the current time and sending it across the network, calling sched_yield() makes a decent attempt at not context switching away between the first and second parts. However, calling send() on a packet, and it actually being sent, are two different things. sendmsg() accepts a flag SCM_DROP_IF_LATE, which adds a deadline when enqueing a packet into the network stack. If it can’t be sent before the deadline, then it will be dropped. This seems like it would help bound how late a packet could be sent out after the current system time was checked.

I don’t see anyone using it for this case. There’s very few uses of it at all, only 10 hits on github code search. Historically this has correlated with kernel features that are either partially implemented or have significant limitations.

PTP

Huygens

[huygens] is an NTP-like approach, offering PTP-like time bounds.

References