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

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Date:	Sat, 24 Dec 2011 20:06:18 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Richard Cochran <richardcochran@...il.com>
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 Fri, Dec 23, 2011 at 10:50 PM, Richard Cochran
<richardcochran@...il.com> wrote:
> On Fri, Dec 23, 2011 at 09:59:04PM -0800, Andy Lutomirski wrote:
>
>> I've written the patch and it's not as big an improvement as I hoped.
>> I'll play with it a bit and send it out soon.
>
> Andy, can you say what the motivation or use case is?

Simplicity and performance.  Many users (e.g. I) use clock_gettime to
measure elapsed time.  We just end up converting to flat nanoseconds
over and over.  This syscall avoids the conversion.

The added padding bits will allow UTC/TAI offset and/or sub-nanosecond
precision in the future if someone wants to add them.

I seem to be saving just over one nanosecond per call on my laptop,
which isn't that impressive.  The real-world improvement may be better
-- calling clock_gettime in a tight loop should give very good branch
prediction success, which will make the cost of the loops that flat ns
avoids seem a little lower than they are.

I'll send patches in the morning.

--Andy

>
> Just curious,
>
> Richard



-- 
Andy Lutomirski
AMA Capital Management, LLC
Office: (310) 553-5322
Mobile: (650) 906-0647
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