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

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Date:	Mon, 18 Nov 2013 15:26:53 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Jiri Olsa <jolsa@...hat.com>
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>
Subject: Re: [PATCH] perf top: Make -g refer to callchains


* 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.

Thanks,

	Ingo
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