linux-kernel - Re: [RFC 0/2] ABI for clock_gettime

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Date:	Wed, 14 Dec 2011 19:37:30 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	john stultz <johnstul@...ibm.com>, linux-kernel@...r.kernel.org,
	Kumar Sundararajan <kumar@...com>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC 0/2] ABI for clock_gettime_ns

On Wed, Dec 14, 2011 at 09:15:29AM -0800, Andy Lutomirski wrote:
> On Wed, Dec 14, 2011 at 8:48 AM, john stultz <johnstul@...ibm.com> wrote:

> > On Wed, 2011-12-14 at 08:46 +0100, Richard Cochran wrote:
> >> Believe it or not, people (from the Test and Measurement field) have
> >> already been asking me about having subnanosecond time values from the
> >> kernel.
> 
> I'm curious how that works.  My personal record is synchronizing time
> across a bunch of computers to within maybe half a nanosecond, but it
> wasn't the *system* clock that I synchronized -- I just calibrated a
> bunch of oscillator phase differences on ADC clocks that I was using.
> I only relied on the system clock being correct to a few tens of
> microseconds, which is easily done with PTP.

On example to take a look at is the White Rabbit project.

It is possible to time stamp events and apply clock corrections at a
very fine resolution, for example with synchronized Ethernet and PTP.

> What about:
> 
> struct sys_timeval {
>     u64 nanoseconds;  /* unsigned.  the current time will always be
> after 1970, and those extra 290 years might be nice. */
>     u64 padding;  /* for later.  currently always zero. */
> 
> That way, once there's both an implementation and a use case, we can
> implement it.  In the mean time, the overhead is probably immeasurably
> low -- it's a single assignment.

Agreed.

> Note that rdtsc isn't good to a nanosecond, let alone sub-nanosecond
> intervals, on any hardware I've ever seen.

But the hardware is coming, sooner or later.

Richard
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