Support Gecko format?

[byroot]

I just discovered the very nice Firefox profiler https://share.firefox.dev/3j3PJoK

The format seem relatively simple and it even support multi-threading: https://github.com/firefox-devtools/profiler/blob/fb5c866af5776e4fec760f4ca39b31848a10706b/docs-developer/gecko-profile-format.md

I'm creating this issue as a bit of a remainder, I'd like to try to add support for it when I find the time.

[tenderlove]

I've started adding support here.

I've currently got some pretty hacky code that will convert Stackprof profiles to Gecko format, but I think we should change the internal logging format to be something more close to Gecko format. The way Gecko stores samples is much more compact than the way we do it, and I think if we change our internal format to be more close to Gecko, then we could support flame graphs "out of the box" without asking the user to do any kind of "raw mode".

[tenderlove]

Actually here is a profile I generated. It's the boot process of a very simple Rails application. The translation script I used is here.

I'll clean it up a little and then send a PR.

One issue is that the profile I linked to won't show you the source code for anything. We can make it show you the source by changing the file names to particular urls. For example, if a file comes from Zeitwork, and it's like this:

/Users/aaron/.rubies/arm64/ruby-trunk/lib/ruby/gems/3.3.0+0/gems/zeitwerk-2.6.7/lib/zeitwerk/gem_loader.rb

If we change the path to this:

git:github.com/fxn/zeitwerk:lib/zeitwerk/loader.rb:v2.6.7

Then the profiler will understand to fetch the source from GitHub. I don't think Stackprof should do that type of translation by default, but I do think we should have a type of "sourcemap" post-processing function in Stackprof. So for example you could download a profile from production, then locally translate the filenames to GitHub urls or whatever.

Second, I think we need to mark Ruby frames as JS frames that way we can filter out native frames from the profile. Probably devs aren't going to change Array#each and it it would be useful to remove those from the profile. I'll tackle this point (it should be easy).