linux-kernel - Re: frame unwinder patches

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Date:	Fri, 5 Sep 2008 07:57:03 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Bernd Schubert <bs@...eap.de>
Cc:	linux-kernel@...r.kernel.org, Andi Kleen <andi@...stfloor.org>
Subject: Re: frame unwinder patches

On Fri, 5 Sep 2008 16:48:52 +0200
Bernd Schubert <bs@...eap.de> wrote:

> On Friday 05 September 2008 16:13:37 Arjan van de Ven wrote:
> > On Fri, 5 Sep 2008 15:52:47 +0200
> >
> > Bernd Schubert <bs@...eap.de> wrote:
> > > > (and if you really care it's 1 line of code to turn it off)
> > >
> > > It is not only this, I think the dwarf2 stack unwinder patches
> > > provide by far better traces than the in-kernel unwinder. At least
> > > ever since I applied these patches to our kernels, I was able to
> > > read the stack dumps...
> >
> > they really wouldn't be different than the ones you get if you
> > remove the "?" lines.
> 
> Well may be, but then there is still the performace degrading, so I
> don't want to have it enabled on our production kernels. I admit I
> never measured what is the difference between of
> CONFIG_FRAME_POINTER=y and =n, but the fact the help text says there
> is a difference already makes me want to disable it (especially,
> since we have to provide benchmarks before we can sell a system).

to be honest, on 64 bit the overhead is quite small (the extra
instructions it adds are optimized for by the modern cpus that you use
in the systems you're selling); on 32 bit the overhead is.. well a
little bigger but not THAT much. yes it loses a register for the
compiler to use, but no it's not a general purpose register, and with
the register renaming that today's cpus do, I'd be surprised if you
could see anything significant.



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