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SubjectRe: Linux 3.1-rc9

* Ingo Molnar <mingo@elte.hu> wrote:

> > > In particular we could try something like:
> > >
> > > (high*2^32 + low)/1e9 ~== ( high * (2^64/1e9) ) / 2^32
> > >
> > > ... which reduces it all to a 64-bit multiplication (or two
> > > 32-bit multiplications) with a known constant, at the cost of 1
> > > nsec imprecision of the result - but that's an OK approximation
> > > in my opinion.
> > >
> >
> > We can do much better than that with reciprocal multiplication.
>
> Yes, 2^64/1e9 is the reciprocal.

So basically, to extend on the pseudocode above, we could do the
equivalent of:

/* 2^64/1e9: */
#define MAGIC 18446744073ULL

secs_fast = ((nsecs >> 32) * MAGIC) >> 32;
secs_fast += (nsecs & 0xFFFFFFFF)/1000000000;

to get to the precise 'timeval.secs' field - these are all 32-bit
operations: a 32-bit multiplication and a 32-bit division if i
counted it right.

(Likewise we can get the remainder as well, for timeval.nsecs.)

So I think if we add 32-bit optimized reciprocal multiplication based
timeval and timespec routines, we can change ktime_t to a simple
scalar type on 64-bit and 32-bit architectures alike.

It would likely be faster as well: the 32-bit ktime operations are
more complex than straightforward u64 operations.

Thomas, what do you think?

Thanks,

Ingo


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