[ruby-core:113474] [Ruby master Feature#18258] Ractor.shareable? can be slow and mutates internal object flags.

14 May 2023

      Issue #18258 has been updated by jeremyevans0 (Jeremy Evans).

Tracker changed from Bug to Feature
Status changed from Open to Closed
ruby -v deleted (3.0.2)
Backport deleted (2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN)

@headius and I discussed this and we agree it is not a bug, though potentially the situation could be improved.  You could avoid the object flag mutation using a temporary hash per call to shareable, but that would likely make it even slower.  It doesn't seem possible to cache that an object is not shareable without creating significant cache invalidation issues.  Having String#freeze set the shareable flag if the string has no instance variables seems like a good idea, but that is more of a feature request than a bug fix.  Changing `Ractor.shareable?` to only do a shallow check and not a full check seems like a large semantic change, not a bug fix, and would make Ractor.shareable? operate differently than Ractor.make_shareable. Maybe a new Ractor.flagged_shareable? method could be added to do a shallow check for the shareable flag, but I'm not sure how useful such a method would be.

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Feature #18258: Ractor.shareable? can be slow and mutates internal object flags.
https://bugs.ruby-lang.org/issues/18258#change-103055

* Author: ioquatix (Samuel Williams)
* Status: Closed
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
----------------------------------------
On my computer, even with a relatively small object graph,`Ractor.shareable?` can be quite slow (around 1-2ms). The following example creates an object graph with ~40k objects as an example, and on my computer takes around 20ms to execute `Ractor.shareable?`. Because the object cannot be marked as `RB_FL_SHAREABLE` because it contains mutable state, every time we check `Ractor.shareable?` it will perform the same object traversal which is the slow path.

``` ruby
require 'benchmark'

class Borked
	def freeze
	end
end

class Nested
	def initialize(count, top = true)
		if count > 0
			@nested = count.times.map{Nested.new(count - 1, false).freeze}.freeze
		end

		if top
			@borked = Borked.new
		end
	end

	attr :nested
	attr :borked
end

def test(n)
	puts "Creating nested object of size N=#{n}"
	nested = Nested.new(n).freeze
	shareable = false

	result = Benchmark.measure do
		shareable = Ractor.shareable?(nested)
	end

	pp result: result, shareable: shareable
end

test(8)
```

I propose we change `Ractor.shareable?` to only check `RB_FL_SHAREABLE` which gives (1) predictable and fast performance in every case and (2) avoids mutating internal object flags when performing what looks like a read-only operation.

I respect that one way of looking at `Ractor.shareable?` is as a cache for object state. But this kind of cache can lead to unpredictable performance.

As a result, something like `String#freeze` would not create objects that can be shared with Ractor. However, I believe we can mitigate this by tweaking `String#freeze` to also set `RB_FL_SHAREABLE` if possible. I believe we should apply this to more objects. It will lead to more predictable performance for Ruby.

Since there are few real-world examples of Ractor, it's hard to find real world example of the problem. However, I believe such an issue will prevent Ractor usage as even relatively small object graphs (~1000 objects) can cause 1-2ms of latency, and this particular operation does not release the GVL either which means it stalls the entire VM.

This issue came from discussion regarding https://bugs.ruby-lang.org/issues/18035 where we are considering using `RB_FL_SHAREABLE` as a flag for immutability. By fixing this issue, we make it easier to implement model for immutability because we don't need to introduce new flags and can instead reuse existing flags.

-- 
https://bugs.ruby-lang.org/