What Are Flame Graphs and How To Read Them

Short tech talk I did for my coworkers on flame graphs.

Further read:
- Blazing Performance with Flame Graphs (video): https://www.usenix.org/conference/lisa13/technical-sessions/plenary/gregg
- FlameGraph: https://github.com/brendangregg/FlameGraph
- ruby-prof: https://github.com/ruby-prof/ruby-prof
- ruby-prof-flamegraph: https://github.com/oozou/ruby-prof-flamegraph

Transcript

1. WHAT ARE FLAME GRAPHS AND HOW TO READ THEM
2. DEFINITION • each box represents a function (a merged stack frame) • y-axis shows stack depth • top function led directly to the profiling event • everything beneath it is ancestry (explains why) • x-axis spans the sample population, sorted alphabetically • box width is proportional to the total time a function was profiled directly or its children were profiled • all threads can be shown in the same Flame Graph (the default), or as separate per-thread Flame Graphs • flame graphs can be interactive: mouse over for details
3. LOOKS • Flame Graphs sort stacks alphabetically • from the bottom frame upwards • increases merging and visualizes code paths
4. EXAMPLE Q: Which function is on CPU the most?
5. EXAMPLE A: f() top edge shows who is on CPU directly
6. EXAMPLE Q: Why is f() on CPU?
7. EXAMPLE A: a() → b() → c() → e() → f() f() was called by e(), e() was called by c(),…
8. EXAMPLE Q: How does b() compare to g()?
9. EXAMPLE A: b() looks like it is running (present) about 10 times more often than g()
10. EXAMPLE Q: Why are we running f()?
11. EXAMPLE A: code path branches can reveal key functions:
 a() choose the b() path
 c() choose the e() path look for branches
12. FURTHER READ • FlameGraph: https://github.com/ brendangregg/FlameGraph • ruby-prof: https://github.com/ruby-prof/ ruby-prof • ruby-prof-flamegraph: https://github.com/ oozou/ruby-prof-flamegraph