linux-kernel - Re: [PATCH] tools, perf: Allow the user to disable time stamps

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Date:	Tue, 25 Mar 2014 21:54:38 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] tools, perf: Allow the user to disable time stamps

On Tue, Mar 25, 2014 at 9:44 PM, Andi Kleen <andi@...stfloor.org> wrote:
>
> From: Andi Kleen <ak@...ux.intel.com>
>
> [post with updated description]
>
> Time stamps are always implicitely enabled for record currently.
> The old --time/-T option is a nop.
>
> The time stamps are needed to handle library mmaps correctly.
> This mainly matters at program startup, but is not useful
> when profiling long running programs.
>
> Allow the user to disable timestamps by using --no-time
>
> This can cause some minor misaccounting, but lowers the
> size of the perf.data nicely:
>
> % perl -e 'while(1) {}' &
> [1] 5827
> % perf record -p 5827 --no-time -c 1000003 sleep 1
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.089 MB perf.data (~3872 samples) ]
> % perf record -p 5827  -c 1000003  sleep 1
> [ perf record: Woken up 1 times to write data ]
> [ perf record: Captured and wrote 0.114 MB perf.data (~4967 samples) ]
>
> 22% smaller data file, even though we got more samples.
> It also seems to speed up perf slightly, that is why we get more
> samples.
>
But that measure of the number of samples is bogus most of the time.
It is an estimate based on a standard sample size.

Timestamp are useful to order samples during reporting.
How do you do with it if you monitor multi-threaded (multi-cpu)
workloads. This is only good for single thread or single CPU
measurements. Or am I missing something?


>
> The defaults are unchanged.
>
> Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> ---
>  tools/perf/builtin-record.c | 1 +
>  tools/perf/util/evsel.c     | 9 ++++++---
>  2 files changed, 7 insertions(+), 3 deletions(-)
>
> diff --git a/tools/perf/builtin-record.c b/tools/perf/builtin-record.c
> index 3c394bf..8e92d3c 100644
> --- a/tools/perf/builtin-record.c
> +++ b/tools/perf/builtin-record.c
> @@ -800,6 +800,7 @@ static const char * const record_usage[] = {
>   */
>  static struct record record = {
>         .opts = {
> +               .sample_time         = true,
>                 .mmap_pages          = UINT_MAX,
>                 .user_freq           = UINT_MAX,
>                 .user_interval       = ULLONG_MAX,
> diff --git a/tools/perf/util/evsel.c b/tools/perf/util/evsel.c
> index 22e18a2..a15efc4 100644
> --- a/tools/perf/util/evsel.c
> +++ b/tools/perf/util/evsel.c
> @@ -613,9 +613,12 @@ void perf_evsel__config(struct perf_evsel *evsel, struct record_opts *opts)
>         if (opts->period)
>                 perf_evsel__set_sample_bit(evsel, PERIOD);
>
> -       if (!perf_missing_features.sample_id_all &&
> -           (opts->sample_time || !opts->no_inherit ||
> -            target__has_cpu(&opts->target) || per_cpu))
> +       /*
> +        * When the user explicitely disabled time don't force it here.
> +        */
> +       if (opts->sample_time &&
> +           (!perf_missing_features.sample_id_all &&
> +           (!opts->no_inherit || target__has_cpu(&opts->target) || per_cpu)))
>                 perf_evsel__set_sample_bit(evsel, TIME);
>
>         if (opts->raw_samples) {
> --
> 1.8.5.3
>
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