Messages in this thread | | | Subject | Re: [PATCH 01/16] math128: Introduce various 128bit primitives | From | Peter Zijlstra <> | Date | Thu, 25 Oct 2012 15:47:29 +0200 |
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On Wed, 2012-10-24 at 16:18 -0700, Linus Torvalds wrote: > On Wed, Oct 24, 2012 at 2:53 PM, Juri Lelli <juri.lelli@gmail.com> wrote: > > From: Peter Zijlstra <a.p.zijlstra@chello.nl> > > > > Grow rudimentary u128 support without relying on gcc/libgcc. > > I missed the part where somebody explains why and what needs this? > It's going to be very expensive indeed on some platforms, so the fact > that it is *sometimes* cheap doesn't necessarily imply it should ever > be used. > > So please, explain what the pressing need is that is so worthwhile > that this is worth it. Maybe it was in a 00/16 cover letter, but not > only was that not sent out to the people who got 01, you'd still want > it in the commit message.
There's two use cases:
1) the proposed SCHED_DEADLINE needs to do some u64xu64 math, it ends up having to multiply a deadline (in usec) with runtime (also in usec).
2) the infrastructure adds mul_u64_u32_shr(), which is something we do a lot of with all the time manipulation, apply a multiplier to some u64 clock value.
We can do better on some archs than we can in generic, so this interface could give a win there.
But yes, in general people should be very very reluctant to use this.
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