runtime/pprof: NewCPUProfile + cpuProfile.Start to allow profile configuration

[rogerlucena]

I think SetCPUProfileRate is, albeit confusing, still functioning. It needs to be called in the right order with StartCPUProfile. Last time I looked (~a month ago), it probably prints some warnings, but it does change the rate.

The pprof package doesn't seem to have a way to set the profiling rate. Not sure if this is really a useful feature.

Originally posted by @cherrymui in #40094 (comment)

The pprof package indeed does not allow us to customize the CPU profiling rate at the moment. This was something critical for my application in google/badwolf#155, since the default profiling rate of 100 was extremely low and did not allow me no see anything in my CPU profile.

To work around this, I had to make a call to runtime.SetCPUProfileRate(800) (with a rate of around 800 hz to see something meaningful and helpful in my CPU profile) before calling pprof.StartCPUProfile. Since there is a hard-coded call to runtime.SetCPUProfileRate(100) inside the pprof.StartCPUProfile, a warning is being printed to my terminal at the moment saying "runtime: cannot set cpu profile rate until previous profile has finished" as this call to runtime.SetCPUProfileRate(100) failed (the boolean cpuprof.on was already set to true in the previous call to runtime.SetCPUProfileRate(800)). This warning is annoying since it is misleading for someone who does not know the internals of pprof.StartCPUProfile, but the important here is that the CPU profiling is done with the first profiling rate that I manually set (800, as I wanted and as wanted from the user point of view).

Given this, what I am trying to say is that the public/exported method runtime.SetCPUProfileRate is still useful, and sometimes even critical for allowing us to use well pprof and to see something relevant on our profiles, depending on the application. I am not alone on this as you can see here (stack overflow) and here (google groups discussion). Even one principal software engineer (L8) I have been working with at Google found it useful when reviewing my PR google/badwolf#155 above and copied the code to use it in one of his applications, on which setting a higher profiling rate was critical as well.

That said, I believe we could, from now on, follow three different paths:

1. The ideal would be for the pprof.StartCPUProfile method already receive as argument the CPU profile rate we want, allowing us to customize it, with this argument having its default value set to 100 if it was not specified by the function call. This would be the ideal, if we had support for default arguments in Go. But, we can achieve something similar if we had a similar method with signature pprof.StartCPUProfileWithSpecifiedRate(w io.Writer, hz int), in addition to the pprof.StartCPUProfile(w io.Writer) method we already have. This way, the runtime.SetCPUProfileRate method would not necessarily need to be an exported method anymore.

2. We could keep runtime.SetCPUProfileRate public/exported, but we would have to fix the problem with the misleading warning message "runtime: cannot set cpu profile rate until previous profile has finished" if we had already manually set the profiling rate before the call to pprof.StartCPUProfile. For this we could:

   2.1. Have an auxiliar boolean method runtime.isCPUProfileRateAlreadySet, and call it inside pprof.StartCPUProfile before the hard-coded call to runtime.SetCPUProfileRate(100) there, trying to set the profiling rate to 100 only if this rate was not already previously set. This way, we would not see any strange warning after a call to pprof.StartCPUProfile if we had already previously set the profiling rate manually. I like this solution because it makes minimal changes on your code, it does not change any signature and it resolves exactly the problem we are trying to solve here, while keeping the warning message for other calls to runtime.SetCPUProfileRate that were not from pprof.StartCPUProfile in the situation above.

   2.2. Make runtime.SetCPUProfileRate return an error in the case the profiling rate was already previously set, allowing the caller to handle it and decide what to do from then on (inside pprof.StartCPUProfile we could ignore this error for example). This would change the signature of runtime.SetCPUProfileRate.

3. We could move the SetCPUProfileRate public signature to the pprof package (with some changes as explained above to avoid the misleading warning message), and deprecate it / make it unexported inside the runtime package.

These were just some of the alternatives that I thought about for now. I would like to hear more from you on what you think about them and if you have any other suggestions for fixing what I explained above.

[gopherbot]

Change https://golang.org/cl/269437 mentions this issue: runtime/pprof: method StartCPUProfile adds a parameter to allow custom cpu rate.

[imxyb]

so we can add method StartCPUProfileWithSpecifiedRate for this issue? @ianlancetaylor

[ianlancetaylor]

I don't think that is the best name, but, yes, something along those lines could work.

[imxyb]

ok, let me think about it

[gopherbot]

Change https://golang.org/cl/269537 mentions this issue: runtime/pprof: method StartCPUProfile adds a parameter to allow custom cpu rate.