shape.c: Implement a lock-free version of get_next_shape_internal #13441
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jhawthorn
approved these changes
May 26, 2025
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|
I got a CI failure on rebase that seems legit: It seems we're rebuilding an object shape during compaction. Not sure exactly when happened, but I suppose |
Whenever we run into an inline cache miss when we try to set
an ivar, we may need to take the global lock, just to be able to
lookup inside `shape->edges`.
To solve that, when we're in multi-ractor mode, we can treat
the `shape->edges` as immutable. When we need to add a new
edge, we first copy the table, and then replace it with
CAS.
This increases memory allocations, however we expect that
creating new transitions becomes increasingly rare over time.
```ruby
class A
def initialize(bool)
@A = 1
if bool
@b = 2
else
@c = 3
end
end
def test
@d = 4
end
end
def bench(iterations)
i = iterations
while i > 0
A.new(true).test
A.new(false).test
i -= 1
end
end
if ARGV.first == "ractor"
ractors = 8.times.map do
Ractor.new do
bench(20_000_000 / 8)
end
end
ractors.each(&:take)
else
bench(20_000_000)
end
```
The above benchmark takes 27 seconds in Ractor mode on Ruby 3.4,
and only 1.7s with this branch.
Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>
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It seems I was right. For now I made it so these objects are pinned, which fixes the issue. That's not ideal, but I have no better solution short term. My hope however is that if we move on with #13289, ultimately there would be no But that's multiple big "ifs". |
peterzhu2118
approved these changes
Jun 2, 2025
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Whenever we run into an inline cache miss when we try to set an ivar, we may need to take the global lock, just to be able to lookup inside
shape->edges.To solve that, when we're in multi-ractor mode, we can treat the
shape->edgesas immutable. When we need to add a new edge, we first copy the table, and then replace it with CAS.This increases memory allocations, however we expect that creating new transitions becomes increasingly rare over time.
The above benchmark takes 27 seconds in Ractor mode on Ruby 3.4, and only 1.7s with this branch.
cc @etiennebarrie @jhawthorn @tenderlove @luke-gruber