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SubjectRE: [RFC] Issue in final aggregate value, in case of multiple events present in metric expression
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Hi Arnaldo,

Kajol reflects this issue for almost two months, got no feedbacks, do you have any comments? That could be appreciated if you can look into it. Thanks a lot!

Please refer to below link:
https://www.spinics.net/lists/linux-perf-users/msg11011.html

Best Regards,
Joakim Zhang

> -----Original Message-----
> From: kajoljain <kjain@linux.ibm.com>
> Sent: 2020年3月24日 16:01
> To: Joakim Zhang <qiangqing.zhang@nxp.com>; acme@kernel.org; Jiri Olsa
> <jolsa@kernel.org>; Andi Kleen <ak@linux.intel.com>
> Cc: linux-kernel@vger.kernel.org; linux-perf-users@vger.kernel.org; Kan Liang
> <kan.liang@linux.intel.com>; Madhavan Srinivasan
> <maddy@linux.vnet.ibm.com>; Anju T Sudhakar <anju@linux.vnet.ibm.com>;
> Ravi Bangoria <ravi.bangoria@linux.ibm.com>
> Subject: [RFC] Issue in final aggregate value, in case of multiple events present
> in metric expression
>
> 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

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