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Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)

From Andi Kleen <andi@firstfloor.org>
Newsgroups linux.kernel
Subject Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf)
Date 2017-07-13 06:50 +0200
Message-ID <u2EeB-XA-1@gated-at.bofh.it> (permalink)
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On Thu, Jul 13, 2017 at 06:28:43AM +0200, Mike Galbraith wrote:
> On Wed, 2017-07-12 at 21:15 -0700, Andi Kleen wrote:
> > On Thu, Jul 13, 2017 at 05:03:00AM +0200, Mike Galbraith wrote:
> > > On Wed, 2017-07-12 at 15:30 -0700, Andi Kleen wrote:
> > > > Josh Poimboeuf <jpoimboe@redhat.com> writes:
> > > > >
> > > > > The ORC data format does have a few downsides compared to DWARF.  The
> > > > > ORC unwind tables take up ~1MB more memory than DWARF eh_frame tables.
> > > > >
> > > > Can we have an option to just use dwarf instead? For people
> > > > who don't want to waste a MB+ to solve a problem that doesn't
> > > > exist (as proven by many years of opensuse kernel experience)
> > > 
> > > Sure the dwarf unwinder works well for crashes, but at the price of
> > > demolishing ftrace/perf utility.
> > 
> > You mean the unwind performance?
> 
> Yeah, it hurts.. massively, has even been known to kill big boxen.

Why was that? 

> 
> > That's a valid concern, but neither ORC nor dwarf are likely
> > to address it. However most usages of ftrace/perf shouldn't be that
> > depending on unwind performance -- just lower the frequency of your
> > events. 
> > 
> > The only possible win is if the win from not using FP code is
> > significant enough. On the x86 side the only modern CPUs that should really
> > care about this are Atoms.
> 
> Nope, they all care.  Measure performance delta of fast/light stuff.

Well if your test cares that much about function overhead you may want to try
LTO. It can get rid of a lot of functions by doing cross file
inlining.

https://git.kernel.org/pub/scm/linux/kernel/git/ak/linux-misc.git/log/?h=lto-411-2

> Maybe I'm expecting too much good stuff to follow, but don't spoil it
> for me, I think I'm looking at a real winner :)

It's somewhat surprising. It would be good to under stand why that
happens. Is it icache misses, data cache misses for the stack, or
simply more instructions executed, or worse tail calls?

-Andi

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[PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-11 17:40 +0200
  Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-12 10:30 +0200
    Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-12 16:50 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-12 21:30 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-14 19:20 +0200
  Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andres Freund <andres@anarazel.de> - 2017-07-12 23:50 +0200
    Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 00:40 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andres Freund <andres@anarazel.de> - 2017-07-13 00:40 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 00:50 +0200
          Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andres Freund <andres@anarazel.de> - 2017-07-13 01:00 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Peter Zijlstra <peterz@infradead.org> - 2017-07-13 09:20 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Peter Zijlstra <peterz@infradead.org> - 2017-07-13 11:00 +0200
          Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-13 11:20 +0200
            Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 14:20 +0200
              Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 14:30 +0200
                Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 14:40 +0200
                Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-14 10:40 +0200
              Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-14 10:30 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Peter Zijlstra <peterz@infradead.org> - 2017-07-13 11:00 +0200
  Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andi Kleen <andi@firstfloor.org> - 2017-07-13 00:40 +0200
    Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 00:50 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andi Kleen <andi@firstfloor.org> - 2017-07-13 06:30 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Josh Poimboeuf <jpoimboe@redhat.com> - 2017-07-13 15:20 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Ingo Molnar <mingo@kernel.org> - 2017-07-13 11:30 +0200
    Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andy Lutomirski <luto@kernel.org> - 2017-07-13 01:30 +0200
    Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Mike Galbraith <efault@gmx.de> - 2017-07-13 05:10 +0200
      Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andi Kleen <andi@firstfloor.org> - 2017-07-13 06:20 +0200
        Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Mike Galbraith <efault@gmx.de> - 2017-07-13 06:40 +0200
          Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Andi Kleen <andi@firstfloor.org> - 2017-07-13 06:50 +0200
            Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Mike Galbraith <efault@gmx.de> - 2017-07-13 07:30 +0200
            Re: [PATCH v3 00/10] x86: ORC unwinder (previously undwarf) Jiri Kosina <jikos@kernel.org> - 2017-07-13 14:10 +0200

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