stackprof: rb_gc_latest_gc_info not implemented (RuntimeError)

[deepj]

To reproduce:

gem install stackprof
ruby -rstackprof

Error

ruby.c:2974:in `rb_tr_error': rb_gc_latest_gc_info not implemented (RuntimeError)
	from ruby.c:4086:in `rb_gc_latest_gc_info'
	from ~/.gem/truffleruby/2.6.5/gems/stackprof-0.2.15/ext/stackprof/stackprof.c:733:in `Init_stackprof'
	from ~/.rubies/truffleruby-dev/lib/mri/rubygems/core_ext/kernel_require.rb:54:in `require'
	from ~/.rubies/truffleruby-dev/lib/mri/rubygems/core_ext/kernel_require.rb:54:in `require'
	from ~/.gem/truffleruby/2.6.5/gems/stackprof-0.2.15/lib/stackprof.rb:1:in `<top (required)>'
	from ~/.rubies/truffleruby-dev/lib/mri/rubygems/core_ext/kernel_require.rb:130:in `require'
	from ~/.rubies/truffleruby-dev/lib/mri/rubygems/core_ext/kernel_require.rb:130:in `require'
	from ~/.rubies/truffleruby-dev/bin/irb:1:in `call'
	from ~/.rubies/truffleruby-dev/bin/irb:1:in `require'
	from ~/.rubies/truffleruby-dev/bin/irb:1:in `<main>'

[eregon]

I'm not sure how realistic it is to try to run stackprof, it seems to use rb_profile_frames() for sampling.
We have a builtin --cpusampler that is likely always going to be far more efficient and should be a good replacement.

So I'd like to close this, because it doesn't seem worth the effort to support rb_profile_frames() if it's inefficient in the end anyway.

If you want a compatible API with StackProf then we could consider adding a TruffleRuby backend in that gem. Please open a separate issue if you want that.

[deepj]

@eregon Hm, my point is more about not implemented rb_gc_latest_gc_info then about the gem itself.

[eregon]

Maybe we could stub that one (it seems to just return a Symbol), but using stackprof would fail anyway at runtime. I'd rather we focus on compatibility with gems we could run, stackprof seems rather unlikely to be useful to run on TruffleRuby as the overhead of profiling that way seems high.

[deepj]

@eregon If I'm not mistaken it's one of flamegraph gem's dependencies. That's why I guess tested this gem. Anyway, I still think it'd be worth to implement this method.

[eregon]

OK, implementing rb_gc_latest_gc_info(:state) as e.g. always returning :none seems easy:
https://github.com/ruby/ruby/blob/fbd7f08e922cdf1a4be7a71aa056659971c81dab/gc.c#L8862-L8863

I don't see it as a dependency in https://github.com/SamSaffron/flamegraph/blob/master/flamegraph.gemspec

[bjfish]

This is now implemented at: 26bac34