Re: [RFC PATCH 00/10] x86: undwarf unwinder

[Josh Poimboeuf]

On Thu, Jun 01, 2017 at 03:50:05PM +0200, Ingo Molnar wrote:
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
> > On Thu, Jun 01, 2017 at 08:08:24AM +0200, Ingo Molnar wrote:
> > > Being able to generate more optimal code in the hottest code paths of the kernel 
> > > is the _real_, primary upstream kernel benefit of a different debuginfo method - 
> > > which has to be weighed against the pain of introducing a new unwinder. But this 
> > > submission does not talk about that aspect at all, which should be fixed I think.
> > 
> > Actually I devoted an entire one-sentence paragraph to performance in
> > the documentation:
> > 
> >   The simpler debuginfo format also enables the unwinder to be relatively
> >   fast, which is important for perf and lockdep.
> > 
> > But I'll try to highlight that a little more.
> 
> That's not what I meant! The speedup comes from (hopefully) being able to disable 
> CONFIG_FRAME_POINTER, which:
> 
>  - creates simpler/faster function prologues and epilogues - no managing of RBP 
>    needed
> 
>  - gives one more generic purpose register to work from. This matters less on 
>    64-bit kernels but it's a small effect.
> 
> I've seen numbers of 1-2% of instruction count reduction in common kernel 
> workloads, which would be pretty significant on well cached workloads.

Ah, you meant runtime performance with FP disabled.  I also dedicated a
whole sentence to that one :-)

  Unlike frame pointers, the debuginfo is out-of-band, so it has no
  effect on runtime performance.

I'll try to flesh that out and maybe come up with some numbers.

-- 
Josh