linux-kernel - Re: [RFC] perf: perf record sets inherit by default

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Message-ID: <20100511150052.GE19192@ghostprotocols.net>
Date:	Tue, 11 May 2010 12:00:52 -0300
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Stephane Eranian <eranian@...gle.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	mingo@...e.hu, Paul Mackerras <paulus@...ba.org>,
	"David S. Miller" <davem@...emloft.net>,
	perfmon2-devel@...ts.sf.net
Subject: Re: [RFC] perf: perf record sets inherit by default

Em Tue, May 11, 2010 at 04:04:17PM +0200, Stephane Eranian escreveu:
> I am confused by the inheritance cmd line option of perf record:

>  usage: perf record [<options>] [<command>]
>     or: perf record [<options>] -- <command> [<options>]
>     -p, --pid <n>         record events on existing process id
>     -t, --tid <n>         record events on existing thread id
>     -a, --all-cpus        system-wide collection from all CPUs
>     -C, --profile_cpu <n> CPU to profile on
>     -i, --inherit         child tasks inherit counters
 
> This leads to believe that by default inheritance in children is off.
 
> However, builtin-record.c says:
> static bool                     inherit    =   true;
 
> If that's the case, what's the point of the -i option?

Humm, since for -C and -a using -i doesn't make sense, I guess it should
be off by default and only be auto-activated if we don't specify any
option, i.e. when using it like:

perf record ./hackbench

What do you think?
 
> Another side effect of inheritance is that in per-thread mode, perf
> creates as many "sessions" as you have CPUs. So on a 16-way processor,
> sampling on cycles, perf creates 16 events and 16 x 2-page sampling
> buffers. That's a lot of resources consumed if I am just interested in
> monitoring a single-threaded workload.
 
> Am I missing something here?

I don't think so, but maybe I'm missing too :-)

- Arnaldo
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