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From: Rich Felker <>
To: Gregor Jasny <>
Subject: Re: [musl] Prefer monotonic clock for DNS lookup timeouts
Date: Thu, 1 Dec 2022 10:11:36 -0500	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Thu, Dec 01, 2022 at 12:24:47PM +0100, Gregor Jasny wrote:
> Hello,
> while looking for a reason for a failed DNS resolve I noticed that
> the mtime function which is used to calculate and decide on the
> timeout uses the wall clock instead of a monotonic clock:
> static unsigned long mtime()
> {
> 	struct timespec ts;
> 	clock_gettime(CLOCK_REALTIME, &ts);
> 	return (unsigned long)ts.tv_sec * 1000
> 		+ ts.tv_nsec / 1000000;
> }
> Is this a bug or intentional?

It was intentional, based on a belief that the monotonic clock might
not be present on all kernels. That seems to be incorrect for the
range of versions we "support" (>=2.6.0) but some archs unofficially
work back to mid 2.4.x or earlier with limited functionality (no
threads). Note for example that clock_gettime has fallback to the
gettimeofday syscall despite all kernels >=2.6.0 having clock_gettime
(though was it perhaps gated under some CONFIG_ for "realtime
features" at some point? this probably calls for some research...)

Switching to monotonic here has been on my radar for a while. I see
two decent ways to do it without any possibility of regression:

1. Have the above mtime() function fall back to CLOCK_REALTIME on
   ENOSYS, or

2. Go through with integrating a fallback for CLOCK_MONOTONIC I've had
   in draft for a long time that works on ancient kernels. It works by
   combining the seconds-resolution time from SYS_sysinfo uptime with
   the finer-grained-but-wrapping jiffy count from SYS_times too get a
   monotonic jiffies-resolution uptime.

The latter is cute/fun but a little bit of work to get right and I'm
not sure it's sufficiently useful to justify doing it. Option 1 seems
very reasonable.


  reply	other threads:[~2022-12-01 15:11 UTC|newest]

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-12-01 11:24 Gregor Jasny
2022-12-01 15:11 ` Rich Felker [this message]
2022-12-30  3:20   ` A. Wilcox
2022-12-30  3:21     ` [musl] [PATCH] dns: " A. Wilcox

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