Comment by edwintorok
10 months ago
You probably already know, but with OCaml 5 the only way to get flamegraphs working is to either:
* use framepointers [1]
* use LBR (but LBR has a limited depth, and may not work on on all CPUs, I'm assuming due to bugs in perf)
* implement some deep changes in how perf works to handle the 2 stacks in OCaml (I don't even know if this would be possible), or write/adapt some eBPF code to do it
OCaml 5 has a separate stack for OCaml code and C code, and although GDB can link them based on DWARF info, perf DWARF call-graphs cannot (https://github.com/ocaml/ocaml/issues/12563#issuecomment-193...)
If you need more evidence to keep it enabled in future releases, you can use OCaml 5 as an example (unfortunately there aren't many OCaml applications, so that may not carry too much weight on its own).
[1]: I haven't actually realised that Fedora39 has already enabled FP by default, nice! (I still do most of my day-to-day profiling on an ~CentOS 7 system with 'perf record --call-graph dwarf -F 47 -a', I was aware that there was a discussion to enable FP by default, but haven't noticed it has actually been done already)
No, LBR is an Intel-only feature.
https://www.phoronix.com/news/AMD-Zen-4-LbrExtV2 LBR is supposed to work on AMD too, except it doesn't. I'll have to open a bug report (it records the data, it just can't parse it afterwards)
Also, if you want LBR on Zen3 or earlier, you're SOL. I noticed that one the hard way.
Stuff like this is making me lean more towards getting an Intel for my next computer, at least on desktop where their worse power efficiency is less of an issue. But then they keep gating AVX-512 due to their E-cores not supporting it... You really can't win these days.