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

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Date:	Fri, 26 Oct 2012 12:04:10 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Juri Lelli <juri.lelli@...il.com>, tglx@...utronix.de,
	mingo@...hat.com, rostedt@...dmis.org, 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


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2012-10-26 at 11:42 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@...radead.org> wrote:
> > 
> > > On Fri, 2012-10-26 at 11:24 +0200, Ingo Molnar wrote:
> > > 
> > > > So can we control this by restricting the users and avoiding 
> > > > the overflow?
> > > > 
> > > > A 2^64 result should be a *huge* amount of space already for 
> > > > just about anything.
> > > 
> > > I _think_ something like: dl_runtime * dl_deadline < U64_MAX, 
> > > might do that. The question is, is this constraint usable? 
> > > Simplified that boils down to about 4 seconds each, which 
> > > sounds pretty much ok for most people -- but such statements 
> > > usually come back to bite you (640kb anybody...).
> > 
> > We could constrain the precision, not the maximum value.
> > 
> > Having a 4 seconds hard limit is one thing, only having 10 nsecs 
> > precision at 40 seconds is another.
> 
> That gets to be rather ugly I think.. for one it might 
> surprise people, secondly you get to have a bunch of 
> conditionals and shifts in that code path.

I don't think a limitation of precision to about 64 bits is a 
"surprise": it's high grade precision of 0.00000005 parts per 
trillion...

( As a comparison, there's ~13 parts per trillion amount of pure 
  gold dissolved in ocean water. )

> Personally I'd prefer to do the simple thing, esp. for a new 
> interface. So either do the hard limit or the u128 thing.

Given that the u128 thing, once it gets converted to machine 
instructions, is not simple *at all*, that leaves us with the 
hard limit.

> If we go with the hard limit, we can always address things 
> when people run into it and complain, at such a time we also 
> have a better view of people's uses and expectations methinks.

Indeed.

Thanks,

	Ingo
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