Messages in this thread | | | Date | Mon, 17 Oct 2011 23:19:51 +0200 | From | Ingo Molnar <> | Subject | Re: Linux 3.1-rc9 |
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* H. Peter Anvin <hpa@zytor.com> wrote:
> On 10/17/2011 11:49 AM, Ingo Molnar wrote: > > Do we have some hard data on this, which we could put into comments > > in include/linux/ktime.h and such? Older versions of GCC used to do a > > bad job of long long handling on 32-bit systems - that might be a > > factor in the performance figures. > > > > But i suspect you are right that the cost is still very much there > > 64/64 division is done bit by bit on most (all?) 32-bit architectures. > > 64/32 division can be done in hardware on some architectures, e.g. x86.
it's 64/32 division - it's the /1000000000 /1000000 /1000 divisions in the large majority of cases, to convert between seconds/milliseconds/microseconds and scalar nanoseconds.
the kernel-internal ktime_t in the 32-bit optimized case is:
union ktime { s32 sec, nsec; };
which is the same as timespec and arithmetically close to timeval, which many ABIs use. So conversion is easy in that case - but arithmetics gets a bit harder.
If we used a scalar 64-bit form for all kernel internal time representations:
s64 nsecs;
then conversions back to timespec/timeval would involve dividing this 64-bit value with 1000000000 or 1000000.
Is there no faster approximation for those than bit by bit?
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.
But it's late here and math is hard - lets go shopping ;-)
Thanks,
Ingo
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