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? http://lore.kernel.org/lkml/[email protected] Thanks Jin Yao
