linux-kernel - Re: [RFC PATCH 0/7] Share events between metrics

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Message-ID: <20201215150812.GA38786@li-24c3614c-2adc-11b2-a85c-85f334518bdb.ibm.com>
Date:   Tue, 15 Dec 2020 09:08:12 -0600
From:   "Paul A. Clarke" <pc@...ibm.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        Andrii Nakryiko <andriin@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Kajol Jain <kjain@...ux.ibm.com>,
        John Garry <john.garry@...wei.com>,
        Jin Yao <yao.jin@...ux.intel.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Kim Phillips <kim.phillips@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-perf-users <linux-perf-users@...r.kernel.org>,
        Stephane Eranian <eranian@...gle.com>
Subject: Re: [RFC PATCH 0/7] Share events between metrics

On Thu, May 07, 2020 at 10:43:43PM -0700, Ian Rogers wrote:
> On Thu, May 7, 2020 at 2:47 PM Andi Kleen <ak@...ux.intel.com> wrote:
> >
> > > > - without this change events within a metric may get scheduled
> > > >   together, after they may appear as part of a larger group and be
> > > >   multiplexed at different times, lowering accuracy - however, less
> > > >   multiplexing may compensate for this.

Does mutiplexing somewhat related events at different times actually reduce
accuracy, or is it just more likely to give that appearance?

It seems that perf measurements are only useful if the workload is in a
fairly steady state.  If there is some wobbling, then measuring at the
same time is more accurate for the periods where the events are being
measured simultaneously, but may be far off for when they are not being
measured at all.  Spreading them out over a longer duration may actually
increase accuracy by sampling over more varied intervals.

Or, is the concern more about trying to time-slice the results in a 
fairly granular way and expecting accurate results then?

(Or, maybe my ignorance is showing again.  :-)

PC