linux-kernel - Re: [PATCH v3 07/12] perf record: init data file at mmap buffer object

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Date:   Mon, 1 Mar 2021 20:44:24 +0900
From:   Namhyung Kim <namhyung@...nel.org>
To:     "Bayduraev, Alexey V" <alexey.v.bayduraev@...ux.intel.com>
Cc:     Alexei Budankov <abudankov@...wei.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Alexander Antonov <alexander.antonov@...ux.intel.com>
Subject: Re: [PATCH v3 07/12] perf record: init data file at mmap buffer object

Hello,

On Mon, Mar 1, 2021 at 8:16 PM Bayduraev, Alexey V
<alexey.v.bayduraev@...ux.intel.com> wrote:
>
> Hi,
>
> On 20.11.2020 13:49, Namhyung Kim wrote:
> > On Mon, Nov 16, 2020 at 03:19:41PM +0300, Alexey Budankov wrote:
>
> <SNIP>
>
> >>
> >> @@ -1400,8 +1417,12 @@ static int record__mmap_read_evlist(struct record *rec, struct evlist *evlist,
> >>      /*
> >>       * Mark the round finished in case we wrote
> >>       * at least one event.
> >> +     *
> >> +     * No need for round events in directory mode,
> >> +     * because per-cpu maps and files have data
> >> +     * sorted by kernel.
> >
> > But it's not just for single cpu since task can migrate so we need to
> > look at other cpu's data too.  Thus we use the ordered events queue
> > and round events help to determine when to flush the data.  Without
> > the round events, it'd consume huge amount of memory during report.
> >
> > If we separate tracking records and process them first, we should be
> > able to process samples immediately without sorting them in the
> > ordered event queue.  This will save both cpu cycles and memory
> > footprint significantly IMHO.
> >
> > Thanks,
> > Namhyung
> >
>
> As far as I understand, to split tracing records (FORK/MMAP/COMM) into
> a separate file, we need to implement a runtime trace decoder on the
> perf-record side to recognize such tracing records coming from the kernel.
> Is that what you mean?

No, I meant separating the mmap buffers so that the record process
can save the data without decoding.

>
> IMHO this can be tricky to implement and adds some overhead that can lead
> to possible data loss. Do you have any other ideas how to optimize memory
> consumption on perf-report side without a runtime trace decoder?
> Maybe "round events" would somehow help in directory mode?
>
> BTW In our tool we use another approach: two-pass trace file loading.
> The first loads tracing records, the second loads samples.

Yeah, something like that.  With the separated data, we can do it
more efficiently IMHO.

Thanks,
Namhyung