Add ability to trace exit locations for YJIT
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
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
Given the following script, we write to a file called
concat_array.dump the results of
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
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
./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
--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.
- 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
- Stackprof is a requirement to to view the backtrace information from
the dump file
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