Copy on write happens in pages. So whilst you are running, if your child processes (please don't call them "threads"), are each only accessing small subsets of the hash, then only small parts are replicated. But when clean up occurs, every scalar has to have its reference count decremented to 0 before it can be released, and that means that all of the hash has to be replicated for every child before it can complete.

In reply to Re: problems with garbage collection by BrowserUk
in thread problems with garbage collection by Anonymous Monk

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