From: daniel@...42.com Date: 2020-11-17T19:04:38+00:00 Subject: [ruby-core:100915] [Ruby master Feature#17278] On-demand sharing of constants for Ractor Issue #17278 has been updated by Dan0042 (Daniel DeLorme). In #17323#note-5, ko1 mentioned there is a possibility to provide "fork" model. So I tried thinking if it could apply here. We can imagine that accessing an auto-shareable constant a) from non-main ractor: is made shareable b) from main ractor: is made shareable, and then a deep-dup copy is made and set aside for use for main ractor only In this case it's no longer non-deterministic, but there are other tradeoffs. Memory usage is double. Most importantly, the constant may have a diverging value in the main ractor. Let's say you have `COUNTERS = Hash.new(0)` and the counters are only incremented in the main ractor; but from the perspective of the non-main ractors the counters would always be zero. I would find this _very_ unintuitive, and likely very hard to debug. I think the forking model could work for class variables because it's less surprising if a _variable_ has a different value in different contexts. In the end I remain convinced the original model I proposed is best. To a certain extent, non-deterministic behavior is a normal part of parallelism. For example in a producer/consumer architecture, if 2 producers generate each 1M 'A's and 1M 'B's, the consumer will see them in a non-deterministic order. No one would claim that's a problem. It's the same thing for this; the _order_ may be non-deterministic (iif there's a race condition) but the end result is identical: an error. ---------------------------------------- Feature #17278: On-demand sharing of constants for Ractor https://bugs.ruby-lang.org/issues/17278#change-88567 * Author: Dan0042 (Daniel DeLorme) * Status: Feedback * Priority: Normal ---------------------------------------- ### Description This proposal aims to reduce (but not eliminate) the need for freezing/sharing boilerplate code needed by ractors. ```ruby A = [1, [2, [3, 4]]] H = {a: "a"} Ractor.new do p A #A is not actually modified anywhere, so ok end.take H[:b] = "b" #H was never touched by ractor, so ok ``` ## Background Ractors require objects to be preemptively deep-frozen in order to be shared between ractors. This has an especially visible and restrictive effect on globals and constants. I tried thinking of a different way, and maybe I found one. So please allow me to humbly present this possibility. ## Proposal A constant would be by default in a "auto-shareable" state (A) which can change atomically to either (B) "non-shareable" if it is modified by the main ractor (C) "shareable" (and frozen) if it is accessed by a non-main ractor In detail: 1. When an object is assigned to a constant, it is added to a list of ractor-reachable objects 2. When the first ractor is created, the objects in that list are recursively marked with FL_AUTOSHARE * after this point, constant assignments result directly in FL_AUTOSHARE 3. In the main ractor, a call to `rb_check_frozen` (meaning the object is being modified) will 1. if FL_AUTOSHARE is set (state A) * [with ractor lock] * unless object is shareable * unset FL_AUTOSHARE (state B) 2. raise error if frozen * ideally with different message if object has FL_SHAREABLE 4. When a non-main ractor accesses a non-shareable constant 1. if object referenced by constant has FL_AUTOSHARE set (state A) * [with ractor lock] * if all objects recursively are still marked with FL_AUTOSHARE * make_shareable (state C) * else * unset top objects's FL_AUTOSHARE (state B) 2. raise error if not shareable ## Result So in the case that these 2 things happen in parallel: 1) main ractor modifies content of constant X 2) non-main ractor accesses constant X There are 2 possible outcomes: a) main ractor error "can't modify frozen/shared object" b) non-main ractor error "can not access non-shareable objects in constant X" ## Benefits In the normal case where non-frozen constants are left untouched after being assigned, this allows to skip a lot of `.freeze` or `Ractor.make_shareable` or `# shareable_constant_value: true` boilerplate. When you get the error "can not access non-sharable objects in constant X by non-main Ractor", first you have to make that constant X shareable. Then this can trigger a secondary error that X is frozen, that you also have to debug. This way cuts the debugging in half by skipping directly to the FrozenError. ## Downsides When you get the error "can not access non-sharable objects in constant X by non-main Ractor" you may want to solve the issue by e.g. copying the constant X rather than freezing it. This way makes it slightly harder to find where X is being accessed in the non-main ractor. In the case of conflict, whether the error occurs in the main ractor or the non-main ractor can be non-deterministic. ## Applicability This probably applies as well to global variables, class variables, and class instance variables. -- https://bugs.ruby-lang.org/ Unsubscribe: