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 -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
