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Notes of pprof

Jun 30, 2024, 6 min read

tags : Systems, Golang

A lot of things are just copypasting information from other sources and organizing them in the way I understand.

Homepage | Official Documentation

When I was reading about pprof for the first time, I wanted to understand two things; what is it? and how to understand these graphs. I think I finally have some hold of both now, so here is my understanding of pprof in form of a blogpost in the simplest way. If you find any mistake, please feel free to comment in this issue. After reading this you should be able to make some sense of that graph and the options that we use in pprof should make more sense. But the official documantations are pretty great too!

First and foremost it is not one single thing, but a collection of tools and packages. It is tightly coupled with the go programming language and is widely used for profiling go programs. This is how I would breakdown pprof for my understanding:

  • go tool pprof: A tool visualization/analysing profile data. Commonly refered to as pprof, I’ll call this go tool pprof throughout the documentation just to avoid any confusion.
  • runtime/pprof: A package that writes runtime profiling data in the format expected by go tool pprof.
  • net/http/pprof: A package that allows any program running a HTTP sever to serve profiling data in the format expected by go tool pprof
  • profile.proto: It is the data representation for profile data, It’s a protocol buffer description. This is the format expected by go tool pprof

Folowing sections give a little explanation to each of these and example to play with aswell!

Profiling

Before we start, It’s important to understand what profiling really is. Profiling refers to the collection of data that can reflect the execution status of the program during the execution of the program. In software engineering, performance analysis (also known as profiling) is a method of analyzing program behavior by collecting program run-time information. It is a method of dynamic program analysis. It is very useful for identifying expensive or frequently called sections of code.

Examples of profiling tools: gperftools, Linux perf tools etc, Interesting to know that pprof can work with perf.data aswell

Profiling with pprof

In case of pprof, each profile data is a collection of samples and is of the format profile.proto. The golang packages runtime/pprof and net/http/pprof helps in generating profile data and the pprof tool can generates reports out of the profile data to further visualize and analyze the data. The profile data can be read from a local file, or over http. Multiple profiles of the same type can be aggregated or compared, the pprof tool makes this very convnient.

profile.proto

profile.proto is a Protocol Buffer v3 description file. It is the format that profile data uses. On disk, it is represented as a gzip-compressed protocol buffer. Eg. profile.pb.gz

Profile data represent data collected through statistical sampling of our programs; we call these samples. Each sample is associated to a point in a location hierarchy/call stack, one or more numeric values, and a set of labels. It is independent of the type of data being collected and the sampling process used to collect that data.

This allows custom pprof profiles. Additionally to what is provided by the runtime, Go users can create their custom profiles via pprof.Profile and use the existing tools to examine them.

go tool pprof

go tool pprof primarily does reporting by display the profile data in multiple forms, such as identifying hottest locations, building graphical call graphs or trees, etc which the user can further analyse to make their program work better or to identify bottlenecks.

It is primarily used in three ways:

  • Report generation: pprof <format> [options] source
  • Interactive terminal: pprof [options] source
  • Web interface: pprof -http[host]:[port] [options] source=

So the callgraph that I mentioned in the start of this post is just one of the many ways go tool pprof can visualize profile data. It probably uses Graphviz/=gv= and its DOT format for it, It also uses D3 for some other visualizations. In addition, it also supports exporting these visualizations into various common formats such as png, gif, pdf etc. The interpretation of the graph is very nicely explained here. Reading this was like switching on the light in a dark room. 💯

The help command to show all options is go tool pprof -h, I think the same options are described much better in the README

Aside on flat and cum

When using the top command/view in pprof there are few words that initially confused me; namely flat, cum and sum.

  • flat: the value of the location itself.
  • cum: the value of the location plus all its descendants.
  • sum: sum is just this line and all lines before this one combined.
b(){
  c() // takes 1s
  do something directly; // takes 3s
  d() // takes 2s
}

cum of b() would be 6s, and flat of b() would be 3s and the sum% will just say how much basically has been covered up to now.

Golang libraries

There are two packages that help profile your go programs:

  • Runtime Profiling : Usually for non-HTTP applications/libbraries etc runtime/pprof
  • Net Profiling: Good for live applications or applications that are already using a http server net/http/pprof

pprof and production

  • The profiler, for the most part, has very minimal CPU overhead. The execution tracer can have a small overhead, but still small enough that we happily run it in production for a few seconds each hour.
  • Your pprof is showing
  • The official debugging doc also has some notes on running pprof in prod.

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