From: Florian Weimer <fw@deneb.enyo.de>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Haribabu Kommi <kommi.haribabu@gmail.com>,
  PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: gettimeofday is at the end of its usefulness?
References: <31856.1400021891@sss.pgh.pa.us>
	<20140515185614.GG25053@momjian.us>
	<CAA-aLv7m=RU0vKm1myt_uRLYmTviqFcfWZCMPwbXdbKoX4YzYg@mail.gmail.com>
	<20237.1465397783@sss.pgh.pa.us>
	<CAJrrPGd2rA6UoPOxUK1WRgKfxRYB7vQZ357zCv+6kx3ced7v4g@mail.gmail.com>
	<CAJrrPGecetyS5m1F8v4jOUFqy0zGr7hR8kgDMiq1HWeOKB3_oA@mail.gmail.com>
	<15740.1482788055@sss.pgh.pa.us>
Date: Thu, 29 Dec 2016 21:02:49 +0100
In-Reply-To: <15740.1482788055@sss.pgh.pa.us> (Tom Lane's message of "Mon, 26
	Dec 2016 16:34:15 -0500")
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* Tom Lane:

> On Linux (RHEL6, 2.4GHz x86_64), I find that gettimeofday(),
> clock_gettime(CLOCK_MONOTONIC), and clock_gettime(CLOCK_REALTIME)
> all take about 40ns.  Of course gettimeofday() only has 1us resolution,
> but the other two have perhaps 10ns resolution (I get no duplicate
> readings in a tight loop).  Other documented clockids include
>     CLOCK_REALTIME_COARSE: about 10ns to read, but only 1ms resolution
>     CLOCK_MONOTONIC_COARSE: about 12ns to read, but only 1ms resolution
>     CLOCK_MONOTONIC_RAW: full resolution but very slow, ~145ns to read
> So CLOCK_MONOTONIC seems to be the thing to use here.  It won't buy
> us anything speed-wise but the extra resolution will be nice.
> However, we need to do more research to see if this holds true on
> other popular distros.

Isn't this very specific to kernel and glibc versions, depending on
things like CONFIG_HZ settings and what level of vDSO support has been
backported?


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