Hello, Thanks a lot for the detailed explanation. I could understand the working of operf and opreport.
Unlike operf, ocount counts each occurrence of the monitored event. In such a case, why ocount also gives varying values of CPU_CLK_UNHALTED even when the monitored code doesn't get changed. Is it because of background processes? If so, then can we get event count for only monitored command? Thank you, On Fri, Jun 16, 2017 at 4:52 PM, Michael Petlan <mpet...@redhat.com> wrote: > On Fri, 16 Jun 2017, dhara buch wrote: > > Hello, > > > > Many days ago you had helped me a lot in using Oprofile in Virtual > Machine. That time, I also had a query regarding > > getting information from 'opreport --symbols --debuginfo' command. > > Hello, > > yeah, I remember... > > > > > Actually, I am profiling a file1.c file with the following commands... > > > > 1. gcc -g file1.c -o file1 > > > > 2. operf ./file1 --events=CPU_CLK_UNHALTED > > > > 3. opreport --symbols --debuginfo > > > > the output contains various lines where one line look follows: > > > > samples % linenr info image name > symbol name > > > > 2 12.5000 rsaimp.c:16 rsaimp MM > > 2 12.5000 rsaimp.c:34 rsaimp > main > > > > When I execute commands 2 and 3 multiple times, each time I gwt the > output for symbole name MM (which is UDF in the > > program) but I don't get output for main every time. > > > This is kind of random, see below. > > > Additionally,'' the sample % also vary every time I file operf command. > I do not understand the working of operf that > > how it gives different outputs for the same program. > > > > How _counting_ works: > > Each time an event happens in CPU (in our case -- CPU_CLK_UNHALTED -- one > CPU tick), its counter is incremented by one. This way, you can get the > whole number of how many times the event happened: > > $ ocount ls > Events were actively counted for 1863267 nanoseconds. > Event counts (actual) for /bin/ls: > Event Count % time counted > CPU_CLK_UNHALTED 2,672,309 100.00 > > This means that there were 2.672 million CPU ticks necessary to execute > `ls`. > > The tool for counting is *ocount*. > > > How _profiling_ works: > > For profiling, events are counted the same way as when counting, but only > once per N event occurrences a "sample" is taken. A sample is a piece of > information about where in the code it happened, what was the context and > potentially call-stack, etc. This sample is saved into the oprofile_data > dir structure. When you finish profiling, opreport generates some overview > from the samples recorded. > > Of course, taking sample causes some overhead. Thus, you cannot take it > each time the event counter increments. For this purpose, there is "sample > rate" value which divides the event counter and takes a sample only once > per "sample rate" occurrences. Thus, it is pretty random, which event > occurrence is picked for a sample. > > Of course, your main() consumes CPU ticks and the counter is incremented, > but sometimes, one of these incrementations produces sample, sometimes not, > thus the function is missing. > > You have 2 samples on each of the line in the report. That is really a low > number, so it is very probable that sometimes the samples hit main() and > sometimes not. > > $ operf ls > Profiling done. > > $ opreport > Using /root/oprofile_data/samples/ for samples directory. > CPU: Core 2, speed 2267 MHz (estimated) > Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit > mask of 0x00 (Unhalted core cycles) count 100000 > CPU_CLK_UNHALT...| > samples| %| > ------------------ > --> 37 100.000 ls > CPU_CLK_UNHALT...| > samples| %| > ------------------ > 27 72.9730 kallsyms > 5 13.5135 libc-2.19.so > 2 5.4054 ld-2.19.so > 1 2.7027 ls > 1 2.7027 ext4 > 1 2.7027 libattr.so.1.1.0 > > You see that out of (estimated) 2.5-3.5 mil of event occurrences (which is > the scale `ls` usually needs on my PC), I got only 37 samples. > > The sample rate for each event are in `ophelp` as "min count". > > Why did I get 37 samples? My default setting of default event is: > > $ ophelp -d > CPU_CLK_UNHALTED:100000:0:1:1 > -------------------^^ > > This means that there is 1 sample per 100000 event occurrences. > > You can tweak this value a bit, but note that oprofile won't let you set > the value to be too low, due to the overhead reasons. > > Let's try 10000 (sampling 10 times more often): > > $ operf -e CPU_CLK_UNHALTED:10000 ls > WARNING: Lost samples detected! See /root/oprofile_data/samples/operf.log > for details. > Lowering the sampling rate may reduce or eliminate lost samples. > See the '--events' option description in the operf man page for help. > Profiling done. > > $ opreport > Using /root/oprofile_data/samples/ for samples directory. > WARNING: Lost samples detected! See /root/oprofile_data/samples/operf.log > for details. > CPU: Core 2, speed 2267 MHz (estimated) > Counted CPU_CLK_UNHALTED events (Clock cycles when not halted) with a unit > mask of 0x00 (Unhalted core cycles) count 10000 > CPU_CLK_UNHALT...| > samples| %| > ------------------ > 429 100.000 ls > CPU_CLK_UNHALT...| > samples| %| > ------------------ > 313 72.9604 kallsyms > 55 12.8205 ld-2.19.so > 34 7.9254 libc-2.19.so > 17 3.9627 ls > 8 1.8648 ext4 > 2 0.4662 binfmt_misc > > You see that both operf and opreport warned about lost samples which means > that the overhead in kernel was too high and some samples were lost. But > probably not that many, since you see that I got 429 samples instead of > 37 which is very roughly 10 times more. > > > Profiling is not for exact measurement of how many CPU ticks happened in > a function, it is rather designed for relative comparison across your code. > > If you are getting too few samples, try to decrease the sample rate as I > did in the example. Have a look at `ophelp` what is the lowest value for > your event. > > Also please note that the min sample rates are rough and depend on the > load, > so under low load, lower rates can be used without issues. When you profile > systemwide on all CPUs, it's very high chance that you'll run into an > overhead > and lose many samples... > > I hope I explained it a bit. > > Cheers, > Michael > > > Thanks, > > > > Dhara Buch > > > >
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