Profiling and creating call graphs for Go programs

August 8, 2013

In trying to get my head around the code of the very interesting GoFlow library, (for flow-based programming in Go), and the accompanying flow-based bioinformatics library I started hacking on, I needed to get some kind of visualization (like a call graph) … something like this:

Call graph

(And in the end, that is what I got … read on … ) :)

I then found out about the go tool pprof command, for which the Go team published a blog post on here .

Being a Go newbie, I must admit I had quite a hard time deciphering the blog post though. Maybe it was just a psychological barrier because of all the technological anechdotes, that made it look harder than it actually was. Anyhow, it didn’t help that go run pprof didn’t produce any output if I didn’t run processing on a large enough file that it would have time to collect data.

Anyways, with this in mind I wanted to make a slightly easier-to-follow instruction for newbies like me, on how to use go tool pprof for profiling and producing call graphs … but then after I published this post, Dave Cheney pinged me about his excellent profile package, which makes the process even easier, so I went away and updated the blog post to include how to do it both with the profile package, AND with the pprof library itself! :)

Ok, so enough blather, let’s get started:

Easy option: Profiling Go programs with the profile package

Easy option Overview: What you will do

First you will need to put a very small code snippet in your code, that will output a profile-file (named [something].pprof in your /tmp/ folder) when you run your program.
This profile file can then be used with the go run pprof command to do various things like output reports of top functions taking time, and not the least, producing a graphical call graph, which is what I was most interested in here.

Easy option Step 0: Install profile

If you have your GOROOT and GOPATH environment variables correctly setup, you should be able to install it with this simple command:

go get github.com/davecheney/profile

… otherwise you’ll have to install it manually from http://github.com/davecheney/profile

Easy option Step 1: Add a line of code, to produce a profile file

Add github.com/davecheney/profile to your import statement. Something like:

import (
        ... your other imports ...
        "github.com/davecheney/profile"
)

Add the following line at the beginning of your main() function:
```
defer profile.Start(profile.CPUProfile).Stop()
```

… the result should be something like:

func main() {
    defer profile.Start(profile.CPUProfile).Stop()

    // ... your main code here ...
}

Easy option Step 2: Build your program as usual

Something like:

go build [your program].go

Easy option, Step 3: Run your program long enough to get some profiling data

Now run your program as usual
- Note: Make sure it rung long enough to gather data! I had to run my DNA-processing code on a 58MB file rather than my 7.8KB test file I was using first, to get predictable results.
```
./[your program]
```

Easy option, Step 4: Copy the cpu.pprof file from /tmp/…

When running the program in the previous step, you will have seen some output like this:

2013/08/08 16:45:58 profile: cpu profiling enabled, /tmp/profile882806532/cpu.pprof

Copy this file to where you are standing right now, with:

cp /tmp/profile[some number]/cpu.pprof .

Read on from step 4 , for the rest of the steps …

Hard option: Using the pprof library directly

Hard option Overview: What you will do

First you will need to put in some code snippets (more than in the easy option) in your code, that will output a profile-file (named [something].pprof) when you run your program.
This profile file can then be used with the go run pprof command to do various things like output reports of top functions taking time, and not the least, producing a graphical call graph, which is what I was most interested in here.

Hard option Step 1: Add some code snippets to your code, to produce a profile file

Add runtime/pprof to your import statement. Something like:

import (
        ... your other imports ...
        "flag"
        "fmt"
        "runtime/pprof"
)

Add this just before your main() function:

// Profiling stuff ... from http://blog.golang.org/profiling-go-programs
var cpuprofile = flag.String("cpuprofile", "", "write cpu profile to file")

Add the following code at the beginning of your main() function:

flag.Parse()
if *cpuprofile != "" {
    f, err := os.Create(*cpuprofile)
    if err != nil {
        fmt.Println("Error: ", err)
    }
    pprof.StartCPUProfile(f)
    defer pprof.StopCPUProfile()
}

… the result should be something like:

// Profiling stuff ... from http://blog.golang.org/profiling-go-programs
var cpuprofile = flag.String("cpuprofile", "", "write cpu profile to file")

func main() {
    flag.Parse()
    if *cpuprofile != "" {
        f, err := os.Create(*cpuprofile)
        if err != nil {
            fmt.Println("Error: ", err)
        }
        pprof.StartCPUProfile(f)
        defer pprof.StopCPUProfile()
    }

       // ... your main code here ...
}

This will add a command line flag -cpuprofile, which you can later use to specify a filename where to write the profiling data.

Hard option Step 2: Build your program as usual

Something like:

go build [your program].go

Hard option Step 3: Run your program long enough to get some profiling data

Now run your program, specifying a filename to the -cpuprofile flag, where to store the profiling data
- Note: Make sure it rung long enough to gather data! I had to run my DNA-processing code on a 58MB file rather than my 7.8KB test file I was using first, to get predictable results.
```
./[your program] -cpuprofile=cpu.pprof
```

The rest (Same for easy and hard option!)

Step 4: Get some nice output from your profile data

Now comes the fun part, where you can do nice stuff with your profile data.

My favourite here was to output the callgraph as a PDF file:

go tool pprof --pdf [my program] cpu.pprof > callgraph.pdf

Step 5: Enjoy your callgraph

Let’s see what that looks like:

Call graph

Not too bad, no? (Find the PDF version below as well)

Step 6: Try some other commands

Some other output you might want to do:

A textual report:

go tool pprof --text [my program] cpu.pprof > report.txt

Check the outher options:
```
go tool pprof 2>&1|less
```

(Isn’t there a nicer way to get a paginated help screen?)

Step 7: Go back to the Go team blog post and learn the advanced options

Now, this might have been an easier start if you are a newbie, but then Dave Cheney’s post on profile, and the Go team blog post on runtime/pprof contains info on more advanced use of the pprof tool, so be sure to go back and study it: