WHAT ARE FLAME GRAPHS AND HOW TO READ THEM

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

LOOKS • Flame Graphs sort stacks alphabetically • from the bottom frame upwards • increases merging and visualizes code paths

EXAMPLE Q: Which function is on CPU the most?

EXAMPLE A: f() top edge shows who is on CPU directly

EXAMPLE Q: Why is f() on CPU?

EXAMPLE A: a() → b() → c() → e() → f() f() was called by e(), e() was called by c(),…

EXAMPLE Q: How does b() compare to g()?

EXAMPLE A: b() looks like it is running (present) about 10 times more often than g()

EXAMPLE Q: Why are we running f()?

EXAMPLE A: code path branches can reveal key functions:
 a() choose the b() path
 c() choose the e() path look for branches

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