[ruby-core:99209] [Ruby master Bug#17031] `Kernel#caller_locations(m, n)` should be optimized

[merch-redmine@jeremyevans.net]

Issue #17031 has been updated by jeremyevans0 (Jeremy Evans).


Reviewing the related code in vm_backtrace.c, you are correct.  This occurs both for `caller` and `caller_locations`.  The entire internal backtrace object is generated by `rb_ec_backtrace_object`, and then passed to a function that looks at specific parts of it to generate the strings or Thread::Backtrace::Location objects.  To fix this would require changing the logic so that `rb_ec_backtrace_object` was passed the starting level and number of frames.

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Bug #17031: `Kernel#caller_locations(m, n)` should be optimized
https://bugs.ruby-lang.org/issues/17031#change-86589

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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`Kernel#caller_locations(1, 1)` currently appears to needlessly allocate memory for the whole backtrace.

It allocates ~20kB for a 800-deep stacktrace, vs 1.6 kB for a shallow backtrace.
It is also much slower for long stacktraces: about 7x slower for a 800-deep backtrace than for a shallow one.

Test used:
```ruby
def do_something
  location = caller_locations(1, 1).first
end

def test(depth, trigger)
  do_something if depth == trigger

  test(depth - 1, trigger) unless depth == 0
end

require 'benchmark/ips'

Benchmark.ips do |x|
  x.report (:short_backtrace    )    {test(800,800)}
  x.report (:long_backtrace     )    {test(800,  0)}
  x.report (:no_caller_locations)    {test(800, -1)}
end

require 'memory_profiler'

MemoryProfiler.report { test(800,800) }.pretty_print(scale_bytes: true, detailed_report: false)
MemoryProfiler.report { test(800,  0) }.pretty_print(scale_bytes: true, detailed_report: false)
```

Found when checking memory usage on RuboCop.




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