RE: [RFC] Issue in final aggregate value, in case of multiple events present in metric expression
From: Joakim Zhang
Date: Sun May 10 2020 - 22:05:16 EST
> -----Original Message-----
> From: Jin, Yao <yao.jin@xxxxxxxxxxxxxxx>
> Sent: 2020å5æ11æ 9:12
> To: kajoljain <kjain@xxxxxxxxxxxxx>; Joakim Zhang
> <qiangqing.zhang@xxxxxxx>; acme@xxxxxxxxxx; Jiri Olsa <jolsa@xxxxxxxxxx>;
> Andi Kleen <ak@xxxxxxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; linux-perf-users@xxxxxxxxxxxxxxx; Kan Liang
> <kan.liang@xxxxxxxxxxxxxxx>; Madhavan Srinivasan
> <maddy@xxxxxxxxxxxxxxxxxx>; Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx>;
> Ravi Bangoria <ravi.bangoria@xxxxxxxxxxxxx>
> Subject: Re: [RFC] Issue in final aggregate value, in case of multiple events
> present in metric expression
>
> Hi Kajol,
>
> On 3/24/2020 4:00 PM, kajoljain wrote:
> > Hello All,
> > I want to discuss one issue raised by Joakim Zhang where he mentioned
> > that, we are not getting correct result in-case of multiple events
> > present in metric expression.
> >
> > This is one example pointed by him :
> >
> > below is the JSON file and result.
> > [
> > {
> > "PublicDescription": "Calculate DDR0 bus actual utilization
> which vary from DDR0 controller clock frequency",
> > "BriefDescription": "imx8qm: ddr0 bus actual utilization",
> > "MetricName": "imx8qm-ddr0-bus-util",
> > "MetricExpr": "( imx8_ddr0\\/read\\-cycles\\/ +
> imx8_ddr0\\/write\\-cycles\\/ )",
> > "MetricGroup": "i.MX8QM_DDR0_BUS_UTIL"
> > }
> > ]
> > ./perf stat -I 1000 -M imx8qm-ddr0-bus-util
> > # time counts unit events
> > 1.000104250 16720 imx8_ddr0/read-cycles/
> # 22921.0 imx8qm-ddr0-bus-util
> > 1.000104250 6201 imx8_ddr0/write-cycles/
> > 2.000525625 8316 imx8_ddr0/read-cycles/
> # 12785.5 imx8qm-ddr0-bus-util
> > 2.000525625 2738 imx8_ddr0/write-cycles/
> > 3.000819125 1056 imx8_ddr0/read-cycles/
> # 4136.7 imx8qm-ddr0-bus-util
> > 3.000819125 303 imx8_ddr0/write-cycles/
> > 4.001103750 6260 imx8_ddr0/read-cycles/
> # 9149.8 imx8qm-ddr0-bus-util
> > 4.001103750 2317 imx8_ddr0/write-cycles/
> > 5.001392750 2084 imx8_ddr0/read-cycles/
> # 4516.0 imx8qm-ddr0-bus-util
> > 5.001392750 601 imx8_ddr0/write-cycles/
> >
> > Based on given metric expression, the sum coming correct for first
> > iteration while for rest, we won't see same addition result. But
> > in-case we have single event in metric expression, we are getting correct
> result as expected.
> >
> >
> > So, I try to look into this issue and understand the flow. From my
> > understanding, whenever we do calculation of metric expression we don't use
> exact count we are getting.
> > Basically we use mean value of each metric event in the calculation of metric
> expression.
> >
> > So, I take same example:
> >
> > Metric Event: imx8qm-ddr0-bus-util
> > MetricExpr": "( imx8_ddr0\\/read\\-cycles\\/ +
> imx8_ddr0\\/write\\-cycles\\/ )"
> >
> > command#: ./perf stat -I 1000 -M imx8qm-ddr0-bus-util
> >
> > # time counts unit events
> > 1.000104250 16720 imx8_ddr0/read-cycles/
> # 22921.0 imx8qm-ddr0-bus-util
> > 1.000104250 6201 imx8_ddr0/write-cycles/
> > 2.000525625 8316 imx8_ddr0/read-cycles/
> # 12785.5 imx8qm-ddr0-bus-util
> > 2.000525625 2738 imx8_ddr0/write-cycles/
> > 3.000819125 1056 imx8_ddr0/read-cycles/
> # 4136.7 imx8qm-ddr0-bus-util
> > 3.000819125 303 imx8_ddr0/write-cycles/
> > 4.001103750 6260 imx8_ddr0/read-cycles/
> # 9149.8 imx8qm-ddr0-bus-util
> > 4.001103750 2317 imx8_ddr0/write-cycles/
> > 5.001392750 2084 imx8_ddr0/read-cycles/
> # 4516.0 imx8qm-ddr0-bus-util
> > 5.001392750 601 imx8_ddr0/write-cycles/
> >
> > So, there is one function called 'update_stats' in file util/stat.c
> > where we do this calculation and updating stats->mean value. And this
> > mean value is what we are actually using in our metric expression calculation.
> >
> > We call this function in each iteration where we update stats->mean and
> stats->n for each event.
> > But one weird issue is, for very first event, stat->n is always 1 that
> > is why we are getting mean same as count.
> >
> > So this the reason why for single event we get exact aggregate of metric
> expression.
> > So doesn't matter how many events you have in your metric expression,
> > every time you take exact count for first one and normalized value for rest
> which is weird.
> >
> > According to update_stats function: We are updating mean as:
> >
> > stats->mean += delta / stats->n where, delta = val - stats->mean.
> >
> > If we take write-cycles here. Initially mean = 0 and n = 1.
> >
> > 1st iteration: n=1, write cycle : 6201 and mean = 6201 (Final agg
> > value: 16720 + 6201 = 22921) 2nd iteration: n=2, write cycles: 6201 +
> > (2738 - 6201)/2 = 4469.5 (Final aggr value: 8316 + 4469.5 = 12785.5)
> > 3rd iteration: n=3, write cycles: 4469.5 + (303 - 4469.5)/3 =
> > 3080.6667 (Final aggr value: 1056 + 3080.6667 = 4136.7)
> >
> > I am not sure if its expected behavior. I mean shouldn't we either
> > take mean value of each event or take n as 1 for each event.
> >
> >
> > I am thinking, Should we add an option to say whether user want exact
> > aggregate or this normalize aggregate to remove the confusion? I try to find
> it out if we already have one but didn't get.
> > Please let me know if my understanding is fine. Or something I can add to
> resolve this issue.
> >
> > Thanks,
> > Kajol
> >
>
> Since you use the interval mode, can this commit fix the issue?
>
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Hi Andi, Jin Yao
Patch can fix this issue, thanks.
Best Regards,
Joakim Zhang
> Thanks
> Jin Yao