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

[kajoljain]

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