Re: boot time on Linux

[Natanael Copa <natanael.copa@gmail.com>]

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Hi, I had a quick look at the thread, and I have a few comments.

1) it is openrc's bootmisc (
https://github.com/OpenRC/openrc/blob/86efc43d0e0d7569f5d2e7a58b8c461ac9f7dae8/init.d/bootmisc.in#L197)
that creates /var/run/utmp. There is no guarantee that this exists. You
can for example run emacs in a alpine docker container.
2) Even if it does exist, there is no guarantee that the timestamp is
correct. There are machines without RTC (Raspberry PI for example), and in
this case the time stamp may end up to be the same every reboot (if
correctly set up it should save the shutdown time for the reboot and set
time to this on next boot, but there is no guarantee).

For emacs' use case I'd say the best solution would be to use uptime from
sysinfo(2) and assume system time is correct (NTP client should be running
at the time emacs is started).

-nc


On Wed, Aug 9, 2023 at 9:31 PM Paul Eggert <eggert@cs.ucla.edu> wrote:

> [For those cc'ed, the thread's at <https://bugs.gnu.org/64937#142>.]
>
> On 2023-08-09 07:29, Bruno Haible wrote:
>
> > And on Alpine Linux, while /var/run/utmp is empty, its time stamp is
> > essentially the boot time.
> >
> > The approach used by Emacs, namely to look at the time stamp of
> > /var/run/random-seed, is therefore essentially one of the best
> approaches.
> > It just needs to also look at /var/lib/systemd/random-seed and - on
> Alpine
> > Linux - /var/run/utmp .
>
> Thanks for looking into this. Clearly Emacs had some catching up to do,
> since it was using a location for the random-seed file that current
> GNU/Linux distros no longer use. To try to fix this I installed the
> attached patch to Emacs master on Savannah.
>
> This patch does not address the problem for Alpine, nor I suspect for
> Android. I suppose Alpine could use the timestamp of /var/run/utmp (or
> is that /run/utmp?) but I don't know how 'configure' would reliably
> detect it's being built or cross-built for Alpine. I'll cc this to
> Natanael Copa, who does the Alpine ports for Emacs, to see whether he
> can give advice.
>
> Also, I don't know how Android records boot time so I'll cc this to Po
> Lu, the main developer for Emacs on Android.



-- 
Natanael Copa

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