stuck provides a visualization for quickly identifying common bottlenecks in running, asynchronous, and concurrent applications.

It is not a replacement for other profiling tools. stuck is very good at one thing, and nothing else. Specifically, stuck shows you, roughly, the deepest stack frame that most threads spend most of their time in. And it does so over time, so that you can see how the hotspots of your program changes as execution progresses. Since that is the one thing stuck does, it's worth visiting in a bit more detail.

Stuck samples the stack of all threads in your application periodically, and every so often it does the following:

1. For each thread, find the stack frame that was most often on the thread's stack. Ties are broken by the depth of the stack frame.
2. For each such stack frame, note down the stack up to and including that frame.
3. Group the data from all the threads by that stack.
4. Order the groups by the number of samples that were collectively taken for the group's stack across all threads.

It then displays these groups, along with their shared stacks and how many threads were a part of each one. What does it look like?

How do you run it?

First, install stuck:

$ cargo install stuck

Then, run a program of some sort that you want to benchmark.

Then, use bpftrace to profile that application. A low sampling rate (hz:1) is probably fine, but adjust as you see fit (e.g., hz:99). Pipe the result into stuck.

$ pid=$(pgrep my_program)
$ sudo env BPFTRACE_NO_CPP_DEMANGLE=1 bpftrace -e 'profile:hz:1 /pid == '"$pid"'/ { printf("%ld %d %s\n", elapsed, tid, ustack) }' | stuck

That's it!

Profiling Rust programs

First, you probably want to enable debug symbols in release mode, so that you don't just end up with unhelpful addresses in your output.

[profile.release]
debug=true

Second, bpftrace requires that your program is compiled with frame pointers. Rust does not do so by default, so you have to set a Rust compiler flag to make it work correctly:

$ env RUSTFLAGS='-Cforce-frame-pointers=yes' cargo build --release

(If you're a gcc/clang person, use -fno-omit-frame-pointer)

Development status

Very much experimental. Probably broken in several.