Issue #19315 has been updated by himura467 (Akito Shitara). mame (Yusuke Endoh) wrote in #note-43:
Please provide a quantitative evaluation of performance and compatibility at the application level. Does this change bring an observable performance improvement? Does memory usage increase or decrease? It would also be useful to observe how often `RSTRING_PTR` is actually called without the GVL in practice, for example by inserting `rb_bug` into that code path and running test suites. Evaluating with yjit-bench might be a good idea.
Thanks, I'll share the results. Eregon (Benoit Daloze) wrote in #note-44:
mame (Yusuke Endoh) wrote in #note-43:
By the way, this did not come up during the meeting, but I think the fact that `RSTRING_PTR` can now trigger GC could itself be a serious compatibility problem. There is probably a fair amount of code written under the assumption that `RSTRING_PTR` never triggers GC, and such code may start to fail with very low probability, depending on subtle behaviors of the conservative GC. Evaluating this risk is very difficult.
Regarding that, I think there is no need for `RSTRING_PTR` to trigger GC, so the implementation could make sure it never does.
Makes sense. I'll work on updating the implementation with these in mind. ---------------------------------------- Feature #19315: Lazy substrings in CRuby https://bugs.ruby-lang.org/issues/19315#change-117621 * Author: Eregon (Benoit Daloze) * Status: Open ---------------------------------------- CRuby should implement lazy substrings, i.e., "abcdef"[1..3] must not copy bytes. Currently CRuby only reuse the char* if the substring is until the end of the buffer. But it should also work wherever the substring starts and ends. Yes, it means RSTRING_PTR() might need to allocate to \0-terminate, so be it, it's worth it. There is already code for this (`SHARABLE_MIDDLE_SUBSTRING`), but it's disabled by default and `RSTRING_PTR()` needs to be changed to deal with this. It seems a good idea to introduce a variant of `RSTRING_PTR` which doesn't guarantee \0-termination, so such callers can then use the existing bytes always without copy. There are countless workarounds for this missing optimization, all not worth it with lazy substring and all less readable: * https://bugs.ruby-lang.org/issues/19314 * https://bugs.ruby-lang.org/issues/18598#note-3 * https://github.com/ruby/net-protocol/pull/14 * Manual lazy substrings which track string + index + length * More but I don't remember all now, feel free to comment or link more urls/tickets. -- https://bugs.ruby-lang.org/