Issue #13772 has been updated by sam.saffron (Sam Saffron).
On Friday I downgraded one of our clusters, this is how it looks.

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Bug #13772: Memory leak recycling stacks for threads in 2.4.1
https://bugs.ruby-lang.org/issues/13772#change-65985
* Author: sam.saffron (Sam Saffron)
* Status: Open
* Priority: Normal
* Assignee:
* Target version:
* ruby -v: 2.4.1
* Backport: 2.2: UNKNOWN, 2.3: UNKNOWN, 2.4: UNKNOWN
----------------------------------------
Per:
https://github.com/rest-client/rest-client/issues/611
gem install rest-client
```
100000.times.each_slice(32) do |slice|
slice.map { Thread.new { RestClient.get('echo.jsontest.com/key/value/one/two') } }.each(&:join)
GC.start(full_mark: true, immediate_sweep: true)
slots = GC.stat[:heap_live_slots]
puts "slots #{slots} #{i+=1} threads #{Thread.list.count}"
end
```
Causes unbound memory growth on 2.4.1, this was not the case in earlier versions of Ruby or Ruby master.
When running through heaptrack and massif-visualizer it looks like all unreclaimed allocations come from thread_recycle_stack
Ruby heaps do not grow during tests, all the growth is around unmanaged memory.
I feel getting this sorted is urgent, we have seen similar leaks at Discourse.
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