Performance monitoring with perf

Question

Disclaimer: I am new to perf and still trying to learn the ins/outs.

I have an executable that is running on my target system running Linux. I would like to use perf in order to profile/monitor its performance over time. For arguments sake I am trying to prove that my CPU utilization that is currently measured from top and collectD can be replaced by monitoring it via perf which will result in much more granular data.

Since we are trying to get a plot over time I have been using perf record -e cycles -p <pid>. Afterwards I can get the data to display via perf report.

Questions:

1. Just displaying the data with perf report shows me a summary of the entire data set that I took correct?
2. If I were to run perf report -D I get a dump of all data. (Just as a side question the timestamp is uptime in ns correct?) Now I would assume that sample is based on the frequency that could be set in perf record correct? I have run into issues by taking the time delta of the timestamp and it appears to be recorded at a random interval.
3. Once I dump the data there is nothing in here that really shouts out "this is your count!!" So the assumption was that the "period" field from the dump is the raw count. Is this true? Meaning that if period = 100, I could assume that for that interval, my program used 100 cycles? Additionally, I am starting to get the feel that this is for not just the application but for each library or kernel call that the program makes. I.e. if a malloc is called a different event will be record outlining that calls cycles taken. So overall how can I derive duration or event + the number of cycles from the event + which event it actually was from this field to get a true measure of CPU utilization?

IF this application of perf is not what it was intended to do then I will also like to know why not? Additionally, I think this same type of analysis would be useful for all of the other types of statistics since you can then pinpoint in time when an anomaly occurred in your running code. Just for reference I am running perf against top collecting at 1s. I am doing this since I want to compare top output to perf output. Any insight would be helpful since as I said I am still learning and new to this powerful tool.

(Linux Kernel version: 3.10.82)

Answer 1

ANSWER #1

Yes mostly. perf report does show you a summary of the trace collected. Samples collected by perf record are saved into a binary file called, by default, perf.data. The perf report command reads this file and generates a concise execution profile. By default, samples are sorted by functions with the most samples first. However, you can do much more detailed profiling also using this report.

ANSWER #2

You should ideally use perf script -D to get a trace of all data. The timestamp is in microseconds. Although, in kernels newer than the one you specify, with the help of a command line switch (-ns) you can display the time in nanoseconds as well. Here is the source -

Timestamp

It is quite hard to tell this without looking at what kind of "deltas" are you getting. Remember the period of collecting samples is usually tuned. There are two ways of specifying the rate at which to collect samples --

You can use the perf record (--c for count) to specify the period at which to collect samples. This will mean that for every c occurrences of the event that you are measuring, you will have a sample for that. You can then modify the sampling period and test various values. This means that at every two occurences of the event for which you are measuring, the counter will overflow and you will record a sample.

The other way around to express the sampling period, is to specify the average rate of samples per second (frequency) - which you can do using perf record -F. So perf record -F 1000 will record around 1000 samples per second and these samples will be generated when the hardware/PMU counter corresponding to the event overflows. This means that the kernel will dynamically adjust the sampling period. And you will get sample times at different random moments.

You can see for yourself in code here:

How perf dynamically updates time

ANSWER #3

Why not ? Ideally you should get the number of event samples collected if you do a perf report and just do a deeper analysis. Also when you do a perf record and finish recording samples, you would get a notification on the command line about the number of samples collected corresponding to the event you measured. (This may not be available in the kernel module you use, I would suggest you switch to a newer linux version if possible!). The number of samples should be the raw count - not the period.

If your period is 100 - it means that for the whole duration of the trace, perf recorded every 100th event. That means, if a total of 1000 events happened for the trace duration, perf approximately collected event 1, 100, 200, 300...1000.

Yes the samples recorded are not only from the application. In fact, you can use switches like this : perf record -e <event-name:u> or <event-name:k> (u for userspace and k for kernel) to record events. Additionally perf records samples from shared libraries as well. (Please consult the perf man-page for more details).

As I said previously, perf report should be an ideal tool to calculate the number of samples of event cycles recorded by perf. The number of events collected/recorded is not exact because it is simply not possible for hardware to record all cycle events. This is because recording and preparing details of all the events require the kernel to maintain a ring buffer which gets written to periodically as and when the counter overflows. This writing to the buffer happens via interrupts. They take up a fraction of CPU time- this time is lost and could have been used to record events which will now be lost as the CPU was busy servicing interrupts. You can get a really great estimate by perf even then, though.

CONCLUSION

perf does especially what it intends to do given the limitations of hardware resources we have at hand currently. I would suggest going through the man-pages for each command to understand better.

QUESTIONS

2. I assume you are looking at perf report. I also assume you are talking about the overhead % in perf report. Theoretically, it can be considered to be an arrangement of data from the highest to least occurrence as you specified. But, there are many underlying details that you need to consider and understand to properly make sense of the output. It represents which function has the most overhead (in terms of the number of events that occurred in that function ). There is also a parent-child relationship, based on which function calls which function, between all the functions and their overheads. Please use the Perf Report link to understand more.

3. As you know already events are being sampled, not counted. So you cannot accurately get the number of events, but you will get the number of samples and based on the tuned frequency of collecting samples, you will also get the raw count of the number of events ( Everything should be available to you with the perf report output ).