[#100309] How to use backport custom field — Jun Aruga <jaruga@...>
Please allow my ignorance.
9 messages
2020/10/06
[#100310] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/06
IkJhY2twb3J0IGN1c3RvbSBmaWVsZCIgaXMgb25seSBhdmFpbGFibGUgZm9yIHRpY2tldHMgd2hv
[#100311] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/06
On Tue, Oct 6, 2020 at 4:44 PM NARUSE, Yui <[email protected]> wrote:
[#100314] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/06
VGhhbmsgeW91IGZvciBjb25maXJtYXRpb24uCkkgY2hlY2tlZCBhZ2FpbiBhbmQgdG8gZWRpdCBi
[#100322] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/07
On Tue, Oct 6, 2020 at 7:25 PM NARUSE, Yui <[email protected]> wrote:
[#100326] Re: How to use backport custom field
— "NARUSE, Yui" <naruse@...>
2020/10/07
SSBhZGRlZCB5b3UgdG8gIlJlcG9ydGVyIiByb2xlIGluIHRoZSBwcm9qZWN0CgoyMDIw5bm0MTDm
[#100327] Re: How to use backport custom field
— Jun Aruga <jaruga@...>
2020/10/07
On Wed, Oct 7, 2020 at 1:42 PM NARUSE, Yui <[email protected]> wrote:
[ruby-core:100479] [Ruby master Feature#17278] On-demand sharing of constants for Ractor
Issue #17278 has been reported by Dan0042 (Daniel DeLorme).
----------------------------------------
Feature #17278: On-demand sharing of constants for Ractor
https://bugs.ruby-lang.org/issues/17278
* Author: Dan0042 (Daniel DeLorme)
* Status: Open
* 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.
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