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Date: Mon, 11 May 2020 09:12:23 +0800 From: "Jin, Yao" <yao.jin@...ux.intel.com> To: kajoljain <kjain@...ux.ibm.com>, Joakim Zhang <qiangqing.zhang@....com>, "acme@...nel.org" <acme@...nel.org>, Jiri Olsa <jolsa@...nel.org>, Andi Kleen <ak@...ux.intel.com> Cc: "linuxkernel@...r.kernel.org" <linuxkernel@...r.kernel.org>, "linuxperfusers@...r.kernel.org" <linuxperfusers@...r.kernel.org>, Kan Liang <kan.liang@...ux.intel.com>, Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>, Anju T Sudhakar <anju@...ux.vnet.ibm.com>, Ravi Bangoria <ravi.bangoria@...ux.ibm.com> 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 incase 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": "imx8qmddr0busutil", > "MetricExpr": "( imx8_ddr0\\/read\\cycles\\/ + imx8_ddr0\\/write\\cycles\\/ )", > "MetricGroup": "i.MX8QM_DDR0_BUS_UTIL" > } > ] > ./perf stat I 1000 M imx8qmddr0busutil > # time counts unit events > 1.000104250 16720 imx8_ddr0/readcycles/ # 22921.0 imx8qmddr0busutil > 1.000104250 6201 imx8_ddr0/writecycles/ > 2.000525625 8316 imx8_ddr0/readcycles/ # 12785.5 imx8qmddr0busutil > 2.000525625 2738 imx8_ddr0/writecycles/ > 3.000819125 1056 imx8_ddr0/readcycles/ # 4136.7 imx8qmddr0busutil > 3.000819125 303 imx8_ddr0/writecycles/ > 4.001103750 6260 imx8_ddr0/readcycles/ # 9149.8 imx8qmddr0busutil > 4.001103750 2317 imx8_ddr0/writecycles/ > 5.001392750 2084 imx8_ddr0/readcycles/ # 4516.0 imx8qmddr0busutil > 5.001392750 601 imx8_ddr0/writecycles/ > > Based on given metric expression, the sum coming correct for first iteration while for > rest, we won't see same addition result. But incase 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: imx8qmddr0busutil > MetricExpr": "( imx8_ddr0\\/read\\cycles\\/ + imx8_ddr0\\/write\\cycles\\/ )" > > command#: ./perf stat I 1000 M imx8qmddr0busutil > > # time counts unit events > 1.000104250 16720 imx8_ddr0/readcycles/ # 22921.0 imx8qmddr0busutil > 1.000104250 6201 imx8_ddr0/writecycles/ > 2.000525625 8316 imx8_ddr0/readcycles/ # 12785.5 imx8qmddr0busutil > 2.000525625 2738 imx8_ddr0/writecycles/ > 3.000819125 1056 imx8_ddr0/readcycles/ # 4136.7 imx8qmddr0busutil > 3.000819125 303 imx8_ddr0/writecycles/ > 4.001103750 6260 imx8_ddr0/readcycles/ # 9149.8 imx8qmddr0busutil > 4.001103750 2317 imx8_ddr0/writecycles/ > 5.001392750 2084 imx8_ddr0/readcycles/ # 4516.0 imx8qmddr0busutil > 5.001392750 601 imx8_ddr0/writecycles/ > > 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 writecycles 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? http://lore.kernel.org/lkml/20200420145417.68641yao.jin@linux.intel.com Thanks Jin Yao
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