linux-kernel - Re: [PATCH] perf top: Make -g refer to callchains

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Message-ID: <20131118174945.GD24375@krava.brq.redhat.com>
Date:	Mon, 18 Nov 2013 18:49:45 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	David Ahern <dsahern@...il.com>, linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Jan Kratochvil <jan.kratochvil@...hat.com>
Subject: Re: [PATCH] perf top: Make -g refer to callchains

On Mon, Nov 18, 2013 at 03:26:53PM +0100, Ingo Molnar wrote:
> 
> * Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > On Mon, Nov 18, 2013 at 09:59:45AM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Fri, Nov 15, 2013 at 06:46:09AM +0100, Ingo Molnar escreveu:
> > > > btw., here's some 'perf top' call graph performance and profiling 
> > > > quality feedback, with the latest perf code:
> > > > 
> > > > 'perf top --call-graph fp' now works very well, using just 0.2% 
> > > > of CPU time on a fast system:
> > > > 
> > > >  4676 mingo     20   0  612m  56m 9948 S     1  0.2   0:00.68 perf                                                                                                        
> > > > 
> > > > 'perf top --call-graph dwarf' on the other hand is horrendously 
> > > > slow, using 20% of CPU time on a 4 GHz CPU:
> > > > 
> > > >   PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                                                                     
> > > >  4646 mingo     20   0  658m  81m  12m R    19  0.3   0:18.17 perf    
> > > > 
> > > > On another system with a 2.4GHz CPU it's taking up 100% of CPU 
> > > > time (!):
> > > > 
> > > >   PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                                               
> > > >  8018 mingo     20   0  290320  45220   8520 R  99.5  0.3   0:58.81 perf      
> > > > 
> > > > Profiling 'perf top' shows all sorts of very high dwarf 
> > > > processing overhead:
> > > 
> > > Yeah, top dwarf callchain has been so far a proof of concept, it 
> > > exacerbates problems that can be seen on 'report', but since its 
> > > live, we can see it more clearly.
> > > 
> > > The work on improving callchain processing, (rb_tree'ing, new comm 
> > > infrastructure) alleviated the problem a bit.
> > > 
> > > Tuning the stack size requested from the kernel and using 
> > > --max-stack can help when it is really needed, but yes, work on it 
> > > is *badly* needed.
> > 
> > agreed ;-)
> > 
> > also there's new remote unwind interface recently added into libdw, 
> > which seems to be faster than libunwind.
> >
> > I plan on adding this soon.
> 
> If the main source of overhead is libunwind (which needs independent 
> confirmation) then would it make sense to implement dwarf stack unwind 
> support ourselves?
> 
> I think SysProf does that and it appears to be faster - its unwind.c 
> is only 400 lines long as it only implements the small subset needed 
> to walk the stack - AFAICS.

I think it's an option.. but it'll simpler to try the libdw
interface first and see if it's good/fast enough..

also I recall discussing the speed with libdw developer
Jan Kratochvil (CC-ed) and AFAICS they're open for
suggestions/optimizations

jirka
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