* Peter Zijlstra <[email protected]> wrote:
> On Fri, 2012-10-26 at 11:42 +0200, Ingo Molnar wrote:
> > * Peter Zijlstra <[email protected]> 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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