Issue #18812 has been updated by k0kubun (Takashi Kokubun). Status changed from Open to Closed https://github.com/ruby/ruby/pull/5970 has already been merged. ---------------------------------------- Feature #18812: Add ability to trace exit locations for YJIT https://bugs.ruby-lang.org/issues/18812#change-104370 * Author: eileencodes (Eileen Uchitelle) * Status: Closed * Priority: Normal ---------------------------------------- Currently, when running yjit with `--yjit-stats` you are able to see method call exit reasons and the top 20 most frequent exits. This is useful to know where to spend time investigating whether an exit should be fixed, but in a larger codebase like Rails, it's next to impossible to know what the Ruby code that is exiting looks like. Aaron Patterson and I aim to fix that with the addition of `--yjit-trace-exits` option and feature. When running with `--yjit-stats` turned on Ruby code can inform the user what the most common exits are. While this is useful information it doesn't tell you the source location of the code that exited or what the code that exited looks like. This change intends to fix that. To use the feature, run yjit with `--yjit-stats` and `--yjit-trace-exits`, which will record the backtrace for every exit that occurs. Users must save the output of `RubyVM::YJIT.exit_locations` to a dump file. That file can then be read by StackProf to see the code that exited and the reason. *Example usage:* Given the following script, we write to a file called `concat_array.dump` the results of `RubyVM::YJIT.exit_locations`. ```ruby def concat_array ["t", "r", *x = "u", "e"].join end 1000.times do concat_array end File.write("concat_array.dump", Marshal.dump(RubyVM::YJIT.exit_locations)) ``` When we run the file with this branch and the appropriate flags the stacktrace will be recorded. Note Stackprof needs to be installed or you need to point to the library directly. ``` ./ruby --yjit --yjit-call-threshold=1 --yjit-stats --yjit-trace-exits -I/Users/eileencodes/open_source/stackprof/lib test.rb ``` We can then read the dump file with Stackprof: ``` ./ruby -I/Users/eileencodes/open_source/stackprof/lib/ /Users/eileencodes/open_source/stackprof/bin/stackprof --text concat_array.dump ``` Results will look similar to the following: ``` ================================== Mode: () Samples: 1817 (0.00% miss rate) GC: 0 (0.00%) ================================== TOTAL (pct) SAMPLES (pct) FRAME 1001 (55.1%) 1001 (55.1%) concatarray 335 (18.4%) 335 (18.4%) invokeblock 178 (9.8%) 178 (9.8%) send 140 (7.7%) 140 (7.7%) opt_getinlinecache ...etc... ``` Simply inspecting the `concatarray` method will give `SOURCE UNAVAILABLE` because the source is insns.def. ``` ./ruby -I/Users/eileencodes/open_source/stackprof/lib/ /Users/eileencodes/open_source/stackprof/bin/stackprof --text concat_array.dump --method concatarray ``` Result: ``` concatarray (nonexistent.def:1) samples: 1001 self (55.1%) / 1001 total (55.1%) callers: 1000 ( 99.9%) Object#concat_array 1 ( 0.1%) Gem.suffixes callees (0 total): code: SOURCE UNAVAILABLE ``` However if we go deeper to the callee we can see the exact source of the `concatarray` exit. ``` ./ruby -I/Users/eileencodes/open_source/stackprof/lib/ /Users/eileencodes/open_source/stackprof/bin/stackprof --text concat_array.dump --method Object#concat_array ``` ``` Object#concat_array (/Users/eileencodes/open_source/rust_ruby/test.rb:1) samples: 0 self (0.0%) / 1000 total (55.0%) callers: 1000 ( 100.0%) block in <main> callees (1000 total): 1000 ( 100.0%) concatarray code: | 1 | def concat_array 1000 (55.0%) | 2 | ["t", "r", *x = "u", "e"].join | 3 | end ``` The `--walk` option is recommended for this feature as it make it easier to traverse the tree of exits. *Goals of this feature:* This feature is meant to give more information when working on YJIT. The idea is that if we know what code is exiting we can decide what areas to prioritize when fixing exits. In some cases this means adding prioritizing avoiding certain exits in yjit. In more complex cases it might mean changing the Ruby code to be more performant when run with yjit. Ultimately the more information we have about what code is exiting AND why, the better we can make yjit. *Known limitations:* * Due to tracing exits, running this on large codebases like Rails can be quite slow. * On complex methods it can still be difficult to pinpoint the exact cause of an exit. * Stackprof is a requirement to to view the backtrace information from the dump file PR https://github.com/ruby/ruby/pull/5970 -- https://bugs.ruby-lang.org/