From: marcandre-ruby-core@...
Date: 2021-02-16T20:29:11+00:00
Subject: [ruby-core:102546] [Ruby master Feature#17592] Ractor should allowing reading shareable class instance variables

Issue #17592 has been updated by marcandre (Marc-Andre Lafortune).


ko1 (Koichi Sasada) wrote in #note-13:
> (2) Performance concern
> 
> To allow accessing ivars from other ractors, every ivar accesses to class/module should be synchronized.

I am surprised this is the case. Would it not be possible to have overwriting an existing instance variable be a single memory write operation? Adding a new instance variable would similarly involve creating a new ivar table and setting it with a single memory write operation?

> Current implementation doesn't need to synchronize this accesses.

That is not completely accurate. Current implementation does not but it is not 100% safe. The following completely artificial code crashes (sometimes):

```ruby
class String
  def self.test
    4000.times.map { |i| eval(<<~RUBY) }.each(&:join)
      Thread.new do
        10.times do
          Thread.pass
          @x#{i} = 42
        end
      end
    RUBY
  end
end

String.test
p String.instance_variables.size # => 4000
```

----------------------------------------
Feature #17592: Ractor should allowing reading shareable class instance variables
https://bugs.ruby-lang.org/issues/17592#change-90447

* Author: marcandre (Marc-Andre Lafortune)
* Status: Open
* Priority: Normal
* Assignee: ko1 (Koichi Sasada)
----------------------------------------
It would be very helpful if Ractor was allowing reading class instance variables from non-main Ractor.


Currently is raises an IsolationError:

```ruby
module Foo
  singleton_class.attr_accessor :config
  Foo.config = {example: 42}.freeze
end

Ractor.new { p Foo.config } # => IsolationError
```

This limitation makes it challenging to have an efficient way to store general configs, i.e. global data that mutated a few times when resources get loaded but it immutable afterwards, and needs to be read all the time.

Currently the only way to do this is to use a constant and use `remove_const` + `const_set` (which can not be made atomic easily).

I think that allowing reading only may be the best solution to avoid any race condition, e.g. two different Ractors that call `@counter += 1`.

The only 3 scenarios I see here are:
0) declare the constant hack the official way to store config-style data
1) allow reading of instance variables for shareable objects (as long as the data is shareable)
2) allow read-write

I prefer 1)



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

Unsubscribe: <mailto:ruby-core-request@ruby-lang.org?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>