Feature #18258: Ractor.shareable? can be slow and mutates internal object flags.

• Status: Closed
• Assignee: ko1 (Koichi Sasada)
• Target version: -

[ioquatix (Samuel Williams)]

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.

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.

[Eregon (Benoit Daloze)]

Yeah that's an alternative design.

Currently Ractor.shareable? semantics are "is it already shareable as in conceptually or not?".
(And the flag is just a cache for the "yes" case)
And it's not "is it already shareable as marked with Ractor.make_shareable or all instances of that class are shareable, or they are leaf frozen objects".

Note that String#freeze could only set the shareable flag if the String has no ivar, otherwise it's incorrect.

I'm not against that design, but it might be compatibility issue.

Anyway, I think no program should check Ractor.shareable?, probably only Ractor internals should check that, and if not shareable then raise an exception and therefore performance before that is not that big a deal.
So maybe one solution is removing Ractor.shareable?, and also in C-API, they seem only useful for Ractor internals, and maybe for debugging (maybe they could be only defined with ruby -d?).

[ioquatix (Samuel Williams)]

I feel from a predictability POV, it would definitely be advantageous to set RB_FL_SHAREABLE in every case it's known ahead of time to avoid unexpected overheads. Even if Ractor.shareable? is not public interface, it's still used internally in many many places...

[Eregon (Benoit Daloze)]

I think for the semantic model it could be easier with the proposed semantics in this issue: only always-shareable objects are shareable without Ractor.make_shareable/freeze for leaf objects with no ivars/deep_freeze/Immutable.
Right now it's probably confusing for users that some objects are magically shareable? if there were enough .freeze.
OTOH, it seems impossible that shareable? returns true except if the user already froze the instance and there are no other non-frozen objects referred in it.

Cons: it would require using Ractor.make_shareable even more, instead of being able to just call enough .freeze.
Also once we have Immutable, those objects should of course be shareable, yet Ractor.make_shareable wouldn't be called on them.

[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 (Charles Nutter) 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.