Sender: Valdis Kletnieks <valdis@vt.edu>
From:   "Valdis Kl=?utf-8?Q?=c4=93?=tnieks" <valdis.kletnieks@vt.edu>
To:     Muni Sekhar <munisekharrms@gmail.com>
cc:     Daniel Lezcano <daniel.lezcano@linaro.org>, tglx@linutronix.de,
        LKML <linux-kernel@vger.kernel.org>,
        kernelnewbies <kernelnewbies@kernelnewbies.org>
Subject: Re: Time: new clocksource
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Date:   Wed, 08 Dec 2021 04:10:14 -0500
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On Tue, 07 Dec 2021 13:03:07 +0530, Muni Sekhar said:

> Which module is responsible for resulting in the absolute time? Is
> absolute time synchronized across multiple systems connected on the
> network?

man -k ntp

Keeping the system clock in sync with external timesources and reality is too
complicated and messy to be done inside the kernel, so it's usually done with a
userspace daemon which on Linux boxes is usually ntpd.

Depending on the clock stability of the hardware, whether you have an RTC chip
that keeps track of time even when powered down, and the timing tolerances you
want on your network (synced to the same minute, or same second, or same 0.001
seconds), it may be sufficient to run ntpdate at an appropriate time during
system boot, or from inside a cron job, or you may want to have a continually
running ntpd process.

Most sane distros have ntpd enabled by default with a reasonable set of
defaults - systems that have non-broken clocks will fairly quickly figure out
the clock drift rate, and your network load to keep the clocks within a few
milliseconds of a national time standard drops to a few packets every 20
minutes or so. In case of a network outage, it will continue to correct the
system clock according to the last known drift rate.

And if you need better than millisecond sync, you're going to be springing
for a GPS receiver with a PPS output and/or a cesium or rubidium clock.