linux-kernel - Re: [PATCH V3 11/12] perf top: switch default mode to overwrite mode

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Date:   Thu, 11 Jan 2018 15:26:17 +0100
From:   Jiri Olsa <jolsa@...hat.com>
To:     kan.liang@...el.com
Cc:     acme@...nel.org, peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org, wangnan0@...wei.com,
        jolsa@...nel.org, namhyung@...nel.org, ak@...ux.intel.com,
        yao.jin@...ux.intel.com
Subject: Re: [PATCH V3 11/12] perf top: switch default mode to overwrite mode

On Thu, Dec 21, 2017 at 10:08:53AM -0800, kan.liang@...el.com wrote:
> From: Kan Liang <kan.liang@...el.com>
> 
> perf_top__mmap_read has severe performance issue in
> Knights Landing/Mill, when monitoring in heavy load system. It costs
> several minutes to finish, which is unacceptable.
> 
> Currently, perf top is non overwrite mode. For non overwrite mode, it
> tries to read everything in the ringbuffer and doesn't pause the
> ringbuffer. Once there are lots of samples delivered persistently,
> the processing time could be very long. Also, the latest samples could
> be lost when the ringbuffer is full.
> 
> For overwrite mode, it takes a snapshot for the system by pausing the
> ringbuffer, which could significantly reducing the processing time.
> Also, the overwrite mode always keep the latest samples.
> Considering the real time requirement for perf top, the overwrite mode
> is more suitable for perf top.
> 
> Actually, perf top was overwrite mode. It is changed to non overwrite
> mode since commit 93fc64f14472 ("perf top: Switch to non overwrite
> mode"). It's better to change it back to overwrite mode by default.
> 
> For the kernel which doesn't support overwrite mode, it will fall back
> to non overwrite mode.
> 
> There would be some records lost in overwrite mode because of pausing
> the ringbuffer. It has little impact for the accuracy of the snapshot
> and could be tolerant.
> The lost events checking is removed.
> 
> For overwrite mode, unconditionally wait 100 ms before each snapshot. It
> also reduce the overhead caused by pausing ringbuffer, especially on
> light load system.
> 
> Signed-off-by: Kan Liang <kan.liang@...el.com>
> ---
>  tools/perf/builtin-top.c       | 44 ++++++++++++++++++++++++------------------
>  tools/perf/ui/browsers/hists.c | 12 +++++++++---
>  2 files changed, 34 insertions(+), 22 deletions(-)
> 
> diff --git a/tools/perf/builtin-top.c b/tools/perf/builtin-top.c
> index 8d19ef7..3c14333 100644
> --- a/tools/perf/builtin-top.c
> +++ b/tools/perf/builtin-top.c
> @@ -283,16 +283,6 @@ static void perf_top__print_sym_table(struct perf_top *top)
>  
>  	printf("%-*.*s\n", win_width, win_width, graph_dotted_line);
>  
> -	if (hists->stats.nr_lost_warned !=
> -	    hists->stats.nr_events[PERF_RECORD_LOST]) {
> -		hists->stats.nr_lost_warned =
> -			      hists->stats.nr_events[PERF_RECORD_LOST];
> -		color_fprintf(stdout, PERF_COLOR_RED,
> -			      "WARNING: LOST %d chunks, Check IO/CPU overload",
> -			      hists->stats.nr_lost_warned);
> -		++printed;
> -	}
> -

should this stay for non-overwrite mode?

jirka