rb_ractor_main_p() need to access to the ractor pointer in TLS.
However it is slow operation so that we need to skip this check
if it is not multi-ractor mode (!ruby_multi_ractor).
This performance regression is pointed at
https://bugs.ruby-lang.org/issues/17100#note-27
This commit introduces Ractor mechanism to run Ruby program in
parallel. See doc/ractor.md for more details about Ractor.
See ticket [Feature #17100] to see the implementation details
and discussions.
[Feature #17100]
This commit does not complete the implementation. You can find
many bugs on using Ractor. Also the specification will be changed
so that this feature is experimental. You will see a warning when
you make the first Ractor with `Ractor.new`.
I hope this feature can help programmers from thread-safety issues.
