Re: [RFC 0/2] ABI for clock_gettime_ns

From: Andy Lutomirski
Date: Sat Dec 24 2011 - 23:07:05 EST

On Fri, Dec 23, 2011 at 10:50 PM, Richard Cochran
<richardcochran@xxxxxxxxx> 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.


> Just curious,
> Richard

Andy Lutomirski
AMA Capital Management, LLC
Office: (310) 553-5322
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