linux-kernel - Re: [PATCH 01/16] math128: Introduce various 128bit primitives

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Date:	Fri, 26 Oct 2012 09:09:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Juri Lelli <juri.lelli@...il.com>, mingo@...hat.com,
	oleg@...hat.com, fweisbec@...il.com, darren@...art.com,
	johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com,
	jkacur@...hat.com, harald.gustafsson@...csson.com,
	vincent.guittot@...aro.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 01/16] math128: Introduce various 128bit primitives

On Fri, 2012-10-26 at 08:39 -0400, Steven Rostedt wrote:
> On Fri, 2012-10-26 at 12:36 +0200, Thomas Gleixner wrote:
> 
> > By all means. nsec precision is a completly academic thought
> > exercise. It's really pointless to even think about anything below
> > microseconds resolution.
> > 
> > We can still have the user space interface handing in the information
> > in nsec resolution, but it's reasonable to scale it down to something
> > useful. Just shift the incoming information right by 10, so you're in
> > the 1us resolution for all the internal math and all your limitation
> > problems are gone. A shift by ten for converting back and forth to
> > nsecs is not a real performance issue.
> 
> Just make sure this is well documented in the man pages, and that should
> eliminate any "surprises". This is a new interface, we can just make
> this part of the ABI. "The units are in nanoseconds, but all
> calculations are performed to the nearest microsecond. Take this into
> account for error analysis". People should be fine with this.

Actually, a shift by 10 is a division by 1024, which is not truly down
to a microsecond. Would just a shift by 9 work as well? This would make
the resolution closer to a half of microsecond. Otherwise things will
probably get screwy if the user passes in 1000 ns, and gets a zero
result.

-- Steve


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