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?
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	<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> <87y3yyfqqe.fsf@mid.deneb.enyo.de>
	<4130.1483045351@sss.pgh.pa.us>
Date: Fri, 13 Jan 2017 21:48:51 +0100
In-Reply-To: <4130.1483045351@sss.pgh.pa.us> (Tom Lane's message of "Thu, 29
	Dec 2016 16:02:31 -0500")
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* Tom Lane:

> Florian Weimer <fw@deneb.enyo.de> writes:
>> * 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).
>
>> 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?
>
> No doubt, but I have yet to find a platform where clock_gettime() exists
> but performs worse than gettimeofday().  Do you know of one?

ppc64le with all the vDSO fixes for clock_gettime?

glibc has some test cases which fail because clock_gettime gives
inconsistent results.  This has been fixed in current kernels, but I
don't know if everyone uses them.


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