linux-kernel - Re: [PATCH 1/4] Add clock_gettime

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Date:	Tue, 27 Dec 2011 08:25:11 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	linux-kernel@...r.kernel.org, Kumar Sundararajan <kumar@...com>,
	john stultz <johnstul@...ibm.com>,
	Arun Sharma <asharma@...com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/4] Add clock_gettime_ns syscall

On Sun, Dec 25, 2011 at 08:51:00AM -0800, Andy Lutomirski wrote:
> 
> This interface intentionally does not support sub-nanosecond
> precision.  For one thing, light only travels about a foot per
> nanosecond, so nanoseconds are really pretty good for networking
> purposes.  For another, 2^64 picoseconds (say) is a short enough
> interval to be inconvenient.  If anyone needs sub-nanosecond
> precision for anything other than profiling, they're welcome to
> figure out an appropriate interface.  For very precise profiling,
> "what time is it" is the wrong question, anyway -- modern CPUs can
> reorder things across time scales much longer than a nanosecond.

This paragraph probably should be updated, since you did add a padding
field for future sub-nanosecond reporting.

Also, on re-reading this, it seems silly to be to introduce a function
called "clock_gettime_ns" and then to say that this function cannot
answer the question, what time is it?

If the interface is a profiling only interface, then the name should
reflect this fact.

Thanks,
Richard
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